The Body at Work: A Treatise on the Principles of Physiology

by Alex Hill

Luminous Glands.

If it be difficult, when considering the dispersal of energy as mechanical work, heat, or electricity, by living tissues, to bring the phenomena into line with those of which physics takes experimental cognizance, how are we to approach the problems involved in the generation of light? Yet the photogenic property of protoplasm is widely distributed. Protozoans and various other invertebrate animals cause the so-called phosphorescence of the sea. The abysmal depths of ocean are lighted by forests of luminous polyps, and traversed by fishes whose heads are furnished with lamps. By her own light the female glow-worm enables her winged mate to keep his tryst. Fireflies (Lampyrus) flash amongst the orange-trees of Italy, and blaze (Pyrophorus) beneath the mangoes of Ceylon.

Luminous organs vary too widely in structure to allow us to pick out, as in the case of electric organs, the features which are common to them all. In Pyrophorus the organ is a double mass of cylindrical cells near the tip of the abdomen. The cells are set vertically to the surface, and are supported by a tubular membrane. Their substance contains a kind of fat. Beneath them there is a layer of cells, not luminous, but evidently a part of the photogenic apparatus, containing chalky granules. The organ is well supplied with nerves and with respiratory tubes (tracheæ).

More interesting than its structure is the study of the peculiar character of the light which the organ emits. It gives a spectrum which extends from the red (beyond Fraunhofer’s line B of the solar spectrum) to the first blue rays (F). It shows no lines. Green rays appear only when the light is bright, and then they are the brightest of all the rays. The light is practically destitute of actinic or chemical rays. A photographic plate may be exposed for several minutes, almost without changing, to the light of a firefly bright enough to enable one to read with ease in a dark room; whereas light of equal brilliance from any other source would change it in the fraction of a second. Nor are heat-rays mixed with the light. Measurements show that the activity of the photogenic organs does not give rise to any greater rise of temperature than would occur in the case of any other gland.

The contrast between the emission of light by an animal and its production in any other manner is very striking when the physical evidence, or want of evidence, of what happens in the protoplasm which produces it is considered. The fact that no heat accompanies the light precludes us from attributing it to oxidation. If a firefly is enclosed in a vessel of oxygen, its lamp burns no brighter—clear evidence that its luminosity has nothing in common with the burning of a match or the glowing of a stick of phosphorus. Nor is the lamp put out when the insect is suddenly exposed to great cold (-100° C.). It continues to shine until the cold kills it. There is no relation between the luminosity of a firefly and the phenomenon termed “phosphorescence” by physicists. Sulphide of calcium—the substance used for rendering matchboxes visible in the dark—returns light which it has absorbed. A firefly’s power of emitting light is in no wise affected by keeping it for a long while in the dark.

Like all other events in vital chemistry, the generation of light by protoplasm is due to a process of fermentation. The luminous organs may be crushed, and the mixture of fermentable substance and ferment extracted with water. The extract is luminous. If an extract is prepared rapidly, and evaporated to dryness in vacuo, the residue glows when moistened with water. That two substances are present in the extract, one (luciferin) fermentable, the other (luciferase) a ferment, is proved by the following experiment: A certain quantity of extract is divided into two portions. One part (A) is allowed to glow until its capacity for emitting light is exhausted. The other portion (B), as soon as it is separated, is heated to 55° to kill the ferment. B still contains luciferin; A contains luciferase, although all its luciferin has been used up. Recombined, the extract is luminiferous.

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CHAPTER XI
THE NERVOUS SYSTEM

Twenty-five years ago a new process was introduced for colouring the elements which by their combination make up the nervous system. With its aid anatomists discovered the inadequacy of their conceptions of nerve-cells. It was already known that a nerve-fibre—that is to say, its essential part, its core—is a part of a cell, the body and other parts of which are situate within the brain or spinal cord, or in one of their dependents, a ganglion. But the new method showed the nerve-cell as more elaborate in form than anything which had been imagined hitherto; and since the word “cell” was often loosely used when the cell-body alone was referred to, it seemed worth while to give the unit of structure a new name. The term “neurone” was introduced to emphasize its functional individuality. The nervous system is an association of neurones.

By the extremely simple expedient of placing a small block of nerve-tissue in bichromate of potassium, and then transferring it to nitrate of silver, jet-black pictures of nerve-cells are obtained showing with amazing completeness all the details of contour of their bodies and all the intricacies of branching of their limbs. The most surprising feature of the process is the absence of confusion in its results. Dyes were in use which stained one kind of cell better than another, or picked out a particular part—usually the nucleus—of every cell. If the chrome-silver process had acted in the same way, a dense black preparation in which no details could be distinguished would have been the result. But instead of treating all cells alike, the process blackens one cell here and another there, leaving hundreds or thousands untouched. It shows no preference for any particular kind of cell. In one section large cells are picked out, in another small ones; in a third no nerve-cells are blackened, but connective tissue is brought into view. When the block of tissue soaked with bichromate of potassium is immersed in a solution of nitrate of silver, the chromate escapes from it into the surrounding liquor much more quickly than the nitrate gets in; and when at last the nitrate of silver enters, it finds that some of the cells have fixed the chromate in their substance. This retained chromate combines with silver. The product is rapidly reduced to a black subchromate. No explanation of the fixing of the chromate by individual cells has yet been offered. It is a remarkable fact that another process which similarly makes choice amongst the elements has since been introduced, giving even more valuable results. Pieces of fresh tissue are placed in a very dilute solution of methylene blue. When staining is satisfactory, nerve-cells alone take up the dye. The selection of individual nerve-cells is not carried so far as it is by the chrome-silver method, but it is exhibited to a certain extent. It is probable that nerve-cells live (in a physiological sense) longer than other tissue-elements. Methylene-blue contains some easily removable oxygen of which the oxygen-starved nerve-cells take advantage. The reduced methylene-blue remains in their substance, so that when the preparation is reoxidized by exposure to air the pattern of the nerve-cells is rendered conspicuous. When a few cells are selected, it is, presumably, because they were the only ones alive at the time when the dye entered the tissue. Preparations made from the wall of the alimentary canal seem to justify this simple explanation. They show patches in which muscle-fibres are stained, patches in which there is no staining, and intermediate zones in which nerve-cells are coloured and muscle-fibres are not. But the hypothesis is inadequate to meet all cases. When first employed, the blue was injected into the animal in successive doses until it killed it. The staining was believed to occur intra vitam. Subsequently it was found that its application to fresh tissue, or, for certain results, to tissue which has been kept for some hours, is equally effective.

Without an understanding of the nature of the two new processes, and of the character of the results which they yield, it would be impossible for the reader to realize the extraordinary advance in our knowledge of the finer structure of the nervous system which has marked the period during which they have been employed.

The chrome-silver process is the more useful for the central nervous system. Methylene-blue gives better results with tissues containing minute nerve-cells and the branches of nerves. The latter method has revealed such a profusion of nerve-twigs as would never have been suspected but for its use. Consider, for example, the lining epithelium of the lungs ([p. 168]). Every one of its flattened cells has its own nerve twig or twigs. They lie between the cells. They give branchlets which enter them. A similar statement might be made regarding the richness of the nerve-supply of the muscle-fibres of the alimentary canal, or of the cells of glands, and possibly of other tissues. Each fresh success achieved in the application of the method makes a further revelation of the abundance in which nerves are distributed, increasing our sense of the dependence of all vital processes upon nervous control, and our appreciation of the unifying and integrating importance of the nervous system.

The term “neurone” is used by certain writers with a view to emphasizing their belief, not in the functional individuality alone of the unit of structure, but also in its anatomical isolation. The peculiarity of the methods of coloration which we have described lies, as already pointed out, in their selecting the cells which happen to be in a particular nutritive condition, and ignoring their neighbours. Hence pictures of separate and discrete units are obtained. This proves the nutritive autonomy of the cells, but it does not necessarily follow that A is not structurally connected with B, and B with C. Impulses are passed along the chain from A to C. Functionally, therefore, they are linked together; but until the question as to the way in which contact is established is settled, it is as well to think of the neurones as anatomically discrete.

It would be impossible in this book to describe all the varieties of neurone, for nothing is so characteristic of these elements as their enormous range both in size and form. It may be truly described as having no limits. Each of the two electric organs of Malapterurus is governed by a single neurone. Its cell-body is a fifth of a millimetre or more in diameter—large enough to be seen with the naked eye—and traversed by capillary bloodvessels. The axon of this nerve-cell—its single nerve-fibre—ramifies to supply a separate branch to each of the 2,000,000 chambers of the electric organ, and each branch breaks up into a bunch of twigs within the chamber. Contrast with such a giant cell as this one of the granules of the retina or cerebellum, the smallest cells to be found in the body, yet each a perfect neurone, exquisitely elaborate in form.

Fig. 19.—A Nerve-Fibre consisting of A, the Undivided, Fibrillated Axon of a Nerve-Cell, with its Various Wrappings.

In segment 1 the wrappings comprise B, a tube of phosphatic fat (myelin), interrupted at H, a node of Ranvier; C, a delicate membrane (sarcolemma); D, connective tissue; E, the rind of the axon; F, a tubular space containing lymph, between the axon and its sheath of myelin; G, nucleus of an enwrapping cell. At I the myelin is seen to be divided into overlapping conical rings. 2, The medullated nerve-fibre, running an isolated course, is merely enclosed in a tube of connective tissue containing lymph. 3, As a “grey” or “non-medullated” fibre, the axon has lost its myelin sheath.

As types for description we may take one of the motor cells of the spinal cord and a granule of the cerebellum. Every nerve-fibre which supplies a group of voluntary muscle-fibres is a thread drawn out from a large cell-body which lies in the grey matter of the spinal cord or of the axis of the brain. The fibres pass out in the anterior root of a spinal nerve or in a cranial nerve. The cell-body may have a diameter of as much as 100 µ (1 µ = 0·001 millimetre). In shape it is like a very irregular starfish, owing to its being continued into several, usually four or five, thick tapering branching limbs or processes, known as dendrites, in addition to its slender thread-like axon. From its origin in a cell-body to its destination in a muscle—it may be a few inches, or it may be a yard away—the axon is an unbroken thread. A short distance from the cell-body it enters a tubular sheath, which protects and insulates it, recalling the covering of gutta-percha in which the wires of a telegraph cable are enclosed. The sheath is of a phosphatic fat, invested and held in place by a delicate transparent membrane, neurilemma. Beneath this membrane nuclei occur at regular intervals, and midway between each two nuclei the sheath is cut across by a septum. Such interruptions or nodes show that the sheath is not a part of the nerve, if the term is used in the most restricted sense. Each internode is a cell which has been wrapped round the nerve for its protection. The axon with its sheath is spoken of as a nerve-fibre. A large number of nerve-fibres bound together by connective tissue constitute a nerve. In some cases the axon before it leaves the spinal cord, but after it has entered its myelin sheath, gives off one or two lateral branches (“collaterals”), which return to arborize in the grey matter of the cord. It does not appear that they are always present in the case of the motor neurones of the spinal or cranial nerves—probably they are usually omitted—but collaterals are important features of the large neurones of the cortex of the cerebrum and cerebellum (Figs. 23, 24). Usually two, three, or four such branches start off at right angles from the axon, and after a time turn back towards the surface, dividing into a few extremely slender branches. Their purpose is an enigma. Possibly they bind a group of cells together in functional unison. Such an explanation would seem reasonable in the case of an arrangement of collaterals on the plan we have just described; but in various situations in the brain cells are seen of which the axons, instead of becoming nerve-fibres, break up completely into collaterals, which branch repeatedly.

Fig. 20.—A Ganglion of a Leech.

Pear-shaped cells are set round a felt-work of nerve-fibrils (neuropil). A neuro-sensory cell is shown with one fibre directed peripherally, branching on the surface; and one directed centrally, ramifying in the neuropil. Several very slender fibrils from the neuropil pass up the stalk of each ganglion-cell. They join a network near its surface. This net is connected by radiating fibrils with a coarser net which surrounds the nucleus. From the central net a relatively stout fibril passes to muscle-fibres.

By various methods it may be shown that dendrites, cell-body, and axon contain fibrils ([Fig. 22]). These neuro-fibrillæ lie parallel to one another in the axon. Where it divides they are distributed amongst its branches. Possibly they also branch. In the neurones of Malapterurus, already referred to, this would appear to be inevitable. The discovery of neuro-fibrillæ seemed to carry us a step nearer to a comprehension of the physics of nervous conduction. They clearly indicate that particles of the substance of a nerve-fibre are oriented in the direction in which impulses pass. It is a structural differentiation similar to the fibrillation of muscle, and probably of the same order—a response to the same demand. But when we examine the arrangement of the fibrils in a cell-body and its dendrites, the appearances which we discover serve to perplex us. They complicate instead of simplifying our mental picture of the conduction of nervous impulses. The coarsest and most distinct neuro-fibrillæ are to be found in annelids, the ganglion-cells of a leech, for example, affording excellent preparations. These cells are pear-shaped, with a single stalk. As is usual in invertebrate animals, they do not exhibit separate dendrites and axon, but dendrites and axon pass out from the cell in the common stalk. The bodies of the cells are set round a felted mass of nerve-filaments, into which their stalks break up. Just beneath the surface of the stalk of one of these cells two or three very fine neuro-fibrillæ are to be seen. A single, much coarser fibril occupies its axis. The fine fibrils join a network at the periphery of the cell-body. The thick fibril is connected with a coarser network which surrounds the nucleus. Radiating threads unite the finer with the coarser net. It has been suggested that afferent impulses ascend the fine fibrils, pass from the finer to the coarser net, and take their exit by the thick fibril, which can be traced into a motor nerve. Such a transit could not, so far as one can imagine, have any effect upon the distribution of the impulses which pass through the neurone; besides, there are reasons for believing that the course taken by impulses which are delivered to the ganglion by sensory nerves is determined by the felt-work in its centre, the neuropil. It is probable that during their passage through the cell-body impulses acquire the energy requisite to discharge the muscles to which the motor-fibre carries them. In vertebrate animals, sensory nerves are branches of neurones of which the cell-bodies lie in cranial or spinal ganglia. They resemble the ganglion-cells of the leech in as much as they are unipolar; both branches, the one which collects impulses from sense-organs, and the one which distributes them to the spinal cord, come off from the cell in a common trunk which afterwards divides, although the unipolar condition of the cell of the spinal ganglion is not primitive, but acquired. In the earliest stages of its growth the cell is bipolar. Its two ends subsequently grow together for a certain distance, the common portion being the vertical limb of the T ([cf. Fig. 21], which shows the growth of a granule of the cerebellum). The body of the cell contains a network not unlike the network of the leech. It is probably related to what may be termed the charge of the neurone, the development of a suitable degree of force in the impulses which pass through it.

The neuro-fibrillæ of a large nerve-cell, such as a motor cell of the spinal cord, are exceedingly slender ([Fig. 22]). They branch and reunite. A certain number gather towards the axon; but the majority pass through the cell from one dendrite to another, or from one branch of a dendrite to another branch. It is very tempting to suppose that neuro-fibrillæ are connected with conduction. When first discovered they were regarded as conducting strands; but it is evident that they are not comparable with telephone wires or other isolated or separate conductors. There are good reasons for regarding dendrites as collecting processes, taking up impulses from the end-twigs of the nerves which branch in the grey matter around them, passing them through the cell-body into the axon. The continuation of neuro-fibrillæ from dendrite to dendrite seems to be irreconcilable with the hypothesis that they are disposed in the lines of conduction.

In common with those of various other types of neurone, the dendrites of spinal motor cells are beset with “thorns.” These projections are not rugosities or serrations, but short, delicate threads which stand out at right angles from the dendrites ([cf. Fig. 1]). About a dozen years ago, the writer made a careful investigation of these structures; at a time when most anatomists regarded them as artifacts. He found that their claim to be regarded as parts of the neurone is as good as that of its axon or its dendrites; although never seen on certain types of cell, the thorns, of cells which carry them, are perfectly definite in arrangement and spacing. In some kinds of cell they are more numerous, in others less. Neuro-fibrillæ, as we now know them, had not been discovered at the date when this investigation was undertaken; but on various grounds the conclusion was arrived at that thorns are the cell-ends of fibrils which pass from the end-twigs of arborizing axons into dendrites. Upon this conclusion was based an hypothesis of conduction which is here submitted, not because there is not much to be said against it—or, at any rate, many a hiatus in knowledge to be filled—but because it happens to be the writer’s own. The chrome-silver and methylene-blue methods which reveal the existence of thorns do not stain neuro-fibrillæ. They colour the soft protoplasm in which fibrils are embedded. By modifying the chrome-silver method in every way which still allows a result to be obtained, it was found that thorns sometimes appear as comparatively long slender filaments, at others as shorter filaments ending in minute knobs, or as filaments bearing two or three dots; or finally no filaments are visible, but the dots are in the position which they would occupy if fibrils were present, but not stained. From this it was argued that the soft protoplasm which during life surrounds the filament as a continuous film, either falls back towards the cell after death or is made to shrink into the cell by reagents. This accounts for the appearance of rod and knob. What is supposed to happen may be illustrated by dipping a wire in treacle. At first, when the wire is withdrawn, it is surrounded with a film. Then the film gathers into droplets. It was suggested that the entrance of impulses into dendrites, their conduction across the space which separates the end-twigs of axons from the dendrites into which their impulses pass, is by means of the thorns, although the thorns are not in themselves conductors. Conduction occurs only when films of cytoplasm surround the thorns. The first effect of impulses is to call out the films, in the same kind of way that a current of electricity converts a row of falling drops into a continuous stream. A succession of impulses, by adding to the number of the filaments which are enveloped in cytoplasm, or by increasing the amount of cytoplasm investing certain groups of filaments, increases the openness of the path. Sleep is a condition in which all paths are open. Hence no impulses are effective. Wakefulness, alertness, depends upon the closing of all paths save those which are actually in use. We may go further. The power of concentrating attention is the power of limiting the spread of nerve-impulses in the brain. Alcohol opens extra paths; the concentrated effort which was making progress with a problem becomes more diffuse. The first effect appears in greater brilliance of thought, gained at some sacrifice of cogency. Unexpected analogies are discovered. Imagination takes a wider range. But as the dose is increased, a condition akin to sleep is set up. Nerve-impulses become ineffective because, many paths being open, they do not attain a sufficient intensity in any set of paths. These few illustrations are given for the sake of showing the need of a theory of the opening and closing of paths. It is not suggested that they favour the particular hypothesis here set forth as to the structural arrangement which provides the paths and regulates their accessibility.

Recent discoveries in the finer structure of the central nervous system have provided many problems which at present appear insoluble. One of the discoveries most difficult to make use of in constructing theory is the existence of extracellular or pericellular nets, which have the appearance of extraordinarily delicate cases of wire-netting immediately surrounding the nerve-cells. It is somewhat remarkable that the spacing of the nets is often very similar to, if not identical with, the spacing of thorns. While some anatomists look upon the nets as nervous, others regard them as pertaining to the connective tissue of the nervous system. At present it is not known how impulses get across from the finest visible twigs of arborizing axons to the dendrites of the neurones which they influence. The wealth of structural detail which recent research has revealed is an embarrassment to anyone who tries to devise a scheme. Not improbably, pericellular nets are intermediate factors in the exchange; or, if not the nets, the structures whose existence is indicated by the appearance of the nets. In the case of many of the finer markings which staining methods bring into view, it is impossible to say whether they indicate the presence during life of the structure as it appears to be, or whether the markings are due to coagulation of plasma or to strain caused by shrinkage in coagulating agents. In a sense this is not of much consequence. Coagulation in a uniform pattern would mean the existence of an architectural substructure which determines the pattern. We may be looking at the cake or at the tin the cake was baked in.

There is a danger of seeing too much in a nerve-cell when examining it under the highest powers of the microscope, and of endeavouring to picture in too much detail the arrangements which regulate the flow of impulses. Its markings are so complicated as to suggest to the mind of the observer that it is itself a microcosm—a nervous system in miniature. Neuro-fibrillæ appear to offer many alternative paths within the cell. It is unlikely that such a way of looking at the unit of structure is the right one. A certain motor cell of the spinal cord is connected by its axon with thirty or forty separate muscle-fibres; but there is no reason for thinking that the fibres ever contract save as a single group. The axon consists of parallel fibrillæ, but these do not appear to be needed as separate conductors; an impulse travels down the fascicle. It does not appear to be necessary in the case of a motor cell, and presumably the statement holds good for the large cells of the cerebellum and cerebrum to picture any arrangement for the simultaneous conduction in its axon of several impulses, or for the conduction of one impulse along one of its fibrillæ and a different one along another. What is necessary is that this particular efferent path Z should be accessible from every other part of the nervous system—from A to Y. If, merely for the sake of filling the space which would otherwise be blank in the mental picture, we imagine a pericellular net connected by thorns with the body and dendrites of the nerve-cell Z, then the net is the meeting-ground of all the routes through which Z is called into action. A nerve-wave from any of the neurones A to Y, breaking upon this net, passes along the thorns into the protoplasm of Z.

In size a granule of the cerebellum presents a marked contrast to a motor cell of the spinal cord; yet it is formed on essentially the same plan. From its minute round body (about 8 µ in diameter) four or five slender dendritic processes are drawn out. Each dendrite ends in a little bunch of twigs, resembling fingers curved over the palm. Its single slender axon runs towards the surface of the cortex. As the granules lie at a considerable depth, this course is, for those which distribute to the most superficial layers, a long one. They pass from the granular to the molecular layer between the big cells of Purkinje. When the axon has reached a certain level in the molecular layer, it divides into two threads which run for a great distance, right and left.

The granules of the cerebellum have a curious developmental history. Every neurone in the body has a lifelong existence. Except for the rare accident of its destruction by disease it occupies its station to the hour of death. But at the time of birth many neurones are still immature. Not all the granules of the cerebellum have yet assumed their permanent form or situation. Beneath the pia mater there is still a layer of minute undifferentiated cells. These, as they grow into granules, elongate, in the first instance, into long spindles. Subsequently they sink down through the molecular layer and between the cells of Purkinje, leaving the poles of the spindle as the right and left divisions of the axon ([Fig. 21]). It is interesting to learn that such a migration is possible. It is also of interest to find that a tiny granule of the cerebellum goes through the same stages in attaining its adult form as one of the large cells of a spinal ganglion.

Fig. 21.—The Growth and Migration of Granules of the Cerebellum.

Half a dozen nuclei of as yet undeveloped granules are seen lying beneath the pia mater. From this level to the bottom of the drawing granules are shown in successive stages of growth. These developing granules, selected from various preparations of the cortex of the cerebellum, were drawn from nature.

There are many different types of neurone. Any attempt to describe them, or to give an account of the various details of structure which recent improvements in technique have enabled anatomists to observe, would fill a lengthy treatise; and would, moreover, be beside our aim, which is limited to obtaining such an idea of the unit of the nervous system as will enable us to form a conception, however crude, of the way in which it works. From the brief account that has been given, it will be evident that anatomists are approaching to an understanding of the mechanism. It will also be evident that they have already more information than they can apply. They are cognizant of many details of structure which they cannot interpret in terms of function; and at the same time are aware of wide gaps in their knowledge regarding facts which are essential to the construction of any scheme. This much is clear: A sense-cell on the surface or beneath it is touched (probably entered) by the ultimate twig of the outer limb of a neurone whose cell-body lies in a spinal ganglion, while its inner limb, as a fibre of a posterior root, enters the spinal cord. In the spinal cord the root-fibre splits into an ascending and a descending division which rain branches into the grey matter over a considerable area above its point of entrance, and a smaller area below it. The finest twigs of these branches are to be seen in the vicinity of the cell-bodies and dendrites of certain other neurones. The axons of these second links arborize in a similar way in the vicinity of large motor cells, whose axons in turn become fibres of anterior roots. (For simplicity’s sake no reference is made to hosts of other neurones which link the ganglion-cell and the motor cell to other cells higher in the cord or brain.) An impulse generated in the sense-cell on the surface of the body runs up the root neurone into the cord, where the ultimate twigs of the posterior root-fibre offer it a wide choice of distribution. Following the path of least resistance, it passes into neurone No. 2. Again, the arborization of No. 2 offers it alternative paths. It makes a choice which lands it in No. 3. No. 3 passes the impulse on to the muscle-fibres with which it is connected. Three points are especially worthy of attention: (1) The impulse has a wide (literally, an unlimited) choice of routes. The skin of the finger is touched. Any muscle may respond, although resistance is so graded as to cause the impulse to seek in the first instance the group of muscles which is most often required to act in consequence of stimulation of the finger. This means, we may suppose, that it follows the chain which, having the smallest number of links, offers least resistance. If it cannot get through to these muscles, owing to the fact that other impulses, acting simultaneously, either increase the resistance in this particular path, blocking its way, or reduce the resistance in an alternative path, it spreads farther afield. (2) Owing to the ramification of the root-fibre which conveys it to the cord, an impulse is not limited to a single line of distribution. It reaches many secondary links. It may therefore influence various effector neurones simultaneously. For example, a stimulus which calls extensor muscles into action, at the same time inhibits their flexor antagonists. (3) The path which it finally takes is accessible to all other impulses. Its root neurone was peculiar to itself. Link No. 2 was more or less a common path. Neurone No. 3 is open to every impulse which traverses the nervous system.

Anatomy justifies the construction of the scheme just outlined. But there are many points regarding structure upon which a physiologist desires information, many details that he wants to see filled in. How is the impulse passed from the arborization of axon No. 1 to the dendrites of neurone No. 2? By what structural arrangement is resistance introduced, and how is it regulated, if it varies? Supposing the resistance to be higher in one path than in another, or supposing that more force is needed to enable an impulse to invade a wider field, how is additional energy supplied? To the first question no answer can be given at present—the mechanism by which impulses are transferred from one neurone to another is unknown; yet it is convenient to find a name for the junction of axon-endings and dendrites. It is termed a “synapse,” on the understanding that the word involves no hypothesis as to its structural nature. It is generally held that resistance is introduced into nerve-circuits at synapses; although this again is a provisional statement. The phenomena for the explanation of which the idea of synaptic resistance was introduced, may be accounted for on a purely anatomical basis of distribution. The extent to which one neurone influences another may depend upon the size of the brush of fibrils with which its axon touches it. If a certain force is needed to discharge a neurone, a nerve-current must either have a sufficiently high potential when it reaches it, or it must act upon it for a sufficient length of time. There is little to choose between the arguments which place the resistance at the synapse and those which transfer it to the nerve-cell body.

As a mechanism the nervous system is unthinkable, unless we picture its units as independent, yet capable of forming associations; as functionally discrete, yet entering into functional continuity. When acting, they act as chains. Impulses run from link to link, from the end-twigs of an axon of one cell to the dendrites of the next. Neurones are so arranged as to make it impossible for impulses to escape backwards out of dendrites into axon-twigs. In this respect the system is valved. But there is no reason for thinking of the substance of the neurone as polarized in any way. The physical accompaniment of an impulse—the electric variation—travels with equal facility up and down its axon.

There is no evidence of any specificity of neurones; on the contrary, it is clear that impulses of every kind—that is to say, from every source, for we recognize no specificity of impulses—can travel equally well through neurones of all forms. At every junction, in passing through each synapse, they are delayed. It takes at least 0·01 second (less if the knee-jerk be a true reflex action) for a message delivered to the cord by a sensory root to reach a motor root. This hundredth of a second—the sum of the delays entailed in fording two or three synapses—is regarded as the minimum reflex time. To it must be added, in considering any particular reflex action, the time taken in travelling up sensory and down motor nerves. Delay indicates resistance. If a sensory stimulus be not sufficiently pronounced to provoke a reflex action, the reflex may be obtained on intensifying it. Prolonging or repeating the stimulus—really the same thing, since sensory impulses are rhythmic, not continuous—has a far more potent effect than increasing its force. The resistance of synapses gives way after a number of impulses have bombarded them. The desire of brushing a fly from the skin, if resisted, becomes intolerably urgent after a time. A persistent outflow of impulses produced by the irritation of a spot in the cortex of the brain overwhelms the nerve-muscle system in an epileptic fit. The following is an experiment illustrating the spread of impulses from their customary path to another less often used: A piece of blotting-paper, wet with vinegar, is placed on the inner side of the thigh of a brainless frog. There is no use in trying the experiment on a frog which retains its brain; the substitution of one action for another would be an exhibition of the adaptation of means to end—a demonstration of the animal’s right of choice. Besides, the frog might choose not to act, and so the experiment would fail. The brainless frog wipes off the blotting-paper with the foot of the same side. This foot is then fixed so that the action cannot be performed, and the blotting-paper replaced. After a longer interval the frog removes it with its other foot. Evidently it is more difficult for the impulses generated by the irritation which the vinegar causes to get across the cord than it is for them to reach motor neurones on the same side. Evidently, too, the continued irritation of the vinegar adds to the travelling power of the impulses. They are strengthened until they are capable of overcoming the resistance in the longer path. “Resistance in conductors” and “potential of current” are terms with which the study of electricity has rendered us familiar; but it must be evident from the experiment just described that these terms are not really applicable to nervous phenomena, convenient though they may be for use in an allegorical sense. Holding the foot does not, by any mechanism which we can recognize, switch off the shorter circuit, yet the impulses abandon it for the longer path. There is no evidence of a struggle to free the foot that has been fixed, coincident with the spread of impulses, as they gather sufficient strength to reach the nervous mechanism of the other leg. The right foot not being available, the impulses choose the route to the left foot. Any attempt to explain this in terms of resistance and potential involves the formulation of a number of subsidiary hypotheses; easy to devise, no doubt, but stultifying to the explanation exactly in proportion as they complicate it. Yet the hypothesis of lines of greater and of less resistance (keeping as far away from electrical analogies as possible) is essential to any explanation of nervous phenomena, and is, moreover, justified by the evidence available. There are two causes in chief upon which it depends: (1) The greater the number of neurones in a linear chain, the greater is the number of synapses to be traversed. If A, B, C are in the same circuit, the sum of their resistance has to be overcome. (2) The greater the number of neurones amongst which a nerve-current has to be subdivided, the smaller the charge available for each of them. Imagine

	B
A
	C

so placed as to divide B and C, the charge delivered by A between. This arrangement has, probably, an anatomical expression which accounts for the relative ease or difficulty of a path, even on the supposition that impulses do not open out as they advance—do not spread along all the branches into which an axon divides—but keep to a given line. The axon of neurone A divides, to branch about B, C, and D; but its representation in the several pericellular nets (the expression may pass for the sake of the simplicity which it introduces into the picture) is unequal. In the vinegar experiment the impulses delivered to the spinal cord by the root-ganglion neurone A pass to neurone B of the posterior horn. B’s axon arborizes more freely about the cell-body of neurone C in the anterior horn of the same side than it does about neurone D in the anterior horn of the opposite side. Hence the impulses generated by the vinegar stimulate C, sufficiently to discharge it, so long as that road is open, more quickly than they stimulate D. That C should be dischargeable only so long as the foot is free implies that the activity of the neurone is in some way conditioned by its relation with the muscles which it innervates. When the foot is held this relation is interfered with, giving to the impulses generated by the continued action of the vinegar time to overcome the resistance of D.

The simile of the opening up of paths is fairly applicable to the results which follow the use of artificial stimuli. Neurones seem to link up in series under the influence of the impulses which bombard them, popping like fireworks united by a common fuse.

Experimental evidence points to the following conclusions: (1) Resistance is offered at a synapse. This resistance must be overcome before an impulse can get through from neurone 1 to neurone 2. (2) The impulse does not, properly speaking, pass from 1, through 2. It infects 2, causing it to discharge a fresh impulse. (3) Time is of the essence of this process. Either the impulses head up at the synapse, or, passing through into the neurone, they produce a cumulative effect within it, which provokes it to discharge. (The latter hypothesis, which is the less likely of the two, transfers the resistance from the synapse to the neurone to be infected.) These conclusions are based upon experiments of the following kind: The minimal stimulus which will evoke a reflex action is determined. A stronger stimulus is then applied. The reflex occurs more promptly, and is more pronounced. But on further increasing the stimulus, it is found that the limit of effectiveness is soon reached. The proportional relation of response to stimulus is much less evident than it is when the experiment is tried with a nerve-muscle. Choosing a reflex action easily provoked, the afferent path is stimulated with an electric current interrupted fifty times a second. The impulses which flow down the efferent path to the muscle follow one another at the rate of about ten a second. A column of nerve-fibres within the spinal cord is stimulated fifty times a second. Again, the discharge into anterior roots has the natural rhythm of about ten. The cortex of the “motor area” of the great brain is stimulated with a rapidly interrupted current. The muscles which it governs contract with their natural rhythm. The cortex is sliced away, and the stimulus applied to the white matter beneath. A similar result is obtained. Evidence such as this points to an independence of action on the part of the neurones which one can express only in terms of resistance and explosion. But there is another line of thought which leads to the development of a picture of the working nervous system which seems at first sight incompatible with the one that we have sketched. The phenomenon of the knee-jerk ([p. 274]) reveals a nervous system so intimately linked together, so homogeneous, so mobile, that no event, however trivial, occurs in any part without sending a vibration throughout the rest. Instead of a multitude of batteries enveloped in a labyrinth of wires interrupted by myriads of switches which are crackling on and off, the image of a sheet of water better figures our conception—a material so frictionless that it is a-ripple from side to side and end to end, from the most distant rivulet which feeds it to the farthest trickle in which it drains away. It is a fluid in a state of infinite commotion, the movements of its particles varying in amplitude from tremulous quiverings which scarcely frost the silver of its surface to waves which, breaking on the muscular system, throw it up in heaps. The vinegar experiment seems to demand a scheme of batteries and wires. The knee-jerk points to a continuous conducting medium. Other phenomena suggest the superposition of the two pictures; the conception of a nervous system consisting of a uniform medium conducting, not indifferently in all directions, but with such freedom that from our point of view the paths are infinite in number; and within this conducting medium nerve-cell bodies and their processes which collect and distribute groups of vibrations sufficiently strong in combination to produce visible effects. In order that one of these neurones may be stimulated to discharging-point, the medium by which it is surrounded must be thrown into such a state of agitation as suffices to infect it. The considerations which point to the formulation of this double or superposed scheme are such as follow: The passage of tone-impulses does not appear compatible with the ideas we have formed on other evidence of synaptic resistance and neuronic discharge. They are too feeble for such a mechanism. The short “reflex time” of the knee-jerk points to the passage of the agitation up a sensory root to the spinal cord, and through a non-resistant medium to the environment of the motor cells which it discharges, missing the neurone or neurones which intervene in the case of ordinary reflex actions. This is an illustration of the way in which tone-impulses, which we imagine as conducted by the non-resistant medium, pass over into discharges which produce visible effects. Again, the phenomena of inhibition appear to require the supposition of extra-neuronic conduction. Whenever a reflex path is in use, all other paths in its neighbourhood are closed. The passage of impulses leading to a particular reflex action is favoured by the suppression of conduction in its vicinity. When A is talking to D through the nerve-telephone, B and C are compelled to hold their peace. Inhibition is a phenomenon of universal occurrence. In relation to various actions, it is sufficiently pronounced to be visible in the effects which it produces. A simple experiment will illustrate this. Holding water in the mouth has no effect upon respiration, but during the act of swallowing respiratory movements are suspended. Whilst the swallowing reflex is occurring the respiratory reflex is inhibited. This might be attributed to the volitional control of respiration, and certainly when attention is being directed to the process volition plays a large part. But if a finger is placed on the pulse, it is possible to detect that, during the act of swallowing, the pulse quickens, owing to the suppression of the slowing action of the vagus upon the heart. Here is a case in which inhibition is in no degree a voluntary action. Nor is it of any value as an adjunct to the particular reflex with which it is associated. It is an illustration of the universal rule that activity of any one spot in the nervous system is the cause of the quieting of the surrounding area. Impulses which reflexly check the heart cannot get through the medulla oblongata whilst the swallowing impulses are traversing it. Inhibition has been described as a drainage of nerve-force into the active area. On the structural side it seems to require the conception of an extra-neuronic substance which, agitated in the vicinity of the cells which are to be discharged, is brought to rest around neighbouring cells. The promulgation through the nervous system of the state which, when it reaches the centres of consciousness, produces pain also seems to call for an hypothesis of extra-neuronic conduction.

Any reference to pain in a work on physiology needs a few words of preface, since popularly the term “pain” is used in various senses. When I see pink geranium and nasturtiums growing in the same flower-bed, I may exclaim: “It is positively painful.” The want of harmony, and at the same time the insufficiency of contrast, of chalky pink and translucent orange, jars my æsthetic sense. Dislikes, however well founded, are ruled out in thinking of the physiology of pain. Further, in defining pain, we must be careful to isolate the real thing, and not to confuse it with sensations which seem to lead up to it. If, putting my finger in a pair of pincers, I touch it as lightly as possible, the first sensation is one of contact; a little harder, and it becomes a sense of pressure; harder still, and all sense of contact or pressure is lost in pain. It is usual to regard pain as sensation carried to excess. But neither is this physiological. An excessively bright light or an excessively loud sound is disagreeable. It causes a sudden movement for the purpose of avoiding it—just such a movement as one would make if one touched a red-hot poker—but it is not, strictly speaking, painful. Not uncommonly in cases of accident or disease of the spinal cord a sharp distinction is drawn between the sense of touch and the capacity for experiencing pain. Below the injury the patient retains his sense of touch undiminished in acuteness, but no blow, or cut, or burn, causes him any pain. The pain caused by squeezing the finger in a pair of pincers is not, therefore, an excess of touch sensation. Pain begins to be experienced in the skin just when the object applied to it is affecting it to an extent which might do harm. If the point of a needle touches it, it causes pain as soon as the pressure is a trifle less than that needed to pierce its surface. A hot object begins to hurt when the temperature reaches 48° C.—almost enough to coagulate the tissue fluids. Pain is not a discriminative sensation. If I hold my arm out at right angles, I am conscious for the first few minutes of its weight, and have, besides, some sense of the traction exerted by the muscle of the shoulder. At the end of ten minutes these sensations are merged in pain, and for some time after lowering the arm the shoulder-muscle aches, much as it does in rheumatism. Pain is an effect upon consciousness, which absorbs, engulfs, and therefore obliterates sensation. To use an ancient phrase, “It is less that I feel pain than that I am pain.” If we speak of the capacity for pain as a sense, we may call it for the purpose of our present argument the “sense of damage.” The nerves of the skin are acutely affected by any agent which is likely to do harm. It is their business to convey to the central nervous system an influence which so affects it as to set up in consciousness the condition of pain. Sensations of damage evoke reflex movements by means of which the part of the body likely to be injured, or the whole body, is removed to a safe distance. It being the duty of the skin to give this warning, a service of nerves sensitive to noxious agents has been developed which scouts in co-operation with the services devoted to the recognition of physical contact and heat and cold ([cf. p. 425]). If, imagining that the fire has not been lighted, I touch an almost red-hot stove, I acquire quite a considerable amount of information of which I am able to make use. I gain an accurate notion of the situation of the stove, and I put the right part of my finger in my mouth. The skin sends to the brain the ordinary sensations of touch and pressure before the condition of pain is established. In seeking for a definition of pain, we must eliminate the two attributes which have characterized all the forms of stimulation which we have considered up to the present time: (1) The tendency to provoke movement; (2) the supply of information. If I am suffering from a whitlow, the last thing that I am disposed to do is to jerk my finger about. Although it enhances the urgency of skin-reflexes, pain, in general, inhibits movement instead of provoking it. This is well illustrated in pleurisy. So long as a man is healthy he is quite unconscious of the fact that at each respiration the lower part of the lung slides on the lining of the chest-wall; but commencing inflammation on the surface of one of the lungs causes intense susceptibility to friction, and the pain produces an effect which the man is quite unable to produce by an effort of will; it stops the movements of the chest on the damaged side. Pain is inhibitory, not stimulant. It is not, properly speaking, a sensation. Frequently being mixed with sensational elements, it conveys topographical information; but pure pain approaches in quality the nebulous sense of distress of a patient who, when asked where he felt it, replied: “Nowhere; but there is a deal of it in the room.”

Sufferers describe pain in figurative language, as “burning,” “stabbing,” “throbbing,” “aching,” and so forth. Two persons afflicted with the same lesion, the same source of pain, use approximately the same terms. Hence we cannot say that pains do not differ in character. But this is not a sufficient reason for assigning any specific quality to pain. It varies in severity, in continuity or intermittence, in suddenness of onset, in the sensations which accompany it, in the emotional tone to which the disturbance of the organ from which it proceeds gives rise, in the tenseness of the part affected and its consequent sensitiveness to a throbbing pulse. All these things make a complex of pain plus sensation, which causes toothache to differ from headache, and both from the pain of burned skin. But they do not give specific qualities to different varieties of pain. This being the case, there is no need to presume the existence of special nerve-endings for the reception of pain, or of a special region of the cortex of the brain for its reception. On the contrary, the evidence is conclusive that the nerve-fibres which serve the more highly specialized senses, which have well-defined connections in the cortex of the brain, do not convey the influence which enters consciousness as pain. It is the innumerable nerves which have no specialized receptors that take up pain. The afferent nerves of the viscera—the vagus and sympathetic—convey no impulses which enter consciousness, so long as the tissues which they supply are healthy. They have no representation in the cortex. The organs with which they are connected (with trivial exceptions, easily accounted for) are absolutely insensitive to injury. Before the virtues of chloroform were known—in the days when, however severe the operation, the patient had to nerve himself to bear it without an anæsthetic—surgeons proved that the liver or the intestines, or practically any other viscus, may be cut or cauterized without the patient being aware that it is being touched. The same is equally true of the brain itself. But if damage in a viscus is set up gradually, its nerves convey to the central system an agitation which has the most pronounced results upon consciousness, and on the way profoundly affects the reflex actions which the spinal cord can carry out, and also its capacity as a conductor. Once in his life, perhaps, a man passes a gall-stone; for generations such a thing may not have happened in his family. Yet the man finds that he is provided with a nervous apparatus which conveys to consciousness intensest pain.

It is difficult to think of pain as travelling along nerves in the form of rhythmic impulses, similar to those which produce in consciousness the effects which we have distinguished as sensations. A few lines above we stated that no impulses which affect consciousness normally travel up the vagus or the sympathetic nerve, limiting the term “impulse,” perhaps unjustifiably. The vagus conveys an influence which enters our experience, as hunger. Probably other states of feeling for which we have no names, which resemble pain and hunger and their opposites, are set up through the agency of visceral nerves.

Fifty years ago attention was called to the difficulty of finding pain-paths amongst the white tracts (nerve-fibres) of the spinal cord. It is as difficult to point them out now as it was then; but the inference that pain travels up the grey matter has given way to the “neurone theory”; under a misapprehension as the writer holds. Pain travels slowly. If one happens to notice a person who unsuspiciously touches a hot surface, one observes that an interval elapses between contact of his finger with the iron and the exclamation with which he “relieves his feelings.” It amounts to more than a second—if the iron is not very hot, to several seconds—whereas the “reaction-time” for touch is only one-seventh of a second. The slowness of movement of pain through the nervous system can on the neurone theory be explained only on the hypothesis that it travels from link to link along a very long chain of very short neurones. That pain is a state of the grey matter rather than a succession of impulses, and that (within the cerebro-spinal axis) the state is transmitted through an extra-neuronic medium, seems a simpler explanation.

The state set up in the segment of the cord in which afferent fibres, conveying pain from viscera, embouch affects its conductivity. It subdues reflex action through the segment, and at the same time facilitates or reinforces the transmission of sensory impulses towards the seat of consciousness. This shows itself in the apparent increased sensitiveness of the skin of the area of the surface supplied by the posterior root which joins the segment of the spinal cord into which the pain influence is also being poured. For example, afferent sympathetic nerves from the cardiac end of the stomach join the sixth and seventh thoracic spinal nerves. Other afferent fibres run up the vagus to the medulla oblongata. When the cardiac end of the stomach is diseased, pain is referred to the skin area supplied by the sixth and seventh dorsal roots. The ordinary inevitable stimuli acting upon this area cause pain. Experimental stimuli which elsewhere would be felt as touch or warmth are painful. The impulses to which they give rise pass through pain-agitated segments of the spinal cord. The vagus nerve carries its pain influence to the medulla oblongata. Now, it happens that the sensory nerve of the face—the fifth—spreads for a considerable distance up and down the axis of the brain. The fifth nerve in consequence pours its sensory impulses into a region which is pain-agitated by those fibres of the vagus which come from the cardiac end of the stomach. Hence disease of that organ gives rise also to an “illusion” of pain—pains and illusions of pain are philosophically indistinguishable—on the surface of the head. The viscera, having no direct access to consciousness, appear by deputy. When the stomach is distressed, it makes its appeal to the whole body politic for considerate treatment through certain nerves which have the privilege of appearing at Court. The message is misread as coming from the front of the chest—“heart-burn”—or from the shoulder, or from the scalp, or from the other skin areas which these nerves serve. When the liver is in trouble, consciousness, having no knowledge of its whereabouts—is it the business of hand and eye to explore another man’s liver, or incumbent upon the mind to accept their findings?—infers that the cry comes from the shoulder. Nor have the tissues beneath the root of the nail, or the muscle of the shoulder, or the pulp of a tooth, any direct representation in consciousness; but since the pain-condition in the grey matter converts it into a microphone, messages from neighbouring structures which otherwise would fail to arouse attention, after traversing the pain-segments of the nervous system, ring out clearly, and hence the mind locates approximately the “pain” of the whitlow, the muscle-ache, the decayed tooth. Sufferers from toothache are familiar with the phenomenon of the spread of pain from a definite spot to the whole jaw or the whole side of the head, dependent upon the spread of the pain-agitation from the segment of the axis of the brain in which the dental nerve ends to neighbouring segments. Our ability or inability to localize a pain does not depend upon the presence or absence of pain-nerves, but upon the existence or non-existence of nerves coming from the same organ, or from its neighbourhood, and capable of conveying impulses to the seat of consciousness. In passing through the part of the spinal cord or of the axis of the brain which is disturbed by the influence exercised by a damaged organ, silent impulses acquire force sufficient to render them audible, and combine with the pain to produce a feeling which consciousness can analyse, to a certain extent. Informed as to its whereabouts by these accentuated sensations, consciousness recognizes a sense of pain limited in its topographical extension.

Sneezing when a bright light falls upon the eye is a curious illustration of the exaggeration of the effectiveness of sensory impulses when they happen to be poured into an agitated segment of grey matter. About one person in every three is affected in this way. A friend of the writer, who was particularly sensitive, rising in the night because he heard his child cry, three times lighted a candle and three times sneezed it out before he could watch the application of match to wick without suffering from a nerve-storm. Some nervous dogs—especially fox-terriers—are very liable to this neurosis. Many persons who do not sneeze feel, when the sunshine stimulates their retinæ, a tickling in the nose. Again the illusion is to be traced to the door of the fifth nerve—the sensory nerve of the whole of the face. The nose is the true tip of the body. Morphologically it is anterior to the eyes. Just as the fifth nerve extends its distribution to the nose, so also its root-fibres extend their connection within the axis of the brain forwards, until they traverse the mid-brain, the primary centre of the optic nerve. A bright light, by stimulating the optic nerve, sets up a commotion in the mid-brain. The ordinary every-moment impulses from the nose, carried by the fifth nerve to this region, ought not to appear in consciousness at all; but owing to the excited condition in which they find the grey matter they assume an importance which does not belong to them, and discharge the reflex action of sneezing, just as they would do had one taken snuff. Several lessons are to be learned from this phenomenon—as, for example, one which cannot be too often impressed, that the impulses which appear in consciousness (or, more accurately, the impulses to which attention is directed) are but a most insignificant fraction of those delivered by sense-organs to the central nervous system. The impulses which give rise to the sensation of tickling in the nose are not exceptional impulses which happened to be started when the light fell on the eye. They were reaching the brain in a steady flow before the agitation of the mid-brain gave to them exceptional force. No consideration regarding the working of the nervous system has a more important bearing than this. We cannot picture to ourselves the activity of the sensory nervous system. Our experience is limited to the scattered sensations which we perceive. Are the sensory nerve-endings incessantly responding to external forces, throwing an almost continuous procession of impulses up each of the millions of nerve-fibres which connect them with the central system? Such a conception is probably nearer to the truth than the conception which we should develop if we trusted to experience. Yet even experience tells us that an infinity of messages is delivered to the brain, of which consciousness takes no account. Changing trains at a roadside station in France, my attention was attracted by an electric bell on the platform, which was ringing continuously. “Why does the bell ring?” I asked the station-master. “To make known that everything goes well,” was the response. “If it stops, something is wrong.” “But do you not become so accustomed to it that you cease to hear it?” “Yes, truly; it rings day and night. One does not pay attention to it until it has stopped.” Sensory impulses generated by the contact of my skin with the chair that I am sitting on are incessantly ringing the bell of consciousness. I should notice them immediately if they stopped. As it is, they do not attract my attention until they ring a little louder than usual, or until some particular group, owing to unrelieved pressure, produces a cumulative effect. Another lesson; that the condition of the nervous system, and therefore its conductivity, is determined at any given moment by the sensory impulses which are reaching it. We cannot describe the effect of a bright light as pain, yet it agitates the grey matter, altering its state, in the same way as the nerve-inflow which we recognize as pain. A wet rag on the forehead does not assuage a headache by cooling the brain ([cf. p. 106]). The headache is “in the scalp.” The cool wet rag diminishes the dilation of the bloodvessels of the forehead, and quiets the impulses from the skin which are pouring into a tract of grey matter pain-agitated by the influences ascending a visceral nerve—usually the vagus.

It is necessary to warn the reader that a reversion to the old idea of “conduction through grey matter”—i.e., otherwise than by a chain of neurones—is unorthodox. It is set forth here because it seems to the writer that the various phenomena which have to be accounted for fit in best with the hypothesis of a double path. If evidence of the anatomical possibility of extra-neuronic conduction is asked for, it may be pointed out that the chrome-silver and methylene-blue methods, upon which our knowledge of neurones is based, do not, in the very nature of the case, show that grey matter consists only of neurones and their obvious branches. As they select particular elements of structure, we can never by their use alone know what they fail to show. Attention may also be called to the fact that the same staining process which reveals pericellular nets ([p. 301]) shows also a structure resembling a network in the substance which intervenes between them. Truly the method is a rough one. It may well be thought that the nitric acid used to fix the tissue may cause strange coagulations with solution of uncoagulated substance; but, as was remarked with regard to the pericellular nets, regular patterns indicate architectural differentiation. But whether these nets do or do not give hints as to the nature of the conducting medium, there is no difficulty in finding sufficient material, after all the substance entering into the formation of the conducting neurones, as we imagine them, has been accounted for. Ex hypothesi, the conducting material is provided by the fibrils of the sensory nerves in their extensions beyond the limits to which the deposit of subchromate of silver extends, when the chrome-silver method of displaying neurones has been used. Sensation-impulses enter neuronic chains. The condition which, when it affects the seat of consciousness, is known as pain, progresses up the vertebrate neuropil.

Energy is developed within the nervous system. The force of impulses is adjusted to the resistance which they have to overcome. Stimulation of the millions of twigs of the vagus nerve in the lungs brings about the gentle movements of ribs and diaphragm which constitute peaceful respiration. A crumb of bread touching the mucous membrane of the larynx stimulates a few of the endings of the same vagus nerve. Like an avalanche, the impulses gather head as they advance, causing, not the diaphragm and intercostal muscles alone to do their utmost, but calling into action half a dozen accessory muscles of respiration. It is difficult to account for this reverberation of the messages which clamour for the ejection of the crumb of bread without figuring them as spreading from neurone to neurone, urging each in turn to deliver its maximal discharge.

Neurones are provided with material which serves as a store of energy. In their cell-bodies, including their dendrites, are to be seen coarse granules of nucleo-protein, which, being fitted in between groups of neuro-fibrillæ, assume an angular form. They are known as Nissl’s corpuscles, or are termed “tigroids,” owing to the spotted appearance which they give to the substance of a cell. If the nerve-cells of birds be examined just after they have alighted from a migratory flight, the granules are found to be few and small. In a bee returning to the hive at evening with its last load of pollen, they are smaller than they were when it commenced its morning’s work. They disappear in certain pathological conditions, and under the influence of various drugs; and since their presence is revealed by staining, their disappearance is spoken of as “chromatolysis.”

Fig. 22.—The Body of a Motor Neurone.

In its centre is a large clear spherical nucleus, with a nucleolus. The body-substance is prolonged into five dendrites and an axon. Neuro-fibrillæ are seen in dendrites and axon. They traverse the body of the cell in all directions, in little bundles which are separated by angular granules of stainable substance (tigroids).

The wasting of tigroids during functional activity proves clearly that nerve-cells do work, in the physical sense. Energy is expended in transmitting messages from receptor to effector, from sensory cell to muscles, from recipient nerve-ending to glands. Have nerve-cells any privileges or duties? Their functions, so far as we have considered them hitherto, are automatic, from a mechanician’s point of view. Their situation and connections determine the direction in which they conduct, and the degree in which they reinforce stimuli impressed upon the nervous system by the environment, including what may be termed the internal environment, food in the alimentary canal, secretions in ducts, and so forth. Have the cells any directive or executive functions? There is no evidence that they have; nor, it must be added, is there any line of reasoning which leads inevitably to the conclusion that they have not. Remembering that, until recently, it was the custom to solve all obscure problems and to shelve all difficulties by conferring human attributes upon nerve-cells and collections of nerve-cells, termed “centres,” a physiologist admits the negative with reluctance. The unconscious argument in the past used to run somewhat thus: “I decide to act or to abstain from action. The nerve-cell is the mechanism by means of which I decide. Therefore the nerve-cell decides.” (In the past a distinction was drawn between the cell-body and its processes, but that, we now see, was absurd.) It is very difficult to relinquish completely this attitude of mind. I feel, I remember, I will. There must be a something which feels, remembers, wills. But a physiologist finds in the nervous system no evidence of a capacity for any function other than that of conduction, with adjustment of the force of current. He can no more discover feeling, memory, or will in a chain of neurones than he can find music in a violin. He hears the strings singing in the breeze. He can twang them with an electric shock. But he has no vision of ghostly performers, no glimpse of the conductor’s baton. Yet he knows, as every sane man knows, that the neurones are the instruments played in the orchestra of mind. He knows that, while all are sounding, some are muted, in order that the others may produce a dominant effect. He knows, too, whenever he decides to continue writing or to close his notebook, that the conductor is raising the baton or allowing it to sink by his side.

A neurone or nerve-cell is a transmitting link. It is scarce a thing to wonder at that physiologists, having wrestled successfully with the superstition of the “pontifical nerve-cell,” are unwilling to reinstate it even as doorkeeper in a free church. It may be that it exercises some discretion in admitting impulses, but until its authority as a guardian of the path which stretches behind it has been established, it is better to regard it merely as a door which swings open whenever pressed with sufficient force.

Is it possible to classify neurones according to their function? They can be classified according to size, and, with some degree of completeness, according to form. But if, as we believe to be the case, size and form are governed by purely physical requirements, the divisions into which the cells fall have no physiological significance. The motor cells of the spinal cord and axis of the brain are large and irregular in shape. Their dimensions are clearly dependent upon the size, thickness rather than length, of the nerve-fibres which are drawn out from them. They discharge impulses to groups of voluntary muscle-fibres at a considerable distance. Small cells could not do the work. Precisely similar reasons can be given for the large size of the cells of Purkinje in the cerebellum, which transmit the elaborated product, as we may term it, of this organ to the great brain; and for the dimensions of the large pyramids of the great brain, which convey its decisions to the spinal cord. The small pyramids of the cortex of the great brain distribute the first crude impressions of sensations to neighbouring (association) areas of the cortex. A cell of Purkinje ([Fig. 23]) has a more complicated, and at the same time a more regular, form than any other nerve-cell. It resembles an exceedingly richly branched espalier pear-tree, set at right angles to the narrow convolutions of the cerebellum; a disposition easily accounted for, when the structure of the cortex of this organ is considered. Its outer layer in which the espalier processes ramify is traversed longitudinally by an infinity of nerve-threads, the bifurcated axons of granules. These granules are small neurones which take up impulses from afferent (“mossy”) fibres, and distribute them to the dendrites of the Purkinje cells—each collecting from a few fibrils only of the sensory channels. (The word “sensory” is used to indicate that sense-organs are their provenance, and not that their messages become sensations.) The numerous spreading branches of a Purkinje cell, disposed in a transverse plane, are obviously arranged to hold up and keep apart these myriads of longitudinal threads. A cerebral pyramid is shaped like a fir-tree. It is placed in a definitely stratified layer. By its branches it collects impulses from the superficial strata, which it transmits through its stem to the white matter beneath the cortex. The various parts of the central nervous system have work of different kinds to do, and we find interposed in the circuits which compose the several parts cells of various types. We speak of the large cells as “motor,” the granules as “sensory,” the small pyramids as “association” cells—such terms indicating the positions which they occupy in the arcs, but not defining their functions. Of specialization of function the physiologist cannot obtain a hint. He cannot classify nerve-cells in groups concerned in reflex action, in feeling, in remembering, in willing, in thought. On the contrary, he can assert with confidence that such distinctions are not to be drawn.

In various situations in the central nervous system a certain type of cell is found for which, in the present state of knowledge, it is impossible to account. We mention these cells lest it should be inferred, from what has been said above, that all neurones can be fitted into a simple scheme of conducting arcs. In the spinal ganglia there are neurones whose axons divide to form “baskets” around other ganglion-cells. In the cerebellum there are similar cells, the axons of which divide into branches, which break up to encase Purkinje-cells. Cells of the same kind are found in a few other situations. In some cases the end-branches which enter into the formation of the baskets are few in number, and thick and clumsy. They grasp the body of the cell which they surround, with gouty fingers, as it were. In other cases the basket is a tangle of fine threads. It is difficult to see what rôle cells of this kind can play in conduction. From the olfactory and optic centres nerve-fibres extend outwards to the olfactory bulb and retina. Here again is an arrangement which does not fit in with any scheme. We might multiply examples. But enough has been said, perhaps, to convey the impression which we wish to leave, that, although experiment abundantly proves that the nervous system consists of an association of sensori-motor conducting arcs, and although anatomical investigation demonstrates the existence of chains of neurones which take part in the formation of such arcs, it is impossible to reduce the system to schemata or to prepare diagrams in which all structural elements are, even hypothetically, fitted into place.

It may be convenient at this point to call attention to the differences which distinguish the sympathetic system—the ganglia and nerves of the viscera and bloodvessels—from the system devoted to bringing sense-organs into connection with the skeletal musculature which we have chiefly considered hitherto. The fibres of the posterior root of a spinal nerve which convey impulses from the skin and muscular sense-organs, and the fibres of its anterior root which convey impulses to skeletal muscles, have a similar diameter of about 15 µ. In addition to these, the roots contain fibres which carry impulses from and take them to the viscera. Those which bring impulses from the viscera vary greatly in thickness, some being as large as the other sensory nerves of the posterior root. The diameter of the fibres which go to the viscera is not more than one-fifth as great as that of the other fibres of an anterior root. Similar slender fibres are found in the vagus nerve. If all organs are removed from an animal’s chest and abdomen, a string of small pearl-like ganglia, united by a longitudinal cord, is seen lying on either side of the bodies of the vertebræ, one ganglion for each segment. This string of ganglia is termed the “sympathetic chain” ([cf. p. 243]). The small medullated fibres of the anterior spinal roots join these ganglia. Some of them arborize about their cells; some pass by them to arborize in ganglia which lie farther afield, on the course of the great bloodvessels and within the viscera. The axons of neurones whose cell-bodies are within a ganglion break up into bunches of non-medullated fibres. In this way the fibres of the sympathetic system are increased in number. Each of its neurones is a multiplying and distributing station. There is no evidence that it in any way serves as a “centre,” takes part in reflex action, or otherwise usurps the functions of the grey matter of the spinal cord. Nerve-cells are thickly strewn between the mucous membrane and the muscular coat, and again between the two layers of the muscular coat of the alimentary canal. It is not so certain that this system has no “central” functions. The remarkable degree in which the wall of the intestines retains its capacity for co-ordinated movement, after all nerves which reach it from the ganglia and through the vagus have been cut, suggests that the plexus of nerves within it does act to some extent as a reflex centre. If we leave the case of the intrinsic nervous system of the alimentary canal open, awaiting further proof, there is no reason for looking upon the sympathetic system as in any degree independent of the spinal cord and brain. It does its work on a large scale, and its work is of a low order. Nature does not need to connect up the viscera and bloodvessels with the central nervous system by means of fibres as thick as those used for skeletal muscles. It is more convenient to provide for the multiplication of the nerves—which must be extremely numerous, owing to the relatively minute size of the muscle-fibres for which they are destined—outside the central system than it would be to include the necessary distributive cells within it. Again, we find that a nerve-cell, when we see it at close quarters, shows no evidence of administrative capacity. Although of a different shape, a ganglion-cell of the sympathetic system is as large and as complex in form and structure as a pyramidal cell of the cortex of the brain; yet the work which it does is of a purely mechanical order. It receives, reinforces, transmits impulses which reach it from the central nervous system.

The often-repeated statement that a nerve-fibre is a drawn out process of a nerve-cell body has prepared the reader to anticipate that it dies when cut off from its central connection. When the axon is dead, the sheath which invests it rapidly loses its tubular character. If the situation of the cell-bodies of a nerve be known, it can be at once foretold on which side of the cut degeneration will occur. Suppose that the median nerve has been severed at the wrist. All nerve-fibres on the distal side of the wound must atrophy, whereas none of the fibres on the proximal side will be affected. The motor fibres have their cell-bodies in the spinal cord, the sensory in the spinal ganglia. Degenerations following lesions in the central nervous system have taught pathologists more about the course of the fibres in the white matter than any other class of observations. Degeneration above the lesion is spoken of as ascending, below as descending—not that it progresses upwards or downwards. It occurs throughout all the stretch of the fibre which has been isolated from its cell-body at the same time, or nearly so. The thought that impulses can no longer ascend or can no longer descend, as the case may be, has given sanction to the expressions “ascending” and “descending” degeneration.

Restoration to functional activity of tracts of fibres which have degenerated in the brain or spinal cord never occurs, but severed peripheral nerves regenerate. Not that fibres join cut end to cut end, however clean the wound. A wound in the wrist which has divided the median nerve may heal in a few days “by first intention,” so far as other tissues are concerned; but the patient does not for two or three months recover the power of using the muscles of the hand which the nerve supplied or the sense of touch in the area of skin to which it was distributed. The ends of the axons on the proximal side of the wound have to grow downwards to establish new connections in the muscles and in the skin. The interval which elapses between the healing of the wound in the wrist and the restoration of sensation and power of movement is occupied in their downgrowth.

The re-connection of regenerated nerves with their terminal apparatus presents to the mind a curious problem. There is no evidence that as function is re-established the brain has to re-learn the situation of the sensory spots on the skin, or to re-acquire skill in using the muscles which again come under its control. From the moment that the outgrowing nerves have recovered their terminal connections skin and muscles have their right representation in the brain, however much the two cut ends may have been twisted in their relation one to another. It seems inconceivable that each nerve-fibre can find its way to its original station; but if it does not, our conception of the mode of working of the nervous system still needs much refining from the telephone-exchange analogy by which we naturally help out our explanations. If a telephone cable has been severed, it can be made useful again only in one of two ways. Either the two segments of every wire that has been cut must be reunited, or the subscribers’ numbers must be redistributed.

The experiment of uniting the proximal segment of one nerve with the distal segment of another of a quite different function gives results which have an even more disconcerting effect upon our theory of the nervous system. The sympathetic cord of the neck and the vagus nerve lie very close together, alongside the carotid artery. The vagus is both afferent and efferent. The sympathetic is wholly efferent—i.e., it conducts impulses, which enter the sympathetic chain within the thorax, in the direction of the head. If both nerves are cut, and the end of the vagus turned round, so that it is in apposition with the upper end of the sympathetic, its regenerating fibres make their way along the sympathetic cord, headwards, to the superior cervical ganglion. They arborize about the bodies of its ganglion-cells, just as the sympathetic fibres used to do. The vagus is a nerve of many functions. Amongst others, it inhibits the contraction of the heart, constricts the bronchi of the lungs, dilates the bloodvessels of the intestines, and helps in regulating the movements of these viscera. After it has taken the place of the upper segment of the sympathetic it dilates the pupil, constricts the bloodvessels of the ear, erects the hairs of the head, as if to the manner born. To take another example, in a monkey the two nerves supplying respectively certain flexor and certain extensor muscles of the forearm were cut, and their ends crossed, so that flexor nerve-fibres grew down to extensor muscles, and extensor fibres to flexor muscles. There was no bungling of reflex actions or of voluntary actions when the new roads were first used. The monkey did not jerk its hand open when it tried to scratch or to grasp a nut.

When experimental data first began to accumulate, physiologists drew diagrams and made models of the nervous system in which they represented it as composed of conducting arcs. The arcs were superposed to indicate that they were of various grades—spinal for ordinary reflexes, bulbar for co-ordinated actions, through the grey matter in the centre of the great brain for “ideo-motor” actions, through the cortex of the great brain for voluntary acts. They spoke of authority and responsibility, comparing the nervous system to an army or a club. It is premature to attempt a theory of the nervous system compatible with recent discoveries regarding its structure and mode of working, but it is clear that the diagrams and metaphors to which we have just referred were misleading. In place of attempting to disarticulate the machine, we ought to emphasize its structural unity. The results obtained by uniting heterologous nerves cannot be explained by reference to a model made of wires and pieces of cork. They do not fit in with any organization of human units or with any postal system or telephonic apparatus for transmitting news. Probably the lines of thought which will prove most fruitful are somewhat as follows: (1) An efferent discharge occurs as the result of the opening of a circuit from a muscle back to the muscle. Afferent impulses—call them sensory, on the understanding that this does not imply that they appear in consciousness—are ceaselessly flowing from receptors to effectors in the muscle. A sensation—in the case of skeletal muscles usually a skin sensation—reinforces them to discharging-point. If the spinal cord has been severed from the brain, the up-and-down flow does not reach beyond its grey matter. It is short-circuited. If the brain is in normal connection with the spinal cord, sensory impulses travel upwards to its cortex (without, save in exceptional instances, arousing consciousness, or, as we should prefer to express it in this connection, without attracting attention) to a degree which varies with the several classes of receptor and with the animal. A monkey reduced to the condition of a “spinal animal”—i.e., with its spinal cord severed from its brain—is less competent than a dog, and a man is far less competent than a monkey. In other words, a man habitually uses his brain more than does a monkey, and a monkey more than a dog. The proportion which brain-weight bears to body-weight roughly indicates the part the brain plays in conducting the traffic of the body. (2) Communication within the nervous system is almost unrestricted. If, before the median nerve was divided at the wrist, receptor A usually initiated a current which passed through the circuit to effector X, and receptor B to effector Y, and if the new fibres which grew downwards lost their way so that the one which used to receive messages from A attached itself to B, and the one which used to transmit commands to X attached itself to Y, A is not thereby cut off from X, or B from Y. Such a mechanical association is restricted to our diagrams. It does not enter into Nature’s plan. The spinal cord is not scored with unchangeable paths. A messenger from A could always reach either X or Y. It was not the path, but the struggle with competing messengers, which directed him to X.

When we endeavour to picture the mechanism of the nervous system, we find ourselves faced by phenomena which appear irreconcilable. One set of observations leads to the conception of closed paths; another set points to an open conductor. The experimental crossing of nerves to which we have just alluded shows that the nervous system is adaptable, to a degree which seems extraordinary to anyone who attempts to compare it with any of Man’s devices for establishing communication. Paths appear to make themselves. On the other hand, the more important, and therefore dominant, reflex actions, such as swallowing, breathing, the maintenance of position, are due to the union of receptors and effectors by lines which are either reserved for their sole use, or, if shared by other currents, it is on the understanding that they have a first and altogether prepotent claim. No competing impulses can divert them or block their way. All reflexes which in the history of the race have established their right to dominance not only seize and hold a route through the nervous system, to the exclusion of all competitors, but, as we have already shown in the case of the swallowing impulse, the traffic in neighbouring routes is suspended for their benefit. At the other end of the scale we find reflexes which may be termed “occasional,” in that, although of frequent occurrence, they exhibit illimitable variability in form. Occasional reflexes require, as a preliminary to their transmission, that the afferent impulses which give rise to them should secure for a time the exclusive use of the motor neurones by which they are carried out. The receptors bring the motor neurones into tune with themselves, and while in tune they will respond to impulses from no others. But the tuning lasts for a short time only. Either receptor or neurone, or both, soon tire. There is no danger of a particular reflex being prolonged to the detriment of the organism as a whole. As an illustration of an occasional reflex, we may cite the scratching movement of a dog. Its skin is punctured by a flea. It scratches the place. A second flea bites it somewhere in the same neighbourhood. The dog does not shift its hind-foot so as to scratch midway between the two bites. It finishes out one scratch before paying attention to its second tormentor. The exact position to which the hind-foot is raised depends upon the position of the irritant; and since this may be shifted over a very considerable surface, the form of the reflex varies equally widely. Each of the very numerous receptors in the skin tunes a slightly different group of motor neurones; and since a second irritant may reinforce the first, instead of making an alteration in the group of neurones which the reflex is discharging, it is clear that there is no fixed path uniting receptor A with neurones X, Y, Z and receptor B with neurones W, X, Y. If, however, the second irritation occurs at a spot lying at a considerable distance from A, in place of reinforcing the scratching movement which A has set going, it weakens and shortens it. The receptor C, which is calling for the discharge of a markedly different set of motor neurones, tends to inhibit those which are already active. These results are tested with precision upon a “spinal dog” and with the aid of an electric needle, the other pole from the battery being a large flat plate placed in contact with the animal’s body. The conception of definite paths, to which the contemplation of permanent reflexes gives rise, is inappropriate to occasional reflexes. The latter show so wide a range of variability and adaptability as to prove that a given receptor may bring any of a great variety of groups of motor neurones into connection with itself; just as a given group of neurones may be played upon by impulses from a great number of different receptors. We have called it a tuning of the motor neurones. One metaphor is as good as another. The physical process which in the brainless frog underlies the preparation for discharging motor neurones in the spinal cord, on the same side as the leg on which vinegar is placed, so long as that leg is free, and on the opposite side, when that leg is fixed, is unknown. We seem to catch a glimpse of a doubleness of action, receptors in the muscles combining with receptors in the skin in determining the paths along which impulses shall be reflected—the efficient muscles sensitizing their own neurones to the tuning influence of impulses from sounding cutaneous nerve-endings. But it is impossible to formulate a working scheme in the present state of knowledge.

Sense-Organs and Nerve-Centres.—A vast amount of labour has been devoted to the study of the external form of the central nervous system and to unravelling its internal structure; to plotting out its various groups of nerve-cells, to disentangling its innumerable tracts of fibres. The surface of the brain and spinal cord has been mapped and measured. Every millimetre of its substance has been cut into sections on the micro-tome. Organs which, fifty years ago, appeared too complicated for investigation have been described in the minutest detail. An immense accumulation of data is available for purposes of reference; yet anyone who submits the theory of the nervous system as it is held at the present day to a general review must allow that the results of anatomical research enter but little into its construction. The reason for this is not far to seek. As knowledge has advanced, the apparent, or rather the expected, complication of the system has given place to ideas of unity and simplicity. Its external configuration and the varied arrangement in “nuclei” of its nerve-cells may, without impropriety, be described as accidental. The form of the body and the consequent location of the clients of the nervous system determine the disposition and degree of concentration of its various business centres. It shows, when followed throughout the whole animal kingdom, extreme variability of its constituent organs, with absolute uniformity of plan. Indeed, from the physiological point of view the term “organ” is scarce admissible. It implies diversity of function in too high a degree. The several parts into which the central nervous system is obviously divisible co-operate so intimately as to preclude us from thinking of them as separate organs.

If the citadel of the central nervous system is to be captured, all lines of approach must be tried. Its outward form must be studied, its minute structure examined with the microscope, its modifications in various animals compared, its development followed, its reactions to artificial stimuli tested, its pathological deficiencies and vagaries watched. Yet, of all the means which have been made use of in attempting to penetrate its secrets, the study of its history, by the methods of comparative anatomy and embryology, has probably contributed most to the development of sound ideas regarding the manner of its working. The first differentiation visible in the blastoderm—the globe of cells into which the ovum divides and out of which the embryo is built—has relation to the formation of the nervous system. If the earliest stages of its growth are followed, and the different phases through which it passes are compared with the forms which it assumes permanently in lower animals, the plan or type upon which it is constructed shows up distinctly. Looking down the line to the earliest vertebrata, we can discern clearly the form of nervous system possessed by their prototype. Not that this “ideal ancestor” ever existed. Experience teaches that it is unlikely that any animal that ever lived was absolutely regular and symmetrical in all its parts; nevertheless, the type can be presented in a perfectly regular scheme. The ideal ancestor of the vertebrata was segmented, like a caterpillar or a worm. Its mouth was not at the anterior extremity of the body, but two (or more) segments behind it. Every segment bore a sense-organ (at one period two sense-organs) on either side. Beneath each sense-organ there was a clump of “grey matter.” Each segment also contained (although not at the earliest epoch) two clumps of nerve-cells and neuropil in a more central situation. These “ganglia” were united by longitudinal and transverse commissures. They received the axons of the cells which lay in the clumps beneath the sense-organs. They gave axons to various muscles. Such is the type out of which the modern nervous system has developed: two separate sense-organs and a complete nervous system for each segment, the sense-organs connected with the ganglion of the same side, the ganglia of the two sides bound together across the middle line, and each row of sense-organs and each row of ganglia united by longitudinal commissures into a chain. From the nervous system as we see it now the majority of these segmental sense-organs have disappeared; but the mode of formation of the cerebro-spinal ganglia shows that they are the clumps of nerve-cells which lay beneath the vanished organs. In the nose and the eye the grey matter retains its original situation in the immediate vicinity of the receiving epithelial cells—as the olfactory bulb and the deeper (anterior) layers of the retina. The ganglia of the auditory nerve lie within the bones of the ear. Spinal ganglia are close to the spinal cord. Auditory and spinal ganglia contain only the cell-bodies of the first collecting neurones (sensory nerves) together with certain curious bracketing cells already referred to ([p. 324]), all the other constituents of the peripheral clumps of grey matter which are found in the olfactory bulb and retina having been withdrawn from the spinal ganglia into the axis of the brain and spinal cord.

The sense-organs in front of the mouth have had from the beginning an immense advantage over the others as observing-stations. Whereas the body-organs collected information regarding the things with which the animal came in contact, and consequently specialized in touch, pressure, temperature, and, in the case of fishes, sensitiveness to the chemical constitution of the medium in which the animal lived, the head-organs specialized in responsiveness to forces acting from a distance—particles suspended in air, vibrations of light, pulsations of sound. Sensitiveness to touch, if it is to be useful, must be widely distributed. The body-organs therefore broke into scattered groups of sense-cells. Touch-spots are scattered all over the surface, although they are set much closer together in the areas of skin which are usually the first to come into contact with external objects than they are elsewhere. The efficiency of the sense-organs of the head—nose, eye, and ear—depended upon their remaining compact. Progress in animal life, as we understand it—the rise from lower to higher forms—has depended upon increasing integration of the body and co-ordination of its functions. The nervous system is the agent which has accomplished this unification. Each step in advance has depended upon the provision of more nerve-tissue for the lacing together of the various parts. We have seen already ([p. 329]) how intimate is the union of receptors and effectors of every kind via the spinal cord and brain. The overwhelming predominance in the direction of action of the nose, the eye, and the ear has led to the accumulation in their vicinity of the ever-increasing grey matter. The cerebral hemispheres, or “great brain,” are pouched outgrowths from the first pair of ganglia directed towards the olfactory pits. The original eyes bore a similar relation to the second pair of ganglia—the epithet “original” implying that the eyes which we now use are not the organs with which our prevertebrate ancestors saw. First one of the original eyes disappeared, and then the other. The vestige of the second is still to be seen in the “pineal body” which is found on the dorsal side of the brain of every vertebrate animal—in a mammal deeply hidden in the cleft between the cerebrum and cerebellum. In place of the pineal eyes two other sense-organs have specialized as eyes. They are constructed on a different plan, being, to put it shortly, pineal eyes turned inside out; for whereas in the pineal eyes, as in most of the eyes of invertebrate animals, the rods and cones, which are the cells of the retina sensitive to light, are directed forwards towards the lens, the rods and cones of our permanent eyes are directed away from the source of light. This change has made it possible to provide more abundantly for their nutrition, and hence a greater power of discriminating separate points in space and of distinguishing colours is conferred upon them. The substitution of other sense-organs for the original eyes has complicated the pictures which are presented to us by a brain in its successive stages of growth; but it does not prevent us from recognizing the general plan. Probably the secondary eyes, like their predecessors, belonged to a pre-oral segment. The sense-organs of a segment behind the mouth developed into ears; and the ear was in its earliest phases, and still is, something more than an organ of hearing. Its semicircular canals give information of displacements in space. Knowledge of the position of its body is, to a fish, of far more importance than its ability to hear breakers on the rocks. Three looped tunnels, opening at either end into a common chamber, are hollowed in the bone which contains the ear ([cf. Fig. 38]). Placed at right angles one to the other, they occupy all three dimensions of space. Open a notebook until, one of its covers lying horizontally, the other is vertical, and place a sheet of paper vertically against the bottom of the pages. A curved line drawn on each of these three surfaces will represent the three semicircular canals. Arrange another notebook in the same way, and let the two rest on the table with the two vertical covers inclining one to the other, anteriorly, at an angle of 90 degrees. The six surfaces will be in the planes of the six semicircular canals. Within each bony canal is a membranous tube, to which nerves are distributed, filled with fluid. When the position of the head is changed, the fluid within the membranous tubes slides on their walls. It is left behind at the moment the movement commences. It overtakes its receptacle when the movement stops. The stimulus received by the nerve-endings is recognized as indicating an alteration in the orientation of the head. If the movement of the fluid is violent, as when one waltzes, the loss of the sense of position disconcerts the brain to such an extent that giddiness results. For a time the quiet assurance upon which so much depends, that one knows how the body stands in relation to its surroundings, gives way to a chaos of sensations. From the nature of the case, the information which the semicircular canals afford relates to change. They give no help in ascertaining the position of the head when it is at rest. This must be the reason, although the connection is not very clear, for the waning of the effect in consciousness when stimulation is prolonged, and also for the very marked after-sensation. At the commencement of a voyage attention may be unpleasantly attracted to the rolling of the ship. After a few days it ceases to be noticeable; yet when the voyager, the night after landing, wakes in the dark, he finds his bed-room as unsteady as his cabin. Rising hurriedly, the attempt to adjust his position to the heaving floor (we speak from personal experience) may result in a heavy fall. Although this phenomenon must be classed with other “after-sensations,” it is so prolonged as to suggest that consciousness, having become accustomed to a world which causes a backward and forward flow of endolymph, misinterprets the absence of sensation as indicative of change.

Taste is, practically, a special kind of smell. A fish’s olfactory membrane, taste-buds, and chemical organs “of the lateral line” serve the same sense, although, no doubt, they are applicable to the analysis of different forms of matter in solution.

Our ideal prevertebrate has now left its primitive undifferentiated condition. In front of its mouth it bears organs with which it searches the world. Close behind the mouth are its auditory and orienting organs. The rest of the surface of the body is endowed with the capacity of recognizing “taste,” temperature, and contact. Smell, sight, and orientation determine the development of the brain.

The cerebrum which has eventually become, as the seat of consciousness, and hence the apparatus of mind, the dominant factor in the nervous system, was in the first instance the part of the brain concerned with the distribution to the muscles of impulses generated in olfactory organs. There is scarcely any indication in a fish’s brain of the representation in the cerebral hemispheres of any other sense, even that of vision.

A bird’s brain presents a striking contrast to the brain of a fish. With the exception of the apteryx and other ground-birds of New Zealand, all birds are apparently destitute of the sense of smell. Vision is the sense upon which their activity depends. It has invaded the cerebrum, converting it into an organ in which sensations of sight are worked up into “mind-stuff.” The optic lobe connection is restricted to the production of reflex actions in which vision is immediately followed by movement.

All the senses are represented in the great brains of mammals. The cerebrum, which owes its existence to its connection with the favourably-situated sense-organ of the nose, and grew in importance when vision invaded it, has now taken in the senses of hearing, taste, and touch. Only what may be termed in general visceral sense, and the sense of orientation, are excluded.

Looking back to the starting-point, we see a segmented animal; its segments of equal value; its nervous reactions unisegmental, although linked in functional sequence. If it starts to walk, owing to stimulation of one of its sense-organs, the impulse to walk spreads from segment to segment. Comparing the latest product of evolution with the earliest, we find that nervous tissue has concentrated at the anterior end of the body. The double chain of ganglia, now condensed into the axis of the brain and the spinal cord, still contain all the effector neurones by which muscles are called into action. Sensory nerves still arborize in the axis, providing the mechanism for actuating motor neurones. But the vast majority of intermediate or intercalated neurones have been attracted to the two huge brain-masses—the cerebellum and cerebrum. In the former all sensations (not conscious) connected with tone, position, orientation and equilibrium are worked into appropriate impulses for the regulation of the muscular system. In the latter all sensations which convey information regarding the relation of the environment, including the body, to the ego—the not-me to the me—are transformed into motor discharges which set a-going the movements (and the thoughts) by means of which the purposes of life are fulfilled; for in the cortex of the great brain alone is the passage of nerve-currents accompanied by consciousness. Concentration of nerve-tissue allows of the combination of sensations. It also facilitates the no less important effect of mutual influence, interference. Sensations are suppressed, and therefore the multitude of reactions to which they would give rise are inhibited, in the interests of restricted and sustained movement or thought.

The Cerebellum.—Sharks and other swift-swimming fishes have large, deeply fissured cerebella, for the cerebellum is the part of the brain which has gathered into itself most of the grey matter associated with balancing, attitude, posture. The cerebellum is in birds large and deeply folded. Developed from the ganglia to which the auditory nerve distributes impulses from the semicircular canals, it has established connections with all the other nervous tissues concerned with sensations of position, strain, or pressure, including the eyes, which afford information regarding the position of our limbs relatively to the trunk, and of the whole body relatively to external objects. Morphologically it is a median growth. The adverb is one of those qualifying terms, convenient in science, which direct thought without confining it. As used above, it implies that anyone who passes before his mind the cerebella of all animals from fishes to Man, and in all stages of growth, from their earliest appearance in the embryo to their condition in the adult, sees the organ as a median prominence surmounting the medulla oblongata. The bulgings of its sides which, in human anatomy, are termed hemispheres, do not disturb its central, unpaired plan of structure. It has, it is true, a lateral appendage on either side (the combined flocculus and paraflocculus of mammalian anatomy), but this lobe, although of great historic interest, is so small, as compared with the median growth, as not to affect our general conception of the form of the organ. By transverse fissures the cerebellum is divided into a series of lobes.

In appearance the cerebellum varies greatly in the different classes and orders of Vertebrata. Yet underlying this variety there is marked unity of plan. A sagittal section of the organ of a shark, of a bird, of a kangaroo, of a dog, of a whale, of Man, shows that it is divided, from before backwards, into the same number of lobes in animals occupying every position from the bottom to the top of the vertebrate scale. A very little effort to grasp the significance of this mystic number, nine, convinces one of the hopelessness of any attempt to correlate the form of the cerebellum with the muscular development or sensory endowments of vertebrates as a sub-kingdom. It is the same for animals with limbs and animals without; animals with well-developed noses or eyes, and animals destitute of one or other of these sense-organs. This uniformity is extremely significant, when contrasted with the wide differences exhibited by the cerebral hemispheres. It shows that, unlike the great brain which mediates between the several senses and the muscular system, the little brain is concerned in bringing about adjustments to the environment which are equally important to all animals, no matter how far they may depart from the common type. The cerebellum is crossed by deep fissures, dividing it into narrow convolutions or folia. The folia are grouped in nine lobes. If the reader has secured as an illustration the brain of a sheep, he will notice that the lateral regions of the cerebellum present a complicated appearance owing to the contortion of the folia, which results from the unequal development on its sides of the several lobes. In its total size the cerebellum keeps step with the cerebrum, the right side of one organ being associated with the left side of the other.

Fig. 23.—Vertical Section of the Cortex of the Cerebellum,
cut Parallel with the Long Axis of a Folium.

A shows three cells of Purkinje, their espalier systems of dendrites being seen in profile. A “mossy fibre” enters the granular layer from the white matter. About a dozen of the granules are shown, each with four or five dendrites and a single axon. The axon bifurcates in the molecular layer, its two branches running for a considerable distance to left and right along the folium. B shows the other nervous elements which are found in the cortex: a cell of Golgi with a ramified axon, a climbing fibre, a basket-cell, of which the axon divides into four branches, and a small stellate cell.

The grey matter which covers the surface of the cerebellum, its cortex, is singularly regular in microscopic pattern ([Fig. 23]). It is divided into three sheets: superficially, the molecular layer in which the dendrites of the cells of Purkinje branch; beneath this, the thin layer in which are situate the cell-bodies of these neurones; thirdly, the layer of small cells, or granules. Cells of Purkinje and granules have been already described ([p. 303]). To these must be added the stellate, bracketing cells of the molecular layer, the axons of which divide to form baskets about a number of Purkinje-cells, and the cells of Golgi of the granular layer. These last are comparatively large cells, which have thornless dendrites, and axons which branch repeatedly in the granular layer, without passing into the white matter which underlies the cortex. Two kinds of nerve-fibre bring impulses to the cortex: (1) “Mossy” fibres, which bear rosettes of filaments which distribute impulses to the granules; and (2) “climbing” fibres or “tendril” fibres, which, passing through the granular layer, cling like ivy to the trunk and principal boughs of the dendritic processes of Purkinje-cells. The axons of the cells of Purkinje undoubtedly carry impulses away from the cortex, but their destination is not certainly known.

The uniformity of structure of the cerebellum suggests that it “acts as a whole.” Anatomy gives no warrant for the expectation that work of different kinds is done by its several lobes. Its simplicity leads one to hope that its mechanism may some day be understood; but at present there are so many gaps in our knowledge that it is difficult, perhaps hardly profitable, to attempt to string together the few anatomical facts of which we are sure.

By means of tracts of afferent fibres the cerebellum has a very extensive connection with the grey matter of the cerebro-spinal axis (including the optic thalamus) into which sensory impulses of all kinds are poured. Experimental results indicate that the organ distributes impulses to the whole length of the cerebro-spinal axis, from the level of the neurones which govern the muscles which move the eyes to its far hinder end. No nerve-roots enter it. Its afferent fibres are the axons of cell-bodies which lie in the posterior horns of the grey matter of the spinal cord and in the corresponding grey matter of the axis of the brain, especially that part related to the nerve from the semicircular canals. Another set of afferent fibres lies at the periphery of the spinal cord, forming one of the best defined of the spinal tracts. It is also one of the oldest, being found in the same situation in all vertebrate animals. Its fibres, which are exceptionally large, are the axons of cells which form a very definite column—the “vesicular column of Clarke”—on the median side of the posterior horn. Further than this we cannot go. We are ignorant of the nature of the sensory impressions collected by the cells of Clarke. The cerebellum also receives through its middle peduncle the axons of cells which lie in the pons Varolii on the opposite side; which cells are discharged by impulses descending from the cortex of the great brain. It is not improbable that it gives to the great brain as many fibres as it receives from it.

If we had no experimental evidence as to the part which the cerebellum plays in the harmonious working of the whole nervous system, we should infer from its structure and connections that it is somewhat mechanical, a co-ordinator of the activities of other parts rather than in itself a functionally independent organ. Pathological and physiological observations very definitely justify this conclusion. They show that the cerebellum is not essential to life. It may be completely destroyed by disease or removed by operation without robbing the individual of any single function or capacity. Disease of the cerebellum does not diminish the patient’s sensitiveness to every kind of stimulus, nor does it deprive him of the use of any single muscle; but it reduces him to the condition of a person who in gait, but not in mind, is habitually drunk. When he walks he staggers from side to side; when he stretches out his hand it trembles. His movements are jerky; his head shakes, his eyes oscillate; he suffers from a feeling of giddiness; his speech comes haltingly. Cerebellar ataxia, which is a rare disease, resembles in many respects the much commoner “locomotor ataxia” produced by disease of the spinal ganglia and the parts of the cord connected with posterior roots; but careful analysis of the symptoms shows that they are due, not to want of the sensations which guide movements, but to inability to regulate the force of muscular contractions. A man suffering from locomotor ataxia falls when he closes his eyes, because, not being able to feel with his feet, he is dependent upon vision for information as to his attitude. When the cerebellum is diseased, the patient is no less unsteady with his eyes open than he is with them closed.

The results of cerebellar disease or injury bring home to us the fact that a nice adjustment of movements is needed to maintain equilibrium. A dog from which the cerebellum has been removed retains all its natural enterprise, all its instincts, all its emotions; but every action which requires it to maintain its centre of gravity in an unstable position gives it trouble. Placed in water so that its body is supported, it swims almost as well as a normal dog. It is, however, easy to lay too much stress upon the balancing function of the cerebellum. The disturbance of this function attracts our attention; yet it is probably but the indirect result of the suppression of activities of a more widespread character. No animal ventures such liberties with its centre of gravity as the biped Man accomplishes, without thinking, every time that he descends a flight of stairs. Yet the cerebellum of the limbless whale, that lives in a medium which decentralizes its gravity, so to speak, bears the same proportional relation to the rest of the nervous system as that of Man. Strangely enough, it is the only cerebellum in the animal kingdom which so closely resembles Man’s that it might be passed off as belonging to a human giant; another reminder of the difficulty of deducing the functions of the several parts of the organ from a study of their relative development. What have a man and a whale in common which determines the identity in form of their cerebella? How has it come about that two cerebra as widely unlike as a man’s and a whale’s should be associated with a common form of cerebellum?

If we apply to grey matter the distinction between sensory and motor nerve-tissue—having no exact terminology, it is difficult to avoid these metaphorical expressions—the cerebellum is essentially a sensory development. It grows from the very margin of the infolding groove, which, when closed, becomes the central canal of the brain and spinal cord, its elements being marshalled in intimate association with sensory root-fibres. Its millions of loops formed by the axons of granules and the collecting processes of Purkinje-cells, are by-paths which tap the conductors of sensory impulses. From some—those, for example, which originate in the muscles and tendons, and in the semicircular canals—more of the impulse is diverted to the cerebellum, from others less. The organ has no motor functions. It does not discharge neurones which control skeletal muscles, or plain muscle, or glands. Yet it influences the passage of impulses through sensori-motor chains, and apparently its influence is universal. It regulates tone, reflex action, voluntary action. There is no part of the nervous system over which its control is not felt. By its action on the apparatus which binds the infinity of receptors which the body contains to its muscle-fibres and other effectors, it unifies the body. The cerebrum, as we shall see, is the organ which unifies the personality. In the progress of evolution two functions which were originally combined have, for convenience of concentration, been divorced. The great brain has been set free from the more mechanical part of the work. That it can perform the functions of the cerebellum as well as its own is proved in cases of congenital deficiency of that organ. In several instances malformation, amounting to a very considerable reduction in the size of the cerebellum, was not detected until after death, there being no symptoms of a sufficiently pronounced character to call attention to it during life.

The Cerebrum.—All observations made on the great brain prior to 1870 showed it as absolutely inexcitable. Surgeons and physiologists agreed that cutting, burning, passing electric currents through its substance, neither yielded evidence of sensation nor movement of any part of the body. Concerning its structure little was known beyond the fact that whereas the grey matter, or cortex, which covers its surface contains nerve-cells, only fibres are to be found in the white matter which constitutes the greater part of its bulk. It seemed a hopeless task to attempt to make anything out of a mass of tissue so uniform in constitution and so irresponsive to experiment. Removing portions of it appeared to cause a general dulling of the intellect without loss of any particular mental quality. Physiologists, therefore, spoke of the cerebrum as “functioning as a whole.” Phrenologists, having classified the various phases of mental activity as “faculties,” discovered “bumps” on the surface of the skull which they correlated with the possession of the several faculties in a marked degree. They parcelled out the brain in organs concerned with different kinds of thought; but their localization of function was anatomically as baseless as their classification of the various aspects of mind, viewed as a system of philosophy, was absurd. In 1870 it was announced that electrical stimulation of certain areas of the cortex of the cerebrum of an animal under the influence of an anæsthetic, and therefore incapable of voluntary action, induces definite movements. Although the surgical applications of this discovery have proved immensely important, its physiological value, as affording a method of investigating the functions of the brain, is extremely small. Yet the discovery gave an impetus to the further study of the cortex, which has been rewarded with many exact results. By the discovery of its excitability to electric currents it was proved that the whole cortex has not exactly the same work to do, or—perhaps this is the safer form of statement—does not do its work in exactly the same way. As soon as it was known that it is divisible into areas differing in function, many methods by which the delimitation of the areas might be attempted were devised. The converging efforts made during the past forty years by comparative anatomists, histologists, physiologists, pathologists, and physicians, have resulted in the acquisition of an accurate, if very restricted, understanding of the construction and mode of working of the apparatus of thought. Of some of the new data the psychologist is able to make use; but so far as the physiologist is concerned, it is the vehicle of mind which is the subject of study, not its contents.

A new subject has been created since 1870. There is therefore nothing to be gained, so far as our present purpose is concerned, from the consideration of views which were current before that date; and since, as must always occur when a science is rapidly advancing, observations which logically should have been the first to be made were not thought of until it became necessary to devise methods of checking results obtained in other ways, we will consider the various sources of our information without regard to the chronological order in which they were opened up.

The cerebral hemisphere contains two large central masses of grey matter, the nucleus caudatus and the nucleus lenticularis, often described as a single structure under the name “corpus striatum.” Their functions are unknown. The nerve-fibres which connect the cerebral hemispheres with the rest of the central nervous system form two thick limbs or crura on the under side of the brain. Each crus turns upwards into its hemisphere, between the nucleus caudatus and optic thalamus (the latter belongs to the “between-brain”) on the inner side, and the nucleus lenticularis on the outer. In this passage the compact crus, which is somewhat flattened, is termed the “internal capsule.” Immediately above the three grey masses the internal capsule disperses as a fountain of fibres which go to all parts of the cortex. Mingled with these radiating fibres are vast numbers of others, proper to the hemispheres, which run tangentially. Some, crossing the median plane, as the corpus callosum, bind the two hemispheres together. Others form tracts which can be followed from one end or pole of the hemisphere to the other. Groups of fibres, dipping but little below the cortex, unite nearly adjacent spots or neighbouring convolutions.

The folding of the cortex beneath fissures is due to the necessity of disposing of a certain bulk of grey matter without increasing its thickness beyond the proper limit. Since the superficial area of a sphere varies as the square of its radius, whereas its capacity varies as the cube, it is possible for a fixed relation to be maintained between the amount of cortex and the amount of white matter in the brain, only by the folds increasing in depth as the size of the brain increases. Fissuring is a response to a mechanical need. This does not imply, however, that the lines along which it takes place are devoid of morphological meaning. The similarity in pattern of the convolutions and fissures in various animals, and the regular progress of their development in each individual, prove the contrary. If they are not absolutely trustworthy as boundaries of areas of separate function—and further evidence will be needed before a decision can be pronounced upon this disputed question—they are in the main satisfactory as landmarks.

As the nervous system grows, the axons of its neurones acquire their fatty (myelin) sheaths in the order in which they come into functional activity. The passage through them of impulses is the stimulus which leads to the deposition of fat. The study of the progress of myelination enabled the anatomist Flechsig to ascertain the situation within the brain of the tracts of fibres related to the several senses, and hence the traffic of the areas of the cortex to which they go. Glistening white streaks appear successively in the pulpy yellowish-pink substance of the interior of the brain. At the time of birth all the fibres which enter or leave the cerebral hemispheres have acquired their myelin sheaths. In the baby’s brain the sense-organs have established all their connections with the cortex. No new fibres will appear in the nerves of the eye, the ear, or the other sense-organs, nor will their end-stations in the cortex be further multiplied. (The use of the expression “end-stations” is legitimate so far as sensations are concerned; notwithstanding that all sensory impulses are retransmitted by neurones in the cerebro-spinal axis.) But the cortex is very far from having finished its growth. It contains a large amount of embryonic tissue, which gradually spreads outwards from the developed areas into the surrounding unoccupied zones. The taking up of new territory, and the consequent increase in the size of the brain, is continued into adult life. The study of progressive myelination enabled Flechsig to divide the cortex into “sensory centres,” and intervening “association-zones”; although, doubtless, the difference in function between the portions which receive sensations direct and the portions in which the products of sensation are worked up is one of degree, and not of kind.

Fig. 24.—Vertical Sections of the Cortex of the Cerebrum—
A, of the Visual Sensory; B, of the Visual Association Area.

Between the two sections are shown the principal types of cell, at the levels at which they are severally found: a, small pyramid; b, medium-sized pyramid; c, large pyramid. The size of a pyramid is an indication of the distance to which its axon extends before branching; the longer its traject, the more widespread, it would seem, is its terminal arborization. The axon of c, one of the very large pyramids found in this association area, passes to the front of the cerebrum, where it breaks up in an association area of the tactual sense of the hand, or of sensations concerned with the regulation of gait, or in a centre for movements of the eyeball. d, a tangential cell of the surface; e, a Golgi cell with ramified axon; f, a polymorph cell, with its axon directed towards the surface. In sensory areas, tangential fibres and granules are more numerous; in association areas, small and medium-sized pyramids.

The structure of the cortex is not quite the same in sensory and association areas; but it is everywhere so far from showing the diagrammatic simplicity which characterizes the cortex of the cerebellum as to make it difficult to summarize the modifications which distinguish its various regions. To a considerable extent its elements shade one into the other, differing in size and in orientation rather than in form. Commonly it is described as divisible into five layers: (1) A thin superficial layer, containing cells of various forms and fibres derived from the cells of the deeper strata. Some of the cells are pluripolar, possessing several axons which run parallel with the surface. Their destination is unknown. They do not appear to form baskets like the cells of the molecular layer of the cerebellum. The dendrites of pyramidal cells extend into this layer. (2) The layer of small pyramids; cells with a branching apical process, root-like dendrites from the basal angles of the pyramid, and an axon which sinks into the white matter. (3) Granules. Carmine or other nuclear stains show that small cells are present in very large numbers, especially in the sensory areas; but since they are not, like the granules of the cerebellum, coloured by the chrome-silver method, their form and the disposition of their axons are unknown. (4) Large pyramids exactly similar in form to the small ones. Their apical processes are very thorny. Their axons give off several collaterals. Pyramids are the most conspicuous elements in the cortex. Properly speaking, they do not occur in layers, but are scattered throughout its whole thickness, although their cell-bodies are not seen in either its most superficial or its deepest strata. The largest are those of which the axons either descend into the spinal cord or pass to a very distant region of the cortex. They are found singly or in small clusters in the deeper levels. (5) Polymorphous cells, some of them pyramids lying on their sides, or even directing their axons towards the surface; some fusiform or irregular cells; some Golgi-cells ([p. 340]). The axons of pyramids enter the white matter, and many fibres from the white matter radiate towards the surface between the pyramids; but the way in which afferent, sensory fibres are connected with the collecting processes, dendrites, of the pyramids is not known. We have already referred to thorns, and to the possible nerve-net ([p. 301]). Sheets of tangential fibres also occur in the cortex. A particularly distinct sheet divides the granules in the visual cortex into two strata. In sections of this region the sheet of fibres appears as a white line, distinctly visible without a lens.

The limits of the several areas can be determined by examining the structure of the cortex; but the individual peculiarities of the various regions are not so marked as to indicate that they have different kinds of work to do; if by kinds of work we wish to imply that one part is “sensory,” another “motor,” a third concerned with “intellectual processes.” On the contrary, its relative uniformity shows unmistakably that all parts are engaged in the same work. Nevertheless, certain broad conclusions can be drawn with regard to the form of the neurones more immediately concerned with sensation, with motion—that is to say, with the discharge to the grey matter of the cerebro-spinal axis of the impulses which call its neurones into activity—and with the secondary processes, called collectively “association,” which occur within the cortex. Granules, as everywhere throughout the nervous system, are receivers and distributors of sensory impulses; although a study of the cerebral cortex does not justify the conclusion that they are necessary links in its sensori-motor arcs. Large pyramids are occupied with the nutrition of fibres which have a long traject through the system. Hence they are “motor.” They constitute a marked feature of the area which is susceptible to stimulation. They occur also in the visual area and elsewhere. Small pyramids are associational; that is to say, their axons do not leave the cerebral hemispheres. They distribute impulses from sensory areas to association-zones, and from one part of an association-zone to another. The layer of polymorphous cells is relatively thicker in animals in which the cortex of the brain exercises less control over action than in animals in which the cortex is supreme—in a rabbit thicker than in a monkey; in a monkey thicker than in Man. This layer is therefore said to be concerned with the lower functions of the cortex, whatever this expression may mean. Since the relative abundance of small pyramids is a test of the supremacy of the cortex, we may speak of them vaguely as concerned with its higher functions. But a surer test of the capacity of the cortex for the elaboration of the raw materials of thought which sensory nerves deliver to it is the relative abundance of the tissue which intervenes between its cells. The number of cell-bodies to be counted in a square millimetre of a section of a given thickness is smaller in Man than in a monkey, in a monkey than in a dog, and in a dog than in a rabbit.

A comparison of the brains of various mammals in which particular sense-organs are either deficient or exceptionally well developed affords the clearest proof of the localization of sensory areas. This, if it were possible to make satisfactory measurements, would be by far the best class of evidence as to the part played by the several senses in an animal’s mental life. Unfortunately, measurement appears to be out of the question; but a glance at a rabbit’s brain, placed by the side of a mole’s, shows that vision is localized in the occipital region. All marine mammals are destitute of the sense of smell; the brain of a dog, compared with that of a porpoise or a whale, shows that the sphenoidal region ([cf. Fig. 25]) is associated with this sense. The brain of an otter exhibits very clearly the area into which impulses arising in the nerve-endings of the sensory bristles of the cheek are poured.

“Nihil est in intellectu quod non prius in sensu fuerit.” The organ of the intellect is the cortex of the great brain, a sheet of grey matter which has developed in connection with the various sense-organs. The cerebral hemisphere of an infant is merely an extension of the nerve-tissue associated with its sense-organs. Such it remains in a microcephalous idiot. In the lower animals its capacity of growth after birth is very small. But in a normal child the inflow of impressions through sense-organs, the experience acquired regarding itself and its surroundings, education, whether accidental or directed, causes the extension of nerve-tissue from the sensory areas into the expansible intervening zones.

There is still some uncertainty as to the nature of the sensations received in the excitable area. They may be termed “kinæsthetic” (sensations connected with movement) without more exact definition. Some physiologists consider that tactile sensations, as well as the obscure sensations, originated in the nerve-endings in muscles, around tendons, or on joint-surfaces, are distributed to the areas, which, when stimulated, are shown to represent fingers, hand, arm, and other parts of the body. Others have sought, though with doubtful success, for a tactile area, independent of the kinæsthetic centres. When first discovered, these centres were termed “motor,” and still this term may be retained, on the understanding that it does not imply that the exchanges which occur in the kinæsthetic centres are of a different nature to those which take place elsewhere. The region which they occupy has become the motor area of the cortex because voluntary movement is possible only under the guidance of sensations of movement. A sound or a retinal image may prompt the movement; but the part of the temporal region, or of the occipital region in which the sound-movement exchange or sight-movement exchange occurs must act through the motor area by opening kinæsthetic-movement arcs. Destruction of a part of the kinæsthetic cortex causes in Man and the higher apes permanent paralysis for the movements directed by the spot destroyed. In lower animals the definition of the movement centres is vague, and their removal produces only temporary results. Their mastery over the muscles is less complete than in the higher apes and Man.

Practically nothing is known with regard to localization of function in the association-zones, with the exception of the localization of the centres for words; but this exception is so remarkable as to suggest that if there were any other faculties, interference with which caused defects as distinct as those which characterize disorders of speech, it would be found that the association-zones are made up of definite centres. As the evidence stands with regard to the broadest continental divisions, we can merely state that it points, although not very clearly, to the connection of the frontal zone, the region in front of the kinæsthetic area, with ideas of personality, of other zones with ideas of environment. Injury to the frontal region has in certain cases resulted in the victim’s losing his knowledge of himself, his name, and his relation to his family. On the other hand, gunshot wounds and other definite injuries have in a large number of cases destroyed portions of the cortex behind the forehead without causing any recognizable intellectual change. It is quite certain that this part of the brain performs no functions which are of a different, or, as it is often called, higher order than those of other association-zones. It has been stated that disease of the zone which intervenes between the visual and auditory areas is more likely to cause hallucinations, disease of the frontal zone delusions. A patient fancies in the one case that he sees things that are not there, or hears voices when no one is speaking; in the other case he imagines himself a king; but evidence connecting localized disease with mental derangement is very scanty. The functional disturbance which causes lunacy is usually of a general character; or, if local to begin with, it becomes general before the death of the patient makes possible the examination of his brain.

Fig. 25.—The Surface of the Left Cerebral Hemisphere, Cerebellum,
and Medulla Oblongata.

Sensory areas are enclosed by broken lines; certain centres in the association-zones are marked by dots. The sensory area of smell is on the inner aspect of the brain; so also is the area of vision which borders the calcarine and retrocalcarine fissures, and only rarely extends on to the external surface, as shown in the diagram. The sensory area of hearing is largely hidden within the fossa of Sylvius, the opening into which is indicated by the dark line above it. The kinæsthetic-sensory areas for the various muscles of the body occupy the territory between the dotted line in front and the bottom of the fissure of Rolando behind. They do not extend on to the posterior wall of this fissure. It is impossible at present to define the boundaries of any of the centres in the association-zones.

Derangements of speech throw a flood of light upon the organization and manner of working of the association-zones; and, owing to the accident of the continuation of the line of the carotid artery by the middle cerebral artery, which supplies the speech centre, there is no other spot in the cortex so likely to be thrown out of gear. A little plasma coagulates on one of the cardiac valves, or about an atheromatous spot in the aorta. Detached by the blood-stream, it is shot into one of the branches of the middle cerebral artery, which it plugs, causing apoplexy. A larger or smaller number of muscles on the opposite side of the body are paralysed. If the plugging occurs on the left side of the brain, it is accompanied by aphasia; but only if it occurs on the left side, owing to the fact—perhaps the most remarkable in connection with the localization of speech—that only on the left side is the cortex trained to utter words. In course of time the patient may recover the power of speaking, but not until he has, with almost as much labour as in childhood, educated the right side to do the work. There are four speech-centres, quite distinct one from the other. Near the visual area is the centre for seeing words, or rather the centre for seeing the meaning of words. If this centre be diseased, a written word is merely a crooked line. Behind the auditory area is the centre for recognizing the meaning of words heard. If it is interfered with, the most endearing or commanding phrases produce no more impression on the hearer than a bird’s song. In front of the hand-area—its localization is less certain than that of the other three—is the centre for writing. In it are associated words heard or seen, with the movements necessary for the making of letters. In the centre first referred to, as being the one most often thrown out of gear, which lies in front of the area for the mouth and throat, words heard or seen are translated into movements of the parts which give them sound. No other actions illustrate so clearly the “law of neural habit.” In the infant’s brain sounds of words are distinguished from other sounds. They are associated with the objects which they name. Movements of the mouth and throat, made at first ineffectively, blunderingly, succeed after a time in securing the thing of which they sound the name to the child’s satisfaction. Thus, two centres are gradually established in his mind. Sounds and ideas of things are associated in the one; words and ideas of the movements necessary to their pronunciation in the other. Either of the four speech-centres may be placed out of action without the others suffering. A man may be able to write without being able to read what he has written. He may read aloud, although apparently deaf to speech. He may be unable to write or unable to speak, although understanding what he reads or hears. Aphasia, when partial, illustrates still further the law of neural habit. The ability to remember nouns, especially proper names, is most easily lost. Few are the people who, as age advances, do not suffer from this failing. Even the names which are most familiar elude the memory. From one point of view this is strange. Nouns-substantive are the words first learned. Of all words they have the most definite objective association. But it is just their definiteness which makes them difficult of approach when the apparatus of mind is working badly. There are so few paths by which they can be reached. Their mental associations are limited. A patient who is recovering from the effects of a lesion which has rendered him partially aphasic may be able to recall adjectives when he cannot recall nouns. He may say, “Give me the black,” when he wants ink, and “Give me the white,” when he needs paper. Or he may retain control of verbs. “Where is the—— what I put on—what I think with?” may be the circumlocution for hat.

Psychologists explain the voluntary production of a movement as the setting flowing of a sensori-motor current. Everyone agrees that it is impossible to think of the impulses which produce movement as originating without sensory antecedents. Hence psychologists picture the nerve-current as originating on the sensory side. Kinæsthetic images of the sensations which will result from the movement are described as being called up in the mind by the agitation of the part of the brain which, by association, is linked with the neurones which discharge impulses to the appropriate spots in the grey matter of the spinal cord. The idea of movement flows over to the muscles. But this conception of the relation of mind to body assumes too much. It postulates an existent mind in which the images of movement-sensations—the memories, that is to say, of the sensations which previously accompanied movement—are stored. The study of the apparatus of mind does not warrant this assumption of an existent mind. It finds nothing in the nervous system but apparatus. There is no mind existent in the brain during sleep. It would appear to be sufficient to describe the origination of a voluntary movement as the opening of the channels which convey the afferent impulses which are ceaselessly pouring into grey matter from nerve-endings in and about muscles into efferent channels. Our conception of the number of sensations which reach the realm of consciousness is ludicrously restricted by our inability to pay attention to more than one sensation at a time—a restriction, it is needless to remark, which is imperative in the interests of consistency of behaviour. Two personalities paying attention to different sequences of sensations would give incompatible orders. One would command the muscles to cause the body to recline; the other would direct them to make it stand up. From myriads of sense-organs impulses are continuously rippling through the cortex of the brain. The term “impulse” is too heavily weighted by its association with the idea of currents which are strong enough to prove effective without the intervention of consciousness; but no other is available. They ring the bell of consciousness, however little may be the attention which their summons secures. Attention cannot be directed to two things simultaneously. It moves, as it were, on a succession of points. On some it rests longer than on others. They make an impression which can be recalled; the rest being passed by so rapidly that they are not remembered, it is as if they had never been perceived. They blend, as a succession of moving lights blend, in producing a background to consciousness. Not recognizing their separateness, we interpret them as fused. A good deal of misleading metaphor has been used, as it seems to the writer, in accounting for the effect upon the mind of impressions which make but a weak demand upon attention. They are spoken of as “marginal” perceptions, from the analogy of the ineffectiveness of impulses generated at the periphery of the retina, as compared with those which give rise to direct vision. A “subconscious,” or even “unconscious,” self is evoked. The self cannot be less than conscious. Self is the passage of attention from sensation to sensation. Its relation to the not-self is temporal, not spatial.

Every sensation which is called up into consciousness, though it occupy attention for the shortest possible time, tends to give rise to movement—is, indeed, in its very nature an impulse flowing through a sensori-motor arc. The circuit for the voluntary execution of a movement is represented as flowing through kinæsthetic-movement arcs. This may be necessary for volitional actions, but it is not essential for reflex actions. A spinal frog will remove an irritant from its back with its hind-leg, after the roots of all the afferent nerves of the hind-leg have been cut. In this case the reflex is direct, from injured skin to muscles of the leg. It is not double—muscular sensations from the leg, liberated into efferent leg-muscle-nerves by skin-sensations originated simultaneously in a part anterior to the segments in which the roots have been cut.

The unit of sensation to which attention can be directed has yet to be defined. Like sensations—sensations which are correlated in experience, that is to say—seem to fuse in consciousness. A sequence of similar sensations appeals to attention. Unlike sensations interfere one with another. The apparent fusion is not a composite neural effect which consciousness views as a single unit. Not even identical images simultaneously formed on the two retinæ produce a superimposed effect upon a particular spot in the brain. Different brain-spots receive the two separate images which the mind views as one. This raises a doubt as to whether perceptions are, properly speaking, fused. It suggests that they are separate points upon which attention rests in rapid succession; but such a hypothesis does not preclude the conception of the production of a composite sensation by impulses coming simultaneously from the same sense-organ—e.g., a unified neural effect as the result of several musical tones.

Every neural agitation which attracts attention has an effect upon the growth of the nerve-strands in which it occurs. Memory is not an existent. It is the repassage of the same strands. There is no such thing as memory. It is the neural apparatus which responds in a similar way to a similar agitation. It is difficult to speak of association and neural habit, the phenomena upon which not only all mental life, but all co-ordinated activities, are based, without using such expressions as “the broadening of the path” or “the thickening of the conductor” by the impulses which pass through it. Apparently these analogies may with safety be pressed curiously far. Chaotic response to stimulation is unknown. Thanks to the nervous system, action exhibits an ordered relation to stimulation. This relation is determined by education, giving the term a connotation wide enough to cover all experience. Nerve-tissue adjusts itself to experience; and since the nerve-matter which takes the pattern is not labile, the process of organization is consecutive and the result permanent. One pattern is not destroyed as another is impressed. Hence temporal associations are formed. What has been thought once will be thought again, if the circumstances in which it was thought recur. What has been done once will be done again under the influence of a similar sequence of stimuli. The conductors are widened every time that they are used. But, so far as concerns the mind, a reversed influence comes into play. The wider the conductor, the less appeal to attention is made by the impulses which pass through it. It is as if currents which have to overcome resistance in a narrow path acquire a higher potential than those which find an open road. And since the making of the road depends upon attention, the limit of broadening is reached when a volitional act becomes a habit. The first time that a piece of music is played consciousness is alert. Marks on the page and movements of the fingers are felt intensely. With each repetition the need for attention subsides.

A skilled movement is impossible in the absence of guiding sensations. I decide to button my coat. Sensation-paths from the muscles of the forearm are opened into motor paths extending from the large pyramids in the arm-centres of the kinæsthetic cortex. But it is not sufficient that the action be started: it must be guided by the sensations which movement produces. If my fingers are numb with cold, I cannot button the coat. The muscles which move the fingers are warm enough beneath the sleeve, but my attempts to will them to move are as futile as they would be if the muscles belonged to some other person. The will has no power over the muscles. It is essential that the sensations which accompany the act of buttoning the coat flow through the same paths as hitherto in the cortex of the brain. Flowing through the same paths, they produce the same effect in consciousness, the same perceptions. In ordinary parlance, one cannot perform any act unless one can remember what it felt like to perform it on a previous occasion. It is almost as sound physiology to describe the voluntary action of fastening a button as commencing in the skin of the fingers as to describe is as commencing in the brain. The act is due to the direction of attention to impulses which flow from muscle to muscle, and from skin to muscle.

All skill in the use of muscles is acquired by the method of trial and error. Familiar movements are tried, combined, modified with a view to the production of a new result. A man accustomed to striking with the right hand forwards endeavours to swing a golf-club with the left hand backwards. For a long time the result is anything but a success. At length the head of the club takes the right curve. It not only hits the ball with its centre, but it carries it through in the right line. The ball travels 120 yards or so towards the green. In golfing terminology, a successful drive is always “an awful fluke”; but the fluke once accomplished, nothing is easier for the golfer than to drive equally well on all succeeding occasions. He need merely remember exactly what it felt like to give the club a perfect swing, and exclude all other sensations while he is passing these memories through his sensori-motor arcs!

The fact that we can deliberately improve an action, fitting it to the attainment of the object of desire, by suppressing wrong and emphasizing right sensations, shows how large a part consciousness plays in the affairs of the nervous system. This brings us to the frontier of physiology. At this boundary the authority of the physiologist ends. He cannot define consciousness; he cannot investigate it. Yet he naturally asks whether the machine which he is investigating is a machine and nothing more. When the possibilities of reflex action were first recognized, thought tended to dethrone feeling and Will in favour of automatism. If the actions of a spinal frog exhibit so distinct a purposive character, why, it was asked, should we assume that the frog with a brain is anything more than a reflex machine? Light, heat, sound are playing upon its sense-organs; surely these stimuli suffice to set going all the sensori-motor currents which lead to the various movements which in their totality constitute the frog’s behaviour! And why assign to a mammal a self-directing authority which we deny to a frog? The increased complexity of its behaviour is more than accounted for by the greater variety of its nervous arcs. All animals, it was argued, including Man, are reflex machines. Their thoughts and actions are the effects of the play upon their nervous systems of forces from the outer world. Each inherits a nervous system of a certain pattern. Its individual development is conditioned by the sensations which pass through it. The sensations are impressed by the environment. Therefore the individual is a puppet, his activities the dance of circumstance. Consciousness is an “epiphenomenon.” Few physiologists or students of animal behaviour take this material view of life at the present day. The fact that it leads inevitably to the conclusion that consciousness is an “epiphenomenon” (Huxley’s term) is its reductio ad absurdum. It is not in harmony with the economy of Nature that an animal should be endowed with the capacity of feeling pain and pleasure, if such endowment is useless to it. It can be useful only by directing activity towards the attainment of pleasure and the avoidance of pain. This admitted, the mechanical theory falls to the ground. There is an “It” which feels, selects feelings, chooses those which have a pleasant tone, wills to perform the acts by which they are attained. It follows that the value of consciousness lies in the prerogative which it confers of adapting action, within certain limits, to circumstance. An animal succeeds in life in proportion as the nervous system which it inherits reacts satisfactorily to its environment. A chick which, after being hatched in an incubator, has been isolated for twelve hours without food, seizes a grain of corn the instant that it sees it. Its brain contains ready-made sensori-motor arcs connecting the spot in its cortex in which the visual impression of the grain is perceived and the motor neurones which control the pecking muscles. A sheep-dog is quickly broken to sheep, because its ancestors have been selected by mankind from amongst dogs that readily adapted themselves to this work. The breeder has selected a pattern of brain with the same success with which, when appearance is the only desideratum, he selects a pattern of coat. Beavers set to work at constructing a dam at the only spot in a valley at which it is possible to create an artificial lake, because for countless ages Nature has ruled out the animals which constructed their dams in unsuitable places. Man also inherits a brain-pattern; but, not being required to shift for himself soon after birth, he goes through a long period of infancy and tutelage, during which, by force of circumstance and his own Will, the pattern is elaborated. His supreme success is due to his capacity for adapting means to ends. He inherits very few instincts. Except as regards organic functions, his spinal cord is subservient in almost all respects to his brain. Most of the actions of an animal are instinctive—a word which has been sadly misapplied. Its connotation is negative rather than positive. Owing to the marked pattern of its brain, an animal finds it difficult to avoid acting in a particular way. As the nights grow longer and its hours for feeding are curtailed, a swallow is impelled by its instinct to go South. It makes the same use of its sensations during its migration, and is as completely dependent upon them for its guidance as a man would be. The lower we descend the scale, the more inevitable do an animal’s movements become; but there can be no doubt but that consciousness is of value to an animal, as to Man, in that it gives to its individuality the capacity, within such limits as Nature has selected, of resisting or modifying its ancestral instincts when they are not absolutely appropriate to the occasion.

Sentience implies personality. “No system of philosophy can extrude the ego.” The difference between the performance of the animal machine as a physiologist studies it, and its behaviour when under the control of its own driver, is the difference between reflex action and choice. The ego interacts with physical forces. It does not come within the province of the physiologist to explain the source of the force which interferes with force. He finds no trace of it on either credit or debit side when making up the body’s accounts. He is unable to enter, “Item, to the development of consciousness ... so much.” He can form no conception of this immaterial manifestent which hovers over the infinitely numerous sensori-motor exchanges which are always occurring in the cortex of the brain, giving to a particular group of agitations, now here, now there, a special quality; but the manifestent is needed to account for the potency of the reinforced agitations which enables them to take possession of the nerve-paths by which muscles are reached.

It is for the psychologist to define the application of the terms “consciousness,” “attention,” “will.” He cannot define the attributes of the ego which these terms connote. The moralist must show the way in which they determine, or should determine, conduct. Yet within the plain limits of physiology, attention, using the word in its every-day sense, modifies the responses of the nervous system in a degree which cannot escape observation. It is astonishing to anyone accustomed to hospital surgery (although even in this field singular exceptions are met with) to see the grave operations which a veterinary surgeon may perform, without the animal showing any evidence of pain, provided its apprehension has not been aroused and its attention directed to what is being done. A horse standing in front of a crib of oats, untied, will hardly whisk its tail while the surgeon is making a great wound in its flesh, and sawing off a bony excrescence. The knife does not come within the experience of a horse. It has no anticipations, and its skin, intensely sensitive to the tickling of a fly or the smart of a whip, is relatively insensitive to a cut. An eminent surgeon of the last generation (the writer, as a student, “dressed” for him in his old age) was in the habit, having arranged that his patient could not see what he was doing, of performing operations of a very painful nature whilst assuring his patient, “I am merely making a thorough examination, in order that I may be perfectly certain of the cuts that I shall have to make to-morrow in the operating-theatre when you are under chloroform.” We are not concerned with the ethics of his method; but the assurance, “Now that’s all over; you will never need to have that operation performed again,” saved many a sufferer from a night of apprehension and a miserable “coming round.”

It was stated, during the South African War, that at Ladysmith the bearer of a critical despatch, who was struck in the palm of the hand by a bullet which traversed the whole length of his forearm, did not discover that he was wounded until he saw the dripping blood, after his errand was successfully accomplished. To deliberately cut oneself with a razor is most painful, yet shaving in the morning, with thoughts concentrated on the doings of the day, it is often the sight of blood which directs attention to the fact that the skin is severed. Of all evidences of self, the power of paying attention is the most noteworthy. We can direct attention to certain sensations, which then become perceptions, and we can deliberately ignore others, within certain restricted limits.

The control of the nervous apparatus by the self is a truth which no student of the physiology of human beings can ignore. Isolated from its relation to all other scientific truths, it has been made the basis of a nescience which, although positively merely foolish, is, negatively, harmful—yet a form of folly which answers well to the needs of persons of a certain category.

It may be objected that the picture of the relation of mind to brain which is here presented—the one, activity, motion, the other a labyrinth of conducting paths—makes all mental phenomena entirely dependent upon current sensations. No results could happen if the sensations were not there. It affords no ground for the explanation of mental images, hallucinations, dreams. A few lines may be spared to show that this objection does not hold. We cannot attempt to explain the conscious control of thought. It is a part of the impenetrable mystery to which we have just referred. But, granted that it obtains, the direction by the ego of afferent nerve-currents through the same strands which formerly vibrated to sensations which drew a picture, and hence the revival of its image, is no more incomprehensible than the liberation of afferent impulses from muscles into efferent channels. Brain-chains are composed of many links. Their interconnection is illimitable. When I recall the appearance of the house in which I lived as a child, I throw into the chain impulses (from somewhere) which traverse the final links, where passage implies consciousness. At the edge of the lace-work of linked threads the impulses light up a pattern which childhood’s experience worked into the apparatus of thought.

If we were to admire the perfection of any special aspect of the brain’s functioning, the rarity of hallucinations might give us cause for wonder. That impulses so seldom leave their own paths is more astonishing than that occasionally, when the brain is excited and its nutritive conditions deranged, the impulses which the ego can direct into channels where they revive an image should sometimes, and with far greater force, make their own way down well-worn paths, lighting up a picture which deceives the ego. Dreams, by contrast, throw up in a strong light the part played by attention in intelligent life. The capacity for alertness is due to the favouring of one set of impulses by suppressing others. The favoured impulses hold the road. Concentration of attention is keeping thought to one line by resisting all temptation to wander into by-paths. The waking condition is the state in which all nerve-ways are closed, with the exception of those which consciousness is using. The more severe the closure, the more vivid is consciousness. In sleep all paths are open. In none is the potential acquired by impulses in the process of overcoming resistance high enough to evoke consciousness. A burst of impulses ascends from the stomach, set a-flowing by undigested fragments of salmon and cucumber, or mounts from the arm on which the sleeper has been lying until its circulation has been arrested. They reverberate through the open corridors of the brain. If they are sufficiently noisy to awaken the sleeper, he, detecting them in this path and in that, supposes them to be on the same errands as the impulses which commonly pass thus. If dreams are analysed, it will be found that, although the combinations of impressions may be uncommon and extremely bizarre, the impressions are selected from the most familiar. The images of which the dream is compounded, which may have lost all normal relations and may have assumed impossible proportions, are those which the mind most frequently conjures up. In the large majority of instances some happening of the day preceding can be recognized as the prompting cause. A remembered dream is the photograph taken by consciousness of the sensations which have bombarded it into activity. Especially if due to impulses originated by visceral discomfort, the dream may have an unpleasant tone. This may take various forms, but the emotion most commonly aroused is fear. The objects visualized may have preposterous dimensions, or they may be not sufficiently distinct for recognition—elusive imps; but most commonly distress is caused by the want of harmony of sensations, due to the absence of kinæsthetic elements. A man is lying on the railway-line; a train is approaching with increasing speed; he cannot get up. He is in the pulpit, but cannot speak. Dreams thus confirm the view set forth above as to the cause of volitional action. Ability to perform an act depends upon the flow through the kinæsthetic centres of the brain of impulses generated in the muscles by which the act is to be, or is being, performed. Kinæsthetic sensations do not under any circumstances play the same part in mental life as sensations from the skin, the eye, or the ear; when the body is passive in bed they are not flowing into the cortex. The dream-photograph shows elements demanding movement, but affords no evidence that movement is in progress.

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CHAPTER XII
SMELL AND TASTE

In Man the chief function of these senses is to guard the entrances to the respiratory and digestive tracts. In this they are not conspicuously efficient, since various poisonous gases, salts, and powders, escape their vigilance. Merely a selection of the substances which occur in air and in food are recognized as having odour or flavour. Smell and taste are only partially distinguished in ordinary parlance. No odorous substance is spoken of as tasteless when taken into the mouth. Its volatile constituents, escaping to the chambers of the nose, are said to afford a certain flavour. On the other hand, it is recognized that substances which stimulate the tongue alone—bitters, acids, sweets, and salts, unmixed with volatile bodies—have no odour.

Biassed as we necessarily are by the paltry rôle assigned to smell in our mental life, it seems a little unworthy of the present functions of the great brain that it should have developed in association with the nose. Yet smell and taste are the oldest of the senses. Their origin goes back to the days of chemiotaxis, when the organism, having no specialized sense-organs, was attracted to its mate or to its food, and repelled from conditions unsuitable for its well-being, by particles in solution acting as chemical stimulants. An amœba is chemiotactically drawn towards its food, one spore of an alga is attracted to another, by the particles of matter which drift across the interval between them.

In the life of many animals smell plays as important a part as that of either of the other senses. One has but to watch a dog “looking” for its master, already full in view, with its nose, to realize that smell is the sense on which a dog chiefly relies. We describe it as looking, because in ourselves the eye has so far outdistanced the other senses as a channel of information that we speak of “looking” when we mean seeking, and say that “we see” when we wish to imply that we understand.

The difference between smell and taste is, in fishes, a difference in the quality of the sensation, and not in its “modality” or kind; but in terrestrial animals the olfactory membrane of the nose has become specialized for the recognition of particles suspended in air, the tongue for substances dissolved in water. The olfactory membrane, which lines the upper two of the three chambers of the nose, is covered with elongated cells of two kinds: (a) Columnar cells, fairly thick; and (b) fusiform cells, each carrying at its free extremity a bunch of exceedingly minute hairs. The fusiform cells are neuro-epithelial cells of the most primitive type. Before nerve-cells, properly so called, appeared, certain favourably-placed epithelial cells were connected by protoplasmic bridges with muscle-fibres, to which they delivered the impulses which were generated in them by external forces. Later some of the neuro-epithelial cells sank beneath the surface, where, as ganglion-cells, they served as intermediaries between groups of sensory cells on the surface and the nerve-net which lay more deeply in the tissues. The olfactory membrane perpetuates the earlier stage; in so far as it consists of elements which are combinations of sense-cells and nerve-fibres. Each of its fusiform cells sends inwards a nerve-filament, which, traversing the submucous tissue of the nose and the bone (cribriform plate) on the base of the skull, between the orbits, enters the olfactory bulb. The olfactory bulb is a part of the local nervous mechanism of smell. It is the ganglion of the nerve of smell plus nerve-elements which in all segments behind the eye have been withdrawn from the neighbourhood of the sense-organ into the central nervous system ([cf. p. 333]).

The way in which odorous particles in air stimulate the fusiform cells is unknown. The quantity which suffices as a stimulant is so small as to put chemical stimulation out of the question. A few grains of musk will scent a room for years. 0·00000004 milligramme of mercaptan (sulphur-alcohol) is recognizable in a litre of air. This is a dilution to 1 in 50,000,000,000. Probably even such figures as these would be thrown into the shade if we could estimate the minimum amount of human effluvium which will enable a dog to follow his master’s trail. Explanations have been sought in alterations in the vibrations of molecules of air caused by the presence amongst them of relatively heavy molecules of volatile substances; but the difficulty of accounting for the generation of nerve-impulses in the sensory cells remains as great as ever. The hairs borne by olfactory cells are so short that it is impossible that they should project beyond the film of moisture on the surface of the membrane. This seems to preclude an answering vibration. Yet an increase in the thickness of this layer and in its density, due to the presence in it of mucus secreted during a catarrh, renders the sense-cells incapable of responding to odorous particles.

Smell in an animal is not a test of the quality of the air it is breathing, but a source of information as to the direction in which it may seek its prey; or, although far more rarely, as to the direction from which the advance of a foe is to be feared. Hunting animals depend for the most part on the nose. Hunted animals rely chiefly on the eye.

If we attempt to analyse our smell-sensations, we find that we can pick out a number of varieties which appear so unlike as to have nothing in common: Putrid meat, burning indiarubber, sulphuretted hydrogen, ammonia, roses, onions, lemon verbena, methylated spirit. Everyone can make for himself a list of typical odours which seem to have specific qualities—odours so distinct that he never confuses one with another. He can also class together scents about which he is often uncertain. The type-odours he can distinguish when present in a mixture; whereas odours which are less distinct reinforce or modify one another. It has been found, by careful experiment, that certain type-odours even tend to neutralize each other. Musk and bitter almonds, for example, if present in small quantities and properly proportioned, produce a very dim sensation, whether supplied as a mixture to both nostrils, or the one assertive odour to one nostril and the other to the other. This last observation is of great importance. It proves that their mutual destruction does not occur on the olfactory membrane. It is not due to physical interference. The sensation of musk is delivered to one side of the brain, the sensation of bitter almonds to the other; but when attention is directed to these two sensations there is found a quality in the one which is irreconcilable with the quality of the other.

In certain persons and under certain pathological conditions, sensitiveness to particular odours, or groups of odours, is absent, while for the rest the sense is normal. Methylated spirit, prussic acid and mignonette, constitute a group which not infrequently drops out. Instances have also been reported of persons unable to smell vanilla (to which some are hyper-sensitive), and of others insensitive to violets, although normally sensitive to the scents of other flowers. The notes sounded in consciousness extend over a long gamut; but there are reasons for thinking that the number of keys on the clavier which odoriferous substances strike is limited. Eleven is the number provisionally adopted. The effect in consciousness varies according as one key or another is struck, or several at the same time with varying degrees of force.

Many attempts have been made to associate the sensation-qualities of the various odours with the chemical or physical properties of their odorants, with but little success as yet. To excite the sense of smell, a gas must be at least a little heavier than air. No volatile body, it is stated, is so heavy as to be odourless; on the contrary, speaking generally, heavy molecules are more stimulating than light. The quality of a smell-sensation would therefore appear to depend upon the period of vibration of the molecules of the substance which evokes it; but, as already stated, a consideration of the apparatus which responds to stimulation by odoriferous particles does not help us to an understanding of the way in which the particles act upon it.

Fig. 26.—Highly Magnified Section through the Wall of a Circumvallate Papilla
of the Tongue, showing Two Taste-Bulbs.

These sense-organs are groups of elongated epithelial cells, set vertically to the surface. Their cells are of two kinds—the one fusiform, slender, bearing each a bristle-like process which projects through a minute pore left between the superficial cells of the general epithelium; the other thicker and wedge-shaped. Nerve-fibres are connected with the fusiform cells.

Taste is far more limited in its range of sensations than smell. The back of the tongue is sensitive to bitters, the tip to sweets and salts, the sides to acids. Mixtures of these qualities are distinctly analysable by the sense of taste. Our sensations of taste do not fuse. Slight differences in the way in which the organs on the different parts of the tongue react to stimulation enable us to recognize that a sapid substance is a mixture. When, with a great flourish of trumpets, saccharin was introduced as a safe sweetener for gouty people, an attempt was made to provide them with saccharin-sweetened jam. The effect of the jam upon the person who consumed it was truly humorous. First a suspicion of tartness, then its adequate suppression, followed by nauseating sweetness. The sense-organs which subserve the sense of taste are clusters of fusiform epithelial cells, collected in “taste-bulbs” ([Fig. 26]). Each gustatory cell bears a minute bristle, which projects through the pore left by the cells of the surrounding epithelium which constitute a globular case for the bulb. As in the nose, eye, and ear, a second thicker variety of epithelial cell is also present. The nerve-fibres of the taste-bulbs are not, as in the olfactory membrane, processes of their cells, but branches of the fifth nerve which ramify amongst them. On the back of the tongue taste-bulbs are much more numerous than elsewhere. They are not as sensitive as the cells of the olfactory membrane; nevertheless, they enable us to detect 1 part of quinine in 2,000,000 parts of water.

Sensations of taste and smell endure for a long time after stimulation, because the odorous or sapid substance remains in contact with the sense-organs. This accounts for the confusion into which a man is thrown if he sip alternately port and sherry. After a short time he cannot tell the one from the other. The organs are quickly fatigued, using the term loosely. How intolerable patchouli would be to the ladies who use it were it otherwise! If for some time one sniffs the odour of mignonette, it ceases to be recognizable; whereas, turning to a rose, the olfactory membrane is found to be as sensitive as usual. When the sense is fatigued for a particular smell, it is dull for others of the same group, thus affording an opportunity of classifying smell-sensations according to their qualities; but the method is difficult to apply. Taste-organs are greatly affected by temperature. Quinine is not tasted just after drinking ice-cold water. Alcohol, ether, or chloroform paralyses the organs much in the same way. Castor-oil slips down the throat unnoticed if the mouth, just before swallowing it, has been rinsed with brandy or with a strong solution of tincture of chloroform.

Englishmen make but little use of their sense of smell. It might teach them much regarding the various emanations from putrid matter which are produced by bacterial action; but, dreading drains, they decline to cultivate proficiency in the exercise of this sense. The nose is valued for the warning it gives of “nasty smells,” but is not allowed to analyse them. Burnt milk, soap-boilings, rancid oils, are taboo, because they are associated with bungling in the kitchen. With moderated ardour, we allow our sense of smell to distinguish foods and beverages, but we are not a race of epicures. The perfumes of flowers are classed as “nice smells.” The idea of greediness is not associated with their enjoyment; besides, they remind us of gardens, sunshine, pretty forms and colours. When bottled, musk, orange-blossom, violets, lavender, are valued not so much for their own sweetness, as for their singular efficiency in obscuring nasty smells. Few persons practise the recognition and distinction of even pleasant odours. Very few, on first coming across a scented herb or shrub, pay sufficient attention to its perfume to impress it on their memories. They note the shape of its leaves and the colour of its flowers, but they are unable to identify it by its odour when they meet with it again. It is not much to be wondered at, therefore, that this slighted sense tends to leave us after middle life. It has been asserted—and probably the statement is justified—that rarely is the olfactory bulb of a man over forty free from signs of atrophy. We have no statistics concerning the brains of Japanese, who regard the sense of smell as one of the chief avenues of pleasure; but it may be that in this respect their brains present a contrast to our own. Yet the deadening of the sense is scarcely noticed, since its results are of little consequence as compared with those which follow loss of sight or loss of hearing. Many a man, as he grows older, declares that the cook of his club has lost his cunning, or frankly asserts that he “no longer cares for kickshaws. Cold beef, beer, and pickles, are good enough for him.” He little suspects that his palate has lost its power of distinguishing the flavours of dainty meats and wines. Others continue to be exacting, because their imaginations still endow food with the qualities which they remember, just as people eat preserved asparagus or tinned peas because they look—however little they taste—like the gifts of Spring.

Taste accompanies the reception of food in the mouth. We have no knowledge of the situation of our own olfactory membranes, and therefore we suppose that a flavour, whether it be due to stimulation of taste-bulbs or olfactory membrane, is in the mouth. The odour of a flower we mentally project to a distance, because we associate the sight of a flower with its perfume. A dog, able to judge the freshness or staleness of a scent, must project its sensations of smell in the same way in which we project our sensations of sight. It forms an estimate, of a sort, of the time that it will take in reaching the source of the scent. Its excitement increases as the trail grows fresher.

Taste and smell are heavily laden with affective tone. When disagreeable, the feeling which they evoke is near akin to pain. It may gather head until, like hunger, it causes the discharge of motor neurones; but under its influence food is ejected, instead of preparation being made for its reception.

Taste and smell are senses which afford us no information with regard to time or space. They give rise to massive sensations. Such sensations, devoid of detail, produce a frame of mind rather than thought. The smell of tobacco does not distract attention. On the contrary, the steady flow of impulses to which it gives rise helps to inhibit, to subdue, the yapping of more exigent sensations. And since sensations of smell have no features of their own, they form a background to sensations of other kinds, entering with them into memory. No two scenes are exactly alike. One cannot recall another. But the scent of syringa is always the same. Wherever smelled, it opens the pathways in the brain in which were first associated a June evening and syringa, with a scene and a situation upon which memory loves to dwell.

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CHAPTER XIII
VISION

The eye is enclosed in a globe of fibrous tissue, of which the front part, or cornea, being transparent, admits light. The epithelial layer which covers the cornea, conjunctiva, is also transparent. No bloodvessels enter these colourless tissues, unless as the result of inflammation due to infection or to exposure to sunshine or dust. For nutrition they are dependent upon the plasma which, exuding from, and returning to, the vessels which surround them, circulates in their tissue-spaces. In advancing years, when the circulation is less brisk, a ring of opaque tissue, arcus senilis, encroaches on the cornea. In the interior of the globe, just behind the cornea, is a projecting shelf, formed of a ring of tissue supported by buttresses, ciliary processes. It is continued inwards as the iris, a muscular curtain. The “hyaloid membrane” lines the back portion of the globe. Continued on the inner side of the ciliary processes, it splits into several layers, which pass, one in front of the lens, others to its edge, to which they are attached, and still another, very thin, behind it. Since it holds the lens in place, the anterior portion of the hyaloid membrane is known as its “suspensory ligament.” Thus the eyeball is divided into three chambers. The anterior is filled with watery lymph, aqueous humour. In it, resting on the anterior surface of the suspensory ligament of the lens, is the iris. The middle chamber contains the lens. The posterior chamber is filled with a liquid jelly, vitreous humour.

By the contraction of the circular fibres of the iris, the aperture of the pupil is diminished, limiting the light which enters the globe. This adjustment occurs when the illumination is bright. It is also brought into action for the purpose of cutting out divergent rays, which would not be clearly focussed when objects near at hand are looked at. The posterior surface of the iris and the inner surfaces of the ciliary processes are covered with dense black pigment. It is this pigment, showing through the uncoloured connective tissue and plain muscle-fibres of which the iris is composed, that gives their colour to grey and blue eyes. In many eyes the iris contains a brown pigment in its substance.

Fig. 27.— Horizontal Section through the Right Eye.

The slight depression in the retina in the axis of the globe is the fovea centralis, or yellow spot; the optic nerve pierces the ball to its inner or nasal side. The lens, with its suspensory ligament, separates the aqueous from the vitreous humour. On the front of the lens rests the iris, covered on its posterior surface with black pigment. On either side of the lens is seen a ciliary process, with the circular fibres of the ciliary muscle cut transversely, and its radiating fibres disposed as a fan.

The back portion of the globe of the eye is covered with a curtain, the retina, formed by the spreading out of the fibres of the optic nerve in front of various layers of nerve-cells and the sensory cells of the organ of vision, rods and cones. The retina lies between the hyaloid membrane, which encloses the vitreous humour, and a layer of pigment which “backs” it, as a photographer backs a plate when he proposes to use it towards a source of light—to take a photograph of a window from within a room. The serrated margin of the retina is somewhat anterior to the equator of the eyeball. The pigment which backs the retina is contained in a sheet of cells which belongs to the pouch of brain that extended outwards towards the eye-pit ([p. 334]). Properly speaking, therefore, it is a layer of the retina.

Fig. 28.—Diagrams showing the Mode of Formation of the Crystalline Lens.

A, A pit in the epithelium on the surface of the head has closed into a hollow sphere. B, The cells of the posterior wall of this sphere are growing forward, as the fibres of the lens which traverse its whole thickness, with the exception of the cubical epithelium on its front.

Three sets of tissues take part in the development of the eyeball. (1) The epithelium covering the surface of the head is depressed as a pit, which gradually closes into a hollow sphere. This sphere, when its cavity is filled up, owing to the great elongation of the cells of its posterior half, becomes the lens. It breaks away from the rest of the epithelium of the surface, which clears to transparency as that part of the conjunctiva termed the “corneal epithelium.” (2) The retina, as already stated, is a hollow outgrowth from the interbrain. As this pouch approaches the lens, its anterior half is pushed back into the posterior half, forming a cup with a double wall. The anterior, or inner, sheet of the bowl of the cup develops into the nervous layers of the retina, the posterior sheet into its pigmented epithelium. (3) Connective tissues are transformed into the other constituents of the globe—cornea, iris, vitreous humour, etc. The globe is complete, except at a spot on the nasal side of its posterior pole where the optic nerve pierces it.

The bloodvessels of the retina, entering with the optic nerve, ramify on its anterior surface. Under ordinary circumstances we ignore the shadows which they cast, as we ignore the blind spot which coincides with the disc of insensitive tissue presented by the end of the optic nerve, and many other imperfections; but it was shown by Purkinje many years ago that by a very simple manœuvre they may be forced upon our notice.

Fig. 29.—Purkinje’s Shadows.

A beam of light traversing the eyeball in the direction A throws a shadow of the vessel v, lying on the front of the retina, upon the sensitive layer at its back. When the light is moved from A to B the shadow moves from a to b. The mind, supposing the shadow to be a dark mark on the nearest wall or screen, infers that this mark moves from A′ to B′.

By making use of Purkinje’s figures, it can be proved that the level in the retina at which undulations of light give rise to the impulses which evoke visual sensations coincides with the back of its anterior sheet—i.e., with the layer of rods and cones. A person stares fixedly at a white sheet in a dimly lighted room while an assistant, by the help of a lens, focuses a strong light on the front of his eyeball, to the outer side of the cornea. The rays, traversing the white of the eye, throw shadows of the retinal vessels on the layers behind them; but this not being the way in which light normally enters the eyeball, the person experimented upon supposes that he sees the shadows in front of him. He mentally projects them on to the white sheet. The pattern of his retinal vessels appears on the sheet in grey streaks. When the spot of light is moved, the shadow-pattern shifts, and in the same direction; since, as the retinal image is reversed, a movement from right to left is interpreted by consciousness as a movement from left to right. Given the angle through which the light is moved, and the apparent displacement of the shadows, it is a simple matter to calculate the distance behind the bloodvessels of the sensitive layer of the eye. So definite are Purkinje’s figures that the shadows of individual blood-corpuscles can be followed, and the rate at which they are moving in the capillaries of the retina calculated.

The retina is the organ of vision. Cornea, iris, lens, vitreous humour, are parts of the camera in which this sensitive screen is exposed; and of the retina, the sensitive layer is the layer of rods and cones. Interest therefore centres in these structures. They are disposed with the utmost regularity on the posterior surface of a thin, reticulated membrane—the outer limiting membrane. But rods and cones are only the outer halves of sensory cells, the inner portions of which, reduced to a minimum in thickness, except where they contain their nuclei, lie in the outer nuclear layer. Rods are the larger elements. Each consists of an outer segment, or limb, of relatively firm substance transversely striated, and liable to break into discs; and an inner limb of much softer substance, again divisible into two parts, the outer longitudinally striated, the inner granular. Cones are almost identical in structure with rods, save that their outer limbs are much smaller, their inner limbs rather fuller. In frogs and various other animals, but not in Man, each cone contains at the junction of its two limbs a highly refracting globule of oil, often brightly coloured, red, yellow, or green.

Fig. 30.—The Retina in Vertical Section—
A, after Exposure to Bright Light;
B, After Resting in the Dark.

The arrow shows the direction in which light traverses the retina. C, Retinal epithelium, with its pigmented fringe. 1, Layer of rods and cones, separated by the external limiting membrane from 2, the layer of the nuclei of the rods and cones. 3, The ganglion-cells of the retina, which are homologous with the cells of the afferent root of a spinal nerve. Their peripheral axons ramify beneath the sensory epithelium (rods and cones and their nucleus-bearing segments), their central axons in 4, the inner molecular layer. D, Collecting cells on the front of the retina; a a a, their axons which conduct impulses to the brain; b, an efferent fibre from the brain.

The layers in front of the rods and cones contain nervous elements accessory to them. In the “inner nuclear layer” are the ganglion-cells of the retina, homologous with the cells of the ganglia on the posterior roots of spinal nerves; but, in the retina, bipolar and extremely minute. On either side of the rather thick layer occupied by the nuclei of these ganglion-cells (and of cells of other types which, for the sake of clearness, we omit) is a felt-work of nerve-filaments in which their two extremities arborize. The most internal, or anterior, layer consists of a single sheet of rather large collecting cells and of their axons, which stream towards the optic nerve. Each cone has its proper ganglion-cell, collecting cell, and efferent fibre. Rods are served in groups by ganglion-cells and collecting cells. From this it may be inferred that a cone is a sensory unit, an inference confirmed, as we shall show presently, by direct evidence. The connections of the rods show that they also are sensory elements, although it may be doubted whether they are sensory units. The optic nerve contains a very large number of fibres—about a million—all small, but some distinctly larger than the rest. The largest very probably belong to the collecting cells of rods. But the retina certainly does not contain a million collecting cells. A considerable residue of fibres is therefore unaccounted for. It is supposed that they are afferent to the retina, but we have no knowledge regarding the nature of the impulses which descend from the brain.

The retinal pigment is not merely a backing for the sensitive screen. It undoubtedly plays an important part in vision. That it is not essential is evident from the fact that albinos, whose eyes appear pink owing to the absence of pigment, and the consequent showing through of the blood in the exceedingly vascular membrane which lies behind the retina, can see; although their visual sense cannot be described as normal. They are exceptionally sensitive to an excess of light. We shall return to this subject after describing the differences in manner of functioning which distinguish rods from cones, differences so marked as to justify us in speaking of two kinds of vision.

During twilight warm tones gradually fade out of the landscape; cold blues and greys predominate. A time arrives when scarlet poppies look black, although yellow and blue flowers and green leaves can still be dimly distinguished. In full daylight colours are seen at their brightest in the high lights; where the light is dim they tend to appear in different shades of grey. At night, if the sky is star-lit, all colours give place to a slightly bluish grey in the high lights, black in the shade. But a not very uncommon abnormality is night-blindness—inability to see at all when the light is not bright enough for the recognition of colours. In persons so affected the rods do not function; for it is with the rods that we see in weak light. They record differences in intensity between the lower limit of their sensitiveness and the higher degree of brightness, at which they are superseded by cones; but they afford no information regarding colour. Their monochrome is interpreted by the mind as a bluish grey, apparently because, since they are insensitive to red rays, the sensations of which they are the source are associated with the blue end of the spectrum. When the cones are stimulated very slightly, the reinforcing grey of the rods enables us to distinguish all other colours, save red, which appears black. In bright light the rods are in a permanent state of exhaustion; they do not contribute to vision. Rods respond to stimulation more slowly than cones. This fact enables us, by a very pretty experiment, to distinguish the two kinds of vision. A disc of green paper about the size of a threepenny-bit is pasted on a red surface. Held at arm’s length in a room lighted by a single candle, the disc looks dull green when the gaze is directed at it; but if the gaze be directed 2 or 3 inches to one side of it, it appears brighter than before, but less distinct and almost grey. The explanation of this is to be found in the fact that at the posterior pole of the eye there is a shallow cup—fovea centralis—which carries cones only, without rods. This small depression is the area of direct vision, the only spot at which we see things quite distinctly. At the fovea the nuclei and nerve-cells of the retina are withdrawn from in front of the cones to the margin of the cup, in order that they may not interfere with the passage of light. The pit and the ring round it contain some yellow pigment. Hence it is usually termed the “yellow spot.” When we are looking straight at the green disc, it is focussed on the yellow spot. It then excites a sensation of greenness; but since this is not reinforced by any rod-sensations, the green is dull. When it is focussed outside the yellow spot, it stimulates rods and the sparse cones which lie amongst them; and the rods being more sensitive than cones to light of low intensity, the disc looks brighter. If, while the observer is still gazing fixedly at a spot to the side of the disc, the red paper be waved rapidly, but gently, to right and left, a brightish grey cover seems at each movement to slip off the dark green disc, and to regain its position a moment later, with a jump. The grey rod-sensation, developing more slowly than the green cone-sensation, is, as it were, left behind. The two are separated at the moment when the paper starts to right or to left.

Astronomers have long recognized that one of the smaller stars which catches the attention when they are not looking directly at it may be invisible when the gaze is directed to the spot where it ought to be. It was visible when focussed on rods, but it is not visible when focussed on cones. In most birds the retina shows cones alone. To anyone who for the first time enters a dovecote at night the experience is very curious. A candle is for him a sufficiently strong illuminant, but it does not give light enough to enable the pigeons to see. Although evidently alarmed by the noise made by the intruder, they allow themselves to be taken down from their perches without making any attempt to escape. If, startled by the touch of a hand, they take to flight, they fly against the wall. Pigeons are night-blind. The retina of an owl bears chiefly rods, the outer limbs of which are exceptionally long.

The outer limbs of the rods are coloured reddish-purple. This colour is quickly bleached by light. If a frog which has been kept for a short time in the dark be decapitated, its head fixed for ten minutes in a situation in which a window is in front of it, then carried to a photographic dark-room, where an eye is taken out by red light, opened, and the retina removed, a print of the window will be seen upon it. Such an optogram may be fixed by dipping the retina in alum.

The retina is easily detached from its pigment-layer. If it has been bleached by exposure to light, it regains its “visual purple” when again placed in contact with its pigment. Evidently the visual purple is renewed from the pigment which lies behind (and around) the rods.

From the cells of the pigment-layer a fringe of streaming processes depends amongst the outer limbs of the rods and cones ([Fig. 30]). In a dull light the processes hang but a short way down; in a bright light they react almost to the outer limiting membrane. They supply pigment to the rods, but their relation to cones is not understood. It is clear, however, that the cones, although they are not coloured, are dependent upon the pigment-fringe, since they always remain in contact with it. Their inner limbs elongate in the dark, lifting them to the pigment, and shorten in bright light. These movements may merely indicate that the cones require a backing of pigment, but it would seem more probable that, like the rods, they absorb a substance which is sensitive to light, although we cannot recognize it by its colour.

The responsiveness of the rods to light is due to visual purple. As every lady is aware, colours, especially mauves and lilacs, are bleached by light. The chemical change affected by light in the colour of the outer limbs of the rods is the stimulant which originates impulses in the nerve-fibres connected with them, and it is generally believed that cones—the more highly specialized sensory cells—are stimulated in the same way. Visual purple is particularly abundant in all animals that range at night, with the exception of the bat. But its absence in the bat does not militate against the theory that it is the cause of night-vision, for it has been shown that a blind bat flies with almost as much freedom, and avoids obstacles—even threads stretched across the room—with as much skill as one that can see. It is guided by the bristles of its cheek. So, too, is the cat, which has the reputation of being able to see in the dark. Undoubtedly a cat’s eye is an exceptionally efficient organ in dim light, just as it is exceptionally sensitive to sunshine—it is provided with an iris which contracts the pupil almost to a pinhole—but the cat trusts to the bristles of its cheek for information regarding the things which block its path.

Most of the peculiarities which distinguish the reactions of the eye from those of other sense-organs can be explained by its mode of stimulation—the initiation of a nerve-current by a chemical change. No stimulus, if sufficiently strong, can be too brief. The retina reacts to an electric spark in the same way as a photographic plate; but, unlike the plate, the retina is restored to its previous condition of sensitiveness in about one-tenth of a second. A visual sensation lasts about one-tenth of a second. This prolongation of the sensation is, however, a mental, not a retinal, effect. The mind continues to see an object which has been illuminated by a flash until the retina is again in a condition to send brainwards a second impulse. Were our sensations coincident in duration with the stimulation of our sense-organs, we should live in a flickering cinematograph. When one is watching a moving point of light—the glowing end of a match, for example—the prolongation of sensation has its disadvantages; the moving point is interpreted as a streak of light. If the illumination be very brilliant, the object seen may give rise to a prolonged after-image. A glance at the sun leaves in the mind for seconds, or even for minutes, the image of a glowing disc. Sensations due to stimulation of the yellow spot last longer than those which originate in the peripheral retina. If, in a train, one is being carried at a certain pace, past a fence composed of upright palings, one sees the separate slats until the eyes are directed towards them, when they fuse into a continuous screen.

The phenomena of negative or complementary images are of retinal origin. The bright image of the sun, if the stimulus has not been too violent, gives place to a black disc. If one closes the eyes after staring at a window, a black surface crossed by bright lines is seen in place of a white surface with dark frames to the panes. If, after staring at a red surface, one looks at the ceiling, a green patch is seen; after yellow, blue. Every colour has its complement, which may be determined in this way. There is much uncertainty as to the exact terms in which this phenomenon is to be accounted for, but little doubt as to its being due to the peculiar mode of reaction of the retina to light. Chemical substances which have been used up have to be restored, and during the period in which they are coming back to what may be termed a neutral condition the retina delivers to the brain impulses of the opposite sign.

Contrasts which are experienced simultaneously are more difficult to understand than those which appear successively. In [Fig. 31] the half of the grey cross which is surrounded by black appears brighter than the half which lies on white paper. A grey cross on a red background looks green; on a green background, red; on yellow, blue; on blue, yellow. If green is on red, it looks greener than if it is on white or black. These simultaneous contrasts are seen best when the strength of the colours is reduced by covering them with tissue-paper. It is as if activity of any one part of the retina is accompanied by activity of the opposite sign in the remainder. But it is unsafe, in explaining our various sensations, to lay too much stress on the mode of stimulation. The mind judges sensations in the light of previous experience. In anatomical language, the effect of sensations upon the personality depends upon the paths which impulses follow in the brain, and the associations which have been established by previous impulses which have followed the same paths. The retina enables us to distinguish tone and colour. By the variations in tone, the juxtapositions of light and shade, we recognize form. All streams of impulses which do not present tone-variations—do not, that is to say, reproduce the details of a scene—are interpreted in terms of colour. Every child discovers that the tedium of the intervals during which it is proper that his eyes should be closed may be relieved by pressing his knuckles against the lids. Although the world is shut out, a phosphene offers itself for his consideration—a yellow or white disc of irregular form with a red margin, changing into lilac bordered with green, and then into yellowish-green with a blue edge. Such, if my recollection can be trusted, were the pictures which I used to see as a boy; but no adjustment of pressure calls them forth with anything like the same vividness now.

Fig. 31.— Simultaneous Contrast.

The shading of the two V’s is exactly similar; but the figure in half-tone on black appears brighter than the figure in half-tone on a white ground.

All the senses show a tendency to rebound after activity, exhibiting contrast-phenomena; but the contrasts of vision are more marked and varied than those of the other senses, as everyone who is curious in the observation of his own sensations is aware. Negative after-images are generally referred to the retina; but various other kinds of after-image and contrast-phenomena must be attributed to the judgments passed by the mind upon the sensations which it receives; and not to physical changes in sense-organs. Positive after-images are well-marked appearances, although less common, perhaps, than the phenomena of reversal of sensation of which we have just written. On waking in the morning, one looks at the window; shifting the gaze to the ceiling, an after-image of the window appears, just as one saw it, with bright panes and dark frame. The “dark adapted eye,” being exceptionally sensitive, yields the same persistent positive after-image as the eye in its usual condition yields, after being directed towards the sun at mid-day. Movement-after-images can be explained only by referring them to misdirection of judgment. If the gaze is fixed on a rock close beside a waterfall, then shifted to a bank covered with grass or bushes, the part of the bank which occupies the lateral part of the field of vision appears to rush upwards, reversing the movement of the water. When the gaze has been fixed upon falling water—a narrow stream sparkling in sunlight—a central strip of the field moves upwards, the margins remaining stationary. If one stares at the spot on the surface of a basin of water on which drops are falling from a tap, and then looks at the floor, it is seen to contract towards the spot looked at, reversing the movement of the ripples in the basin. These observations reveal a fact of great importance in the physiology of vision. It is, probably, impossible truly to fix the gaze. The muscles of the eyeball keep the retinal field in constant movement—larger movements with minute oscillations superposed. When, as in watching a waterfall, movement has for a time taken a definite direction, its cessation is judged to mean reversal.

The anatomical unit of sensation is a cone. The fovea centralis, the only part of the retina capable of receiving sensations sufficiently discrete for reading, contains cones alone. If the gaze be directed but a very few millimetres on to the white margin of the page, letters lose their form. In the fovea the centre of one cone is 3·6 µ distant from the centre of the next. Two stars are visible as separate stars if they subtend an angle of at least 60 seconds with the eye. Their images on the retina are then 4 µ apart. Parallel white lines ruled on black paper, held at such a distance as causes them to subtend angles of 60 seconds with the eye, appear not straight but wavy, showing that their images are taken up, not by a continuous substance, but by the mosaic of cones. So far the explanation of the visual unit is strictly anatomical; but it must be added that trained observers can recognize the separateness of objects which subtend angles of much less than 60 seconds—not more than 5 or 6 seconds. This can be accounted for only on the hypothesis that images far closer together than the width of a cone produce a specific effect in passing across the anatomical unit.

In 1807 Thomas Young, the physicist, formulated a theory to account for colour-vision. He supposed that the retina contains three kinds of apparatus—a, b, and c—each especially responsive to a particular kind of light, all three slightly stimulated by rays of all colours. (Young imagined three kinds of nerve, but modern supporters of his theory suppose three different substances chemically changed by light.) A prism spreads out the rays which are combined in white light into a band in the order of their wave-lengths—those which have the longest wave-length (0·8 µ) and the slowest rate of vibration (381 billions to the second) at one end, those which have the shortest wave-length (0·4 µ) and the most rapid vibration (764 billions to the second) at the other: between these two extremes every intermediate grade of length and rapidity. These are a mere fraction—a small group—of the waves which the æther transmits, but they are all that we can see. The long, slow vibrations give rise to sensations which we describe as red; the short, rapid vibrations we describe as violet. Our names for the tints which intervene are singularly old-fashioned and unsatisfactory, but all persons agree that they recognize in the spectrum a certain number of definite colours. Some normal-sighted persons say twelve, others eighteen. It is largely a question of terminology.

Many considerations show that it is quite unnecessary to imagine that the retina is affected in a different kind of way by every kind of light, or by each of several groups of waves. If the red of the spectrum is mixed with yellow, we receive an impression of orange, which is identical with the impression produced by waves of the mean length of red and yellow; orange and green give yellow; yellow and blue, green. Any two complementary colours yield white. By taking three colours—say, red, green, and violet—we obtain, when they are duly mixed, not white light only, but light of any other tint, although not of spectral purity, since it is mixed with white. Young considered that all the conditions of colour-vision would be satisfied, all our various sensations provided for, if the retina contain three kinds of apparatus which light, according to its quality, affects in varying degrees; and with this theory of three kinds of apparatus—a, b, and c—the theory of three elementary or fundamental colour-sensations is indissolubly linked. The colour x produces its intensest effect when a is stimulated, with the least possible stimulation of b and c; y is the reaction of b, z of c. Recent studies of the curves of intensity give us the tints of x, y, and z as carmine-red, apple-green, and ultramarine blue.

The blending of sensations is illustrated with the well-known colour-top. But perhaps the most striking proof that three elementary colour-sensations are adequate to produce our visual world is afforded by photographs taken with the three-colour method. Three plates are exposed—(a) behind a red screen, (b) behind a greenish-yellow screen, (c) behind a blue screen. They are fixed in such a way that the portions acted upon by light are rendered insoluble, whereas the rest of the film can be dissolved away; a is then stained red, b greenish yellow, c blue. The three are superposed, and the result appears to the eye as an exact reproduction of the subject of the photograph in all its hues. It shows every shade of orange and green and violet. It is as bright—that is to say, as full of white light—as the original.

Various objections may, however, be brought against Young’s theory. Of these, the most weighty are: (1) The retina does not contain three kinds of apparatus, as Young supposed; nor can we find three kinds of photochemical substances, as required by the theory in its modern form. If we could find them, a fresh difficulty would arise; for we have no reasons for supposing that one and the same nerve-ending can receive stimuli of three different kinds. (2) The theory offers no explanation of negative after-images—the complementary colours experienced when the eye is closed after staring at a brightly coloured object. (3) It does not adequately account for the various deficiencies of colour-blindness.

It is well recognized that there are various degrees of colour-blindness, and that the colour-vision of persons considered normal presents different grades of refinement. Nevertheless, the abnormalities of colour-blind persons are so marked that cases fall into definite classes. Those whose cones do not function—which means that their yellow spots are either undeveloped or diseased—see all things grey. They are totally colour-blind. Excluding these, the colour-blind may be grouped in one or other of two divisions—(a) those who confuse red and green, (b) those who confuse yellow and blue. One person out of every thirty-five is red-green blind. The proportion is even higher if males only are considered, showing how very unfortunate is our choice of warning signals. A man who is red-green blind cannot tell the port from the starboard light. Blue-yellow blindness is, on the other hand, extremely rare. According to Young’s theory, colour-blindness is due to the absence of one of the three sets of visual apparatus. But cases do not altogether conform to this hypothesis. We knew an amateur water-colourist, since deceased, who derived intense pleasure from the beauties of Nature, and showed no mean skill in reproducing them with his brush, notwithstanding the fact that he was red-green blind. Each night his sister arranged his paint-box for him, and only rarely did he use vermilion to fill in a foreground of lush green grass. But this mistake, when he made it, did not destroy his own satisfaction in the picture. It was clear that red had a value for him, although he confused it with green. It is impossible for a normal person to see through the eye of one who is colour-blind, and there is no other means of comparing his sensations with our own. The mistakes which the colour-blind make in sorting coloured objects and in naming mixtures of light selected from various parts of the spectrum show the range of their deficiency, but give us no information regarding the qualities of the sensations which they retain.

The test of colour-sensitiveness usually employed is the grading of a large number of wools of different tint. The order in which the colours should be arranged is not a matter of opinion. They must be placed in the order in which they occur in the spectrum—i.e., arranged according to their wave-lengths. In the cases of colour-blindness which are most frequently met with the defect may be described as due to an absence of the sense of redness, or as an absence of the sense of greenness. The two conditions can be distinguished. But since the eye is not dark for red (although in certain cases vision is very weak for the red end of the spectrum) or dark for green, the abnormality cannot be adequately accounted for on structural grounds. It is not explicable on the hypothesis that one of three sets of responsive sense-organs (or nerve-fibres) or photochemical substances is absent from the eye. Again, it is generally agreed that the sensations of white, yellow, and blue of the red-green colour-blind are similar to those of normal persons. This is not in harmony with the theory of the omission from their eyes of one of three pieces of colour-apparatus.

Professor Hering, of Leipsic, adopting the generally accepted view that light effects chemical changes in substances contained in the retina, to which changes stimulation of nerve-endings is due, formulated a theory of colour-vision which many physiologists prefer to Young’s. He imagines that the retina contains three kinds of pigment, each of which is, as he believes all living substance to be, in a constant state of change. It is at the same time being built up and destroyed. Using the terms which connote the opposite directions of metabolism, the pigment is simultaneously undergoing anabolism and katabolism; the two processes, when the retina is at rest, maintaining equilibrium. When light acts upon either of the substances, it hastens, according to its quality, either the one process or the other; and the chemical change, whether it be constructive or destructive, stimulates the endings of optic nerves. Hering assumes, therefore, that there are six elementary qualities of visual sensation—red, green, yellow, blue, white, black. Red, yellow, white are due to anabolism of the visual substances; green, blue, black are due to their katabolism. The installation of yellow amongst the unanalysable colours is a relief to many minds. It is almost impossible to think of yellow as a compounded colour. White also, we feel, is not a compounded colour, despite our knowledge that a prism scatters from it all the hues of the rainbow. Black, many persons assert, gives them a definite sensation, and not merely a sense of rest. (Parenthetically, it may be observed that the feeling that a colour is pure or mixed is not to be trusted. It may be based upon the chromatic aberration of the eye, or it may be reminiscent of the paint-box. We know that we cannot make yellow by mixing red and green pigments, hence we feel that it is pure. Of green we are not by any means sure; gamboge and Prussian blue come into our minds.) Except when the light which falls upon the retina is giving rise to one of the four pure colour-sensations, all three substances are simultaneously affected, although one may be undergoing katabolism while the other two are being built up, or vice versa. Hering accounts for simultaneous contrast by assuming that the activity of any one part of the retina induces an opposite kind of change in the remainder, and especially in the vicinity of the primarily active part. When a certain patch is developing a sensation of red, the rest of the retina develops a sensation of green.

The great merit of the theory is, however, to be found in its offering an explanation of complementary after-images. The green patch seen with closed eyes after one has stared at a red object is due to the rebound of metabolism. In returning to a condition of chemical equilibrium the retinal substance acts as a stimulant which evokes the antagonistic colour. But it is a theory which makes very large assumptions. It assumes, for example, the possibility of the existence of a substance which is built up by light from one end of the spectrum, and decomposed by light from its centre. Not that Hering regards the existence of three retinal substances as essential to his theory. He is prepared to transfer to the brain the seat of the substances, or the substance, which, by their, or its, anabolism and katabolism, produces antagonistic colour-perceptions; but in this he is abandoning physiology for metaphysics. We have no warrant for imagining that there exists in the brain any substance which, by undergoing physical changes of various kinds, produces various psychical effects. The problem to be solved is physiological. Rays of light of different wave-lengths excite the retina to discharge impulses which are variously distributed in the brain. The effects which they produce in consciousness depend upon their distribution. The impulses to which the longest rays give rise evoke sensations of red, those due to the shortest, sensations of violet. And what is true of the retina as a whole is true, apparently, of each individual cone. In what way does light act upon a cone? It is one of the most fascinating problems in physiology. Round it our thoughts revolve whenever we are trying to form conceptions of the nature of stimulation, sensation, and perception. Each of the two theories which we have expounded above helps to group together certain of the more striking phenomena of colour-vision, but neither gives a satisfying explanation of their causation.

The sensitiveness of the retina is in a remarkable degree adjusted to the intensity of the light. When a dark room is entered, the pupil dilates; but one’s power of distinguishing objects continues to increase after the pupil has reached its maximum size. At the end of ten minutes the eye may be twenty-five times as sensitive as it was when the room was entered. This adaptation to darkness is due in large degree to the substitution of rods for cones as the organs on which vision chiefly depends. But it cannot be wholly due to this, since it occurs when one is working with a red light. Probably the red used in a “dark-room” is not sufficiently near the end of the spectrum to be completely without influence upon visual purple, but it is a colour to which rods are comparatively insensitive. Other evidence also points to an adaptation of cones as well as of rods.

Fig. 32.—The Formation of an Image by the Refracting Media of the Eye.

x, The common centre of curvature (nodal point of the several media). Rays which pass through this point are not deflected. y, The principal focus of the system. All rays which are parallel to the optic axis converge to this point. The image of the point A is formed at a, the spot at which a ray parallel with the optic axis meets an unbent ray—the image of B at b.

Accommodation of the eye for distance is brought about by a mechanism which allows the lens to change in shape. It becomes more convex when a near object is looked at than it was when adjusted for an unlimited distance, which is its condition when the eye is at rest. Adjustment for near objects involves muscular action, and is accompanied by a sense of effort, however slight. Whilst the eye is at rest the lens is mechanically compressed against the anterior layer of its suspensory ligament. Accommodation for near vision is effected by the ciliary muscle, which is placed in the shelf of tissue which projects into the interior of the eyeball. This muscle is made up of a ring of circular fibres, and to the outer side of this, of fibres which radiate backwards and outwards. The longitudinal, or radiating, fibres obtain their purchase by attachment to the firm wall of the globe just beyond the cornea. They spread into the front of the loose chorioid membrane which lines the eye behind the retina. By the joint action of these two sets of plain muscle-fibres the suspensory ligament is slackened, and the extremely elastic lens, previously compressed, bulges forwards. The radius of curvature of its anterior surface changes from 10·3 millimetres for distance to 6 millimetres for vision at the “near point.” It was stated, in connection with the development of the lens ([p. 374]), that the cells of the posterior half of the hollow sphere out of which it is formed grow forwards into extremely long fibres, which traverse its whole thickness. These fibres are bent like the segments of a carriage-spring. Their anterior ends rest against the flattened ligament of the lens; the vitreous humour, which is always under tension, compresses their posterior ends. When removed from the eye, the lens becomes rounder than it is in situ, even when accommodated for near objects. But in later life it grows stiff. It ceases to bulge forwards when its ligament is slackened. Hence it becomes necessary to aid the presbyopic eye with convex glasses when it is used for near objects, although for distant vision it remains as effective as ever. If the ciliary muscle is constantly and completely relieved of the labour of accommodation, it grows lazy, or rather wastes from want of use. A person who relies on spectacles loses his power of accommodation; but ophthalmologists agree that self-focussing, if it give rise to a sensation of strain, is bad for the eyes. In myopic persons the eyeball is too deep; objects are focussed in front of the retina. In hypermetropia (“long sight”) the eyeball is too shallow; objects are focussed behind the retina. Concave glasses correct the one condition, convex glasses correct the other. Glasses are also very commonly called for to neutralize another defect—regular astigmatism—which may be present by itself, or may accompany insufficient length or too great length of the optic axis. It is due to unequal curvature of the cornea. Usually the curvature is sharper in the vertical than in the horizontal meridian ([cf. p. 269]); as a consequence, points in a vertical line are focussed in front of points in a horizontal line. Cylindrical glasses, not lenses, are required to correct this defect. And here it may be well to call attention to the fact that rays of light are more sharply refracted by the surface of the cornea than they are by the crystalline lens. The lens has a high index of refraction (1·45), but it does not lie in air (the index of refraction of which is 1), but between two humours which have about the same index as water—namely, 1·336. The bending by the combined action of the cornea and the lens of rays of light which come from a source so distant that they may be considered as parallel brings them to a focus on the retina, when the lens is at its flattest. When the lens is at its roundest, rays which diverge from a point only 5 inches in front of the eye are focussed on the retina. The lens is therefore essential for accommodation, but, after its removal for cataract, vision, even for near objects, is rendered possible by the use of convex glasses.

Fig. 33.

A, The normal eyeball, in which, when the ciliary muscle is relaxed, parallel rays are brought to a focus on the retina. B, A hypermetropic eyeball. Its depth being less than normal, parallel rays are not brought to a focus on the retina when the eye is adjusted for distant vision without the aid of a convex glass. C, A myopic eyeball. Its depth being more than normal, a concave lens is needed to diminish the convergence of parallel rays.

A star or a distant gas-lamp is seen as a point of light with rays. Usually this figure, which has given origin to the expression “star-shaped,” shows three greater rays alternating with three lesser rays. Such an image is not produced by a point of light near to the eye, since it is due to the puckering of the lens when flattened against its ligament. It brings into evidence the three axes on the front of the lens and the three axes which alternate with them on the back, with regard to which the lens-fibres are disposed.

As an adaptation of living tissues to optical purposes the eye is above admiration, yet it presents many defects, which an optician corrects in the instruments which he manufactures. A remarkable fact in the physiology of vision is our unconsciousness of the imperfections of its organ. An unusual experiment is needed to bring them to our notice. If we look through a common glass lens uncorrected for unequal refraction of rays of different wave-lengths, we recognize that a bright object is shown with a colour-fringe, yet we take no cognizance of the colour-fringes which surround the images of all bright objects focussed upon our retinæ. If we think about the matter, we recognize a feeling that blue in a window of stained glass appears farther away than red; but this might well be due to association. Blue glass is chiefly used for the sky. If we look at a bright object through purple glass, we her red with a blue fringe or blue with a red fringe, according as the eye is focussed for red or for blue. The purple glass having absorbed all intermediate rays, we become aware that we cannot focus the two extreme ends of the spectrum at the same place. Since a greater effort of accommodation is needed to focus red, we judge that the bright object is nearer to us when it appears red than when it appears blue.

Spherical aberration is another fault of the lens. The rays which enter its margin are brought to a focus sooner than those which pass through its centre. This is due to the fact that its surfaces are regularly curved, whereas a glass lens is corrected by grinding it flatter towards the margin. This defect is partly corrected by the cornea, which has an ellipsoidal surface, and partly by the greater density of the centre of the lens. Yet it is still necessary for the eye to be “stopped down” by the iris when a near object is looked at, although less light is entering the eye than when it is directed to the horizon—a condition which would lead a photographer to open his iris-diaphragm.

Of all the imperfections of the eye which the mind ignores, the most remarkable is the gap in the field of vision, due to the gap in the sensitive layers of the retina, which occurs where the optic nerve enters it—the blind spot. Hold this page of the book 10 inches from the face, keeping the lines of print horizontal. Close the left eye and look at X with the right eye. The black disc disappears, because its image is focussed on the blind spot. Since the picture on the retina is reversed, it is clear that the optic nerve enters the globe to its inner side, and slightly above its horizontal meridian. But, unless we employ an unusual test, we are quite unconscious of the fact that a definite hole is punched in the picture. The mind fills it in, and the way in which it does so is extremely suggestive. It lies about it—in a downright ingenuous fashion if it is confident of credence, in a more subtle way if a simple falsehood is likely to be challenged. In place of the black disc make nine conspicuous crosses:

Hold the paper in such a position that X falls upon the blind spot. It ought to disappear, but the mind assures you that there is a cross at that spot. The mind completes the field. In place of the crosses use noughts and crosses, thus:

Now let X fall on the blind spot, and allow the eye to go just a little out of focus. The four marginal crosses draw inwards:

The mind contracts the field. Still denying the gap, but not having sufficient data from which to invent an object, the fraudulent nature of which would not be found out the instant that the gaze is shifted, the mind lies regarding the position on the paper occupied by surrounding objects.

Is it quite fair to the mind to say that it lies about the blind spot? The mind judges sensations in the light of experience. An association of previous sensations teaches me that the wall of the room is not pierced by a round hole a foot in diameter opening into outer darkness. Many sensations to me the fact that the designs on a wall-paper succeed one another with unbroken regularity. Fixing my gaze on one of them, I cannot by any effort of attention efface the pattern which happens to be focussed on the blind spot. I know that I shall see it the instant that I move the eye. If I let my eye roam until the face of my wife falls on the blind spot, its image disappears. I know its lineaments far better than I know the pattern on the wall-paper, but I cannot fill it into the picture. Her hands are visible, and the work which is resting in her lap, but in a mysterious way the background draws together where the face should be. My mind refuses to pass a false judgment; but it also refuses to see that there is a gap.

This exceedingly instructive observation teaches the relativity of sensations. It shows that a sensation has no objective value until judgment has been passed upon it by the mind. The meaning of this we express in figurative language, none other being available. We speak of a new sensation as being compared with sensations previously received—taken into the picture-gallery of the mind, and placed in its due position amongst the infinitely numerous records which are stored there. If we try to make a nearer approach to correlating physical with psychical activity, we say that sensation has no value save that which it acquires from its temporal relation in the sequence of sensations to which attention is directed, and that this value depends upon the relation which similar sensations have possessed in former sequences. There is no gap in binocular vision. An object focussed on the inner (nasal) side of the right eye, where the blind spot is situate, is focussed on the outer (temporal) side of the left eye. The left eye sees the object to which the right eye is blind. Since we have almost invariably used two eyes in the past, experience teaches that there is no gap in the field of vision. Hence the new group of sensations which alleges that there is a gap must be corrected. The field must be filled up in the way which experience shows to be most likely. The retina is a sheet of rods and cones, each of which has a nervous connection with the brain proper to itself. The retinal field is associated with the brain-field. But this does not imply that we may think of the mind as having a spatial distribution on A or button B in the retina causes bell A´ or bell B´ to ring in the brain, but it does not follow that perception A´´ or perception B´´ will be heard in the mind. It will be heard if this is the association established by custom, since mind is the product of experience. But the new sensation is creating precedent as well as being judged by it.

Point A in the right retina is associated by experience with point a in the left, and point B with b. These are termed corresponding points, because they are similarly stimulated in binocular vision. The mind, therefore, judges that it receives the same information from each pair of corresponding points. The position of corresponding points will be understood if the right retina is imagined as put inside the left, precautions being taken to make the yellow spots coincide, and to avoid twisting the retinal cups in taking them out of the eyeballs. Great care is taken to maintain the points in correspondence during the various movements of the two eyeballs. In addition to the four recti muscles which move the eyeball upwards, downwards, to right and left, two oblique muscles give it the requisite amount of rotation. We have learned to give the same value to the impulses from two corresponding points. But under changed conditions the correspondence changes. When a squint develops in childhood, it follows one of two courses; either the obliquity of one of the eyeballs increases until it looks towards the nose, and its images cease to interfere with the images in the dominant eye—they are ignored by the mind—or a fresh correspondence is established between points in the oblique eye and points in the eye which looks straight forward. If we are severely critical, we find, from a study of the form of the eyeball, that it is impossible that the same rods and cones should occupy corresponding points in different positions of focus and with different degrees of convergence of the eyeballs. To permit of this the retinal cups would need to change in shape. But again mechanical correspondence is of little consequence. In the light of experience the mind judges that points correspond. When we are gazing at a flat surface, the mind judges that corresponding points are giving it similar information. It does not see a flower on a wall-paper twice as bright or twice as red with two eyes as with one. If the eyes are normal, the impression received through the two is precisely the same as the impression received through either singly. But when we are looking at solid objects, the image on one retina is not the same as the image on the other. One eye sees farther round the object on the one side, the other on the other; and it is just this disparity in the pictures, aided by the feeling that the eyes are converging, that gives the impression of solidity. Correspondence of points, on the other hand, is not necessarily sufficient by itself to convince the mind that the pictures presented by the two eyes are identical. When a flat triangle such as this is regarded with the two eyes, its black lines fall on corresponding points; but the figure is associated in the mind with other sensations—sensations of movement and touch. Notwithstanding the identity of the retinal images, the mind tries to see them as disparate. The figure troubles the eyes. At one moment the meeting-point of the three central lines projects forwards, at the next it recedes. That similarity of retinal images counts for something is shown by closing one eye. The uncertainty of shape of the figure is rendered more troublesome. It changes still more rapidly from convex to concave. When the point seems to be in front of the page, the accommodation of the eyes is adjusted for nearness; when behind the page, for greater distance. But the illusion that the object occupies three dimensions is not dependent upon the sense of contraction of the ciliary muscle. When the paper is moved towards the eye, its centre recedes; it is left behind until the ciliary muscle has had time to contract. When it is moved away from the eye, it projects until the ciliary muscle has had time to relax. Accommodation follows judgment, not judgment accommodation. The mind is extremely suspicious of the veracity of its newsagents. Disparateness of images, convergence of the eyeballs, shifting of accommodation for the various levels of an object in space, should be indisputable evidence of solidity or of hollowness. Conversely, the absence of either factor should be conclusive proof of flatness. But the mind does not trust to isolated sensations; it looks for associations of sensations. When the finger hints, “I could touch that sharp point,” it is useless for the eye to aver that there is no point to be touched.

If two exactly similar photographs are placed in a stereoscope, the fact that the eyes are not converged gives to the common picture an appearance of depth, notwithstanding the fact that corresponding points on the two retinæ are stimulated. If the two photographs have been taken, as they should be taken for this purpose, with a double camera, the disparity of the retinal images immensely enhances the impression of solidity.

It is impossible to exaggerate the dependence of sensation on judgment. At birth a child commences the long process of education which enables it to associate the sensations derived from its retinal images with the movements which place it in contact with things. It discovers that, when it is necessary to make the eyes converge, the object is near at hand. It also associates the voluntary action of contracting its ciliary muscle with nearness. Unconverged and unaccommodated eyes come to mean distance. So, too, do indistinctness due to absorption by the atmosphere, blueness due to the same cause, a small image on the retina. But there are obvious limits to its power of ascertaining the distance of an object, and therefore, conversely, of its power of estimating size. We have no idea of the size of the retinal image of the sun. Very few people would be prepared to believe that the angle which the sun subtends with the eye barely exceeds half a degree. (The first finger, viewed in profile, at arm’s length, covers one degree of arc.) A disc of paper of the right size, placed at the right distance, looks far too small to represent the sun. The most brilliant of orbs bulks larger than this in our minds. Everyone who for the first time looks at the sun through well-smoked glass, or, better, through a flat-sided vessel filled with ink and water, is astonished that it looks so small. Nor are we prepared to accept the evidence of a camera that the sun at the zenith does not produce a smaller image on the retina than the sun when rising above the horizon. Yet if a photographic plate is exposed to the rising sun, and again, without changing its focus, to the sun at the zenith, the two images are practically equal. There is a slight difference due to the greater refraction of rays passing tangentially through the atmosphere, but it is so slight as to bear no relation to the difference between our two judgments of size. When the sun is rising behind trees and houses, we compare it with objects which we know to be large and distant; yet it looks almost as large when rising out of the sea. One of the causes of the illusion is our conviction that the sky is flattened; and this, again, is due partly to its paler tint—its less substantial blueness—near the horizon, and partly to our impression that it is spread out over a flat earth. When the sun is in what we deem to be the more distant part of the vault of heaven, we judge it to be farther from us, and therefore larger than when it is above us. Yet the last word has not been said in explanation of a phenomenon which has been studied by mankind since the dawn of science. Helmholtz attributed the apparent greater distance, and consequent greater size, of the sun and moon when near the horizon to the indistinctness of their discs. When its image is so reflected from the zenith as to cause the moon to appear to rest upon the horizon, it does not, he said, increase in size. In answer to Helmholtz’s explanation, it may be objected that, when at midnight he brought the full moon down from the zenith, he did not bring with her the conditions of light and colour by which she is customarily surrounded when floating on the horizon. If, when watching the moon which has just risen, vast in diameter, out of the sea, one interposes between it and the eye a sheet of paper in which a small hole has been made, and looks at the moon with one eye through the hole, it instantly shrinks to the size which it appears to have at the zenith. It is not even necessary to blot out the whole of its trail of light on the sea. At the same time, it appears to retreat to a great distance. This shows how complicated are the associations upon which judgments of size and distance are based, and to how small an extent they are determined by the size of the image on the retina. This observation is most surprising if made one or two nights after full moon, when twilight is already dim at moon-rise.

Our estimate of the distance away from us of an object on the horizon is based upon the time and effort which experience tells us we should need to spend in reaching it. The untried appears shorter than the tried. Anyone who compares his feeling of the number of yards he would have to climb up a pole reaching to the zenith with his feeling of the number of steps he would need to take to reach the horizon will recognize that the horizon appears to him to be the farther away.

Fig. 36.—A Symmetrical Arch, divided by a Vertical Line, A,
which passes through its Apex.

In representing a solid object an artist conveys theidea that light is falling obliquely upon it. One side of the object, therefore, is more strongly illuminated than the other. By depth and gradation of shade he indicates the extent to which the thing projects forwards, if solid, or falls back, if hollow. He makes the margin of a ball hazy, in the expectation that the spectator will look at the spot nearest to him—an artifice which he may easily press too far, since the eyes wander restlessly over a flat surface. In representing distance he is dependent upon giving to the various objects in his picture sizes equivalent to the sizes of their images on the retina, making them brighter or paler and more or less distinct. Yet he cannot hope to simulate the convincing evidence of distance which is afforded by our sense of the degree of convergence of our eyes. Hence, as Francis Bacon pointed out, a picture appears more real when one eye is closed than when both are open. Its middle distance at once falls back.

Fig. 37.—Two Horizontal Lines of Equal Length—the One with Diverging,
the Other with Converging, Terminal Lines.

Innumerable are the illustrations which may be given of errors of sensory judgment, but none are more striking than the various figures which may be drawn with converging or diverging lines. The mind under-estimates acute and over-estimates obtuse angles. It is impossible to convince oneself that in [Fig. 36] the line A bisects a symmetrical arch. Equally difficult is it to believe that in [Fig. 37] the line with diverging terminal segments and the line with converging terminal segments are of exactly equal length. In the Ruskin Museum at Sheffield there is a sketch by the master of the façade of a church which shows a vertical tower to one side of a triangular pediment, or, rather, this is what the sketch was meant to show, and does show, when measured on an architect’s table. In effect the tower appears to be leaning towards the pediment. Errors of judgment of this type have been attributed to the curvature of the lines of a rectilinear image on the retina, the mind judging the distance between two points by the length of the chord, and not the length of the arc which joins them. This is very simply illustrated by the example of the apparently greater length of a filled space than of a vacant one.

A B looks longer than B C. If A B C be represented as a curved line, the arc A B will, of course, be longer than the chord B C. But it is not safe to suppose that the mind compares the length of an arc with the length of a chord. Judgment is based upon experience, and probably the illusion is due to more subtle causes than the curvature of the retina. The mind does not look at the retina. If it did, it would find the reversal of the picture the least of the inaccuracies which it had to correct. It would find it very difficult, for example, to superpose in its stereoscope the photographs of a vertical tower taken simultaneously by the right eye and the left. The curved images on the retina of the vertical lines which define the angles of the tower, as seen with one eye, could not be made to correspond with the images focussed by the other eye. The Greeks felt this when they settled the form of a column. The canon of the swelling entasis and increasing taper above it did not destroy the appearance of uniform thickness which the shaft presented. It gave to the eye just the slight help which it needs to enable it to picture the shaft as of the same thickness from base to capital.

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CHAPTER XIV
HEARING

The ear, like the eye, records amplitude of vibration; loudness. It also records rapidity of vibration, musical pitch, which corresponds with colour. But it seems to have a more difficult task than the eye, since it has to analyse, or at any rate has to transmit information regarding the form of compound vibrations. The meanings of these distinctions may be illustrated by reference to a tracing on the cylinder of a phonograph. A needle attached to the posterior surface of the thin metal plate against which one speaks scratches the surface of a rotating cylinder of hardened wax. Examined with a lens, the record is seen to be an irregularly changing line. The depth of the marks is a measure of loudness. Their varying number in a given time indicates the changing pitch of the voice which produced them. Their form is a record of the quality of its tone. The work of the ear, so far as it consists in the estimation of the amplitude and rapidity of pulsations of sound, is easy to describe, but the acoustics of form are complicated.

Light is transmitted as vibrations of æther. They are transverse to the direction in which the light is travelling. Sound cannot travel through a vacuum, since it is dependent upon displacements of material particles. The particles move forwards and backwards in the direction in which sound is progressing. Sound is a sequence of pulsations, alternate condensations and rarefactions of the media which conduct it. Their particles are first pressed together, and then rebound to positions farther apart. A sequence of to-and-fro movements, each smoothly continuous throughout the whole duration of a pulsation, would produce a pure musical tone. Tuning-forks carefully bowed settle down after a few seconds into unbroken oscillations, which convey to the air the to-and-fro movements of pure tones. Such tones vary in nothing but loudness and pitch. If their pulsations are slow, we speak of the pitch as “low”; if they are rapid, we say that their pitch is high. But if the sound produced by tuning-forks (and low-toned stopped organ-pipes) be omitted from the list, no pure tones reach our ears. The notes of flutes, fiddles, trumpets, pianos, have each a certain “quality” characteristic of the instrument. Even in a violin the G string has not the same timbre as the D string. Owing to the elasticity of the substances which originate and of the substances which transmit sound, its pulsations are not simple to-and-fro movements, uninterrupted from beginning to end. Each pulsation is partially broken at intervals; and the quality of the sound depends upon the number and relative accentuation of these partial interruptions. Sound travels through air at the rate of 1,100 feet per second. This figure, divided by the number of vibrations per second of a tone, gives the wave-length in air of a tone of that particular pitch. For example, the middle C has a vibratory rate of 256. Its wave-length is, therefore, somewhat over 4 feet. The lowest tone of an organ has a wave-length of 37 feet; its highest of 3½ inches. These figures give no information, however, regarding the movement of the particles which pass on the sound. When air is transmitting a note—say the middle C—its separate molecules do not move through a distance of 4 feet. Each molecule moves but a short distance, varying with the loudness of the tone; but the “wave” of crowding runs straight forward from the piano-string to the ear, the molecules at the end of each stage of 4 feet taking on a backward movement, so that the crowding, so far as the molecules of that particular section are concerned, returns to its starting-point. Between the piano-string and the ear there is a crowding and forward movement at 0, 4, 8, 12 ... feet; a spreading and backward movement at 2, 6, 10, 14 ... feet. Most illustrations which are intended to aid the mind in forming a definite picture of the transmission of sound are liable to be misinterpreted, because they translate rectilinear movements into waves. They represent the movements of the string, and not the movements of the molecules of air between the string and the ear; but with the aid of the imagination one may picture the positions of the particles in this path. The pulse, we will suppose, has just reached the limit of 12 feet. Half-way from its 8-foot halting place the molecules are again crowded, although not so densely. One-third of the distance from the same point there again appears a tendency to crowd. This latter point marks an interval of one-third of this wave plus the wave which led up to it. At the end of the ninth foot there is a crowding, though less marked—this wave plus the two preceding waves, divided into fourths. Within these intervals are other points at which the molecules have closed together, the distances from a nodal point depending upon the number of waves involved, and, speaking generally, growing less marked as the number increases. Such are the very complex pulsatile movements which reach the ear.

Every musical sound produced by a piano, a violin, or other instrument, is compounded of a fundamental or prime tone, and overtones, partial tones, or harmonics. The following table shows the more important partial tones which accompany the prime tone when the middle C on a pianoforte is struck:

Note.	Number of Vibrations.		Interval.	Ratio.	Number of Overtone.
C‴	2,048				7th
			Super-Second	8/7
B″♭	1,792				6th
			Sub-minor third	7/6
G″	1,536				5th
			Minor third	6/5
E″	1,280				4th
			Major third	5/4
C″	1,024				3rd
			Fourth	4/3
G′	768				2nd
			Fifth	3/2
C′	512				1st
			Octave	2/1
C	256				Fundamental

The quality of a musical note depends upon the number and relative loudness of its overtones. When several notes are sounded simultaneously, they blend into a chord or harmony, provided the intervals which separate them are equal to the intervals which separate the simpler overtones. Each of the notes yields overtones. The tones blend into a concord. Their partials are in unison. The variations in air-pressure of the compound tone are strictly periodic. If the ratios of the frequencies of its constituent notes are simple the product is a rich, full sound, such as a common chord.

At least one other character of the pulsations of sound must be taken into consideration if we wish to picture the nature of the force to which the ear responds. Tones which reach it from several instruments simultaneously are not necessarily in unison, or even in harmony. The overtones of a single note sounded on a piano or violin—the statement does not hold good for bells, nor is it strictly true of flutes or horns—must necessarily bear a simple proportional relation to their prime tone. They divide the grand pulsation into fractions “without a remainder.” But the vibrations of two tuning-forks which are slightly out of unison interfere one with the other at regular intervals. They produce “beats.” Everyone is familiar with the curious effect which is produced upon the eye when one row of railings is seen through another, or one expanse of wire-netting behind another. Sets of lines which occupy nearly the same positions in the line of sight combine to make a large pattern, which overlies the smaller pattern of the rails or netting. The same thing happens with sounds which coincide at considerable intervals, although in the case of sounds interference is as marked as reinforcement. If whilst a tuning-fork yielding 101 vibrations per second is singing another of 100 vibrations is brought into play, the vibrations of the second fork are superposed on those of the first. At a certain moment the forward movement of molecules of air induced by the first fork is reinforced by a forward push from the second. But half a second after this coincidence of phase an opposite result is produced—50½ vibrations of No. 1 have passed, but only 50 of No. 2. No. 2 is going backwards (inwards), whilst No. 1 is moving forwards (outwards). The same molecules are impelled backwards by No. 2 and forwards by No. 1. The result is a pause. The compound sound produced by the two forks reaches the ear in throbs. If the forks were vibrating at the rates of 101 and 99, there would be two pauses and two beats in every second; if at the rate of 202 and 198, four. The number of beats per second equals the difference in frequency of vibration of the tones. A pianoforte tuner does his work best if he has a musical ear, yet he may discharge his duties with competence without one. Having struck a note, he sounds its octave, holding both keys down, and listens for the beat. If the first note gave no beat with his tuning-fork, the second is in time when it likewise gives no beat with the first. We have met a tuner who did his work in this way; but it must be admitted that his tempering of the intervals of the octave with which he commenced, and consequently of the other octaves above and below it, left something to be desired. The result might have been satisfactory had he been provided with twelve tuning-forks.

The question as to whether beats, when sufficiently rapid, blend into a tone has been much discussed, without a decision. Probably they do not. The complementary question as to the cause of dissonance is also not completely closed. Two notes harmonize, as we have seen, when the ratio of their frequencies is a simple fraction. Musicians are not quite agreed as to the level of numerical complexity at which a compound tone first produces a feeling of discomfort. A good deal depends upon its position in the scale and the instruments which are combining to produce it. A minor third (⁶/₅) is on the safe side. This is the first chord in our list of intervals in which a beat can be detected. Slow beats, however, do not distress us. It is the rapid beats of conflicting overtones which give a harsh, rough character to a compound note. The level at which a line is drawn between harmony and dissonance seems to depend to a considerable extent upon musical education, using the term in its widest sense. In primitive music—Hungarian, Scotch, Welsh—intricate minor chords predominate. The minute subdivision of the octave in Indian music is quite incomprehensible to a European ear. Musical cultivation tends to eliminate complex fractions. It is, however, to be noted that the history of Western music also shows the influence of an opposite tendency. Later generations have admitted as harmonies combinations which earlier generations could not tolerate.

Pitch, quality, harmony, and dissonance are distinguished by the human ear. These are the attributes of musical or periodic sounds. In a separate class must be included noises of all kinds, termed in acoustics “aperiodic,” because the vibrations which cause them are not rhythmic. The teeth of a policeman’s rattle may click a hundred times a second, but it does not make music. Even with a rapidity of interruption greater than this (at least 500 times per second) a succession of noises fails to blend into a smooth, continuous sound. The ear recognizes the loudness, duration, and even to a very high frequency the repetition of unmusical sounds.

The ear as a sense-organ can be followed down the zoological scale to jelly-fish. In its primitive form it is a chamber lined with epithelial cells bearing hairs, containing an otolith, or ear-stone. Otoliths are rounded calcareous masses which play an important part in the ears of all animals up to fishes. Even in man they are found in the more subdivided form of otoconia. Contact of the otoliths with the sensory hairs originates impulses in the nerves with which primitive ears are abundantly provided. Advisedly we use the word “ear” in place of “auditory organ.” In all animals this organ affords information of a double nature-movement of the external medium in which the animal lives, and movements of the animal in the medium. When the animal moves, its sensory hairs are displaced with regard to the otolith; when the water in which it is swimming pulsates, its otoliths are shaken against the sensory hairs. Displacements of the animal and agitations of the water produce similar effects. The ear in this stage is an organ of touch. It might well be questioned whether an animal fitted with a piece of sensory apparatus of this kind is endowed with a sense which we may properly, after reflecting upon our own sensations, term “hearing.” It is, however, stated that certain transparent crustaceans, in which the functioning of the ear-organs may be watched through a lens, show in these organs hairs of varying length which vibrate to tones of different frequency. This observation apart, it might be doubted whether fishes hear, if we mean by the word “hearing” the recognition and discrimination of tones of high frequency—musical tones. Their ears serve equally to inform them of the changes in position of their heads and of the tremblings of the sea. The shocks transmitted through the sea are near akin to the slower vibrations of sound, if the fishermen of the Mediterranean are justified in their practice of beating a wooden clapper which rests upon the seat of the boat as they row backwards and forwards in front of a curved net. They believe that the fish are frightened by the noise; but it matters little whether we describe the fish as hearing a noise, or as feeling the percussions of the clapper conducted through the water. To the more rapid vibrations of the clapper, the fish are probably insensitive. The cochlea, which we have every reason for regarding as the organ by which sound is analysed, is not possessed by fishes. It makes its first appearance in reptiles. Birds, it is evident, are able to distinguish musical tones. Their cochleæ are very short, and are destitute of “rods of Corti.” For a moment this appears surprising, but it must be remembered that the range of tones which any bird discriminates is very short, however nicely it may value the notes within its range. In mammals the ear is clearly divided into three parts, to which the three functions which have grown out of the specialization of the sense of touch are allocated. (1) The semicircular canals are concerned with the sense of orientation. (2) The utricle and saccule reverberate to noise—the rumbling of trains, the boom of guns, the beats of dissonant musical tones. We do not know how to classify the agitations of the atmosphere which surrounds us and of the earth on which we stand, nor can we point with any certainty to the groups of stimuli which for us have taken the place of the grinding of stones on the beach and slapping of rocks by waves. (3) The organ of Corti in the cochlea discriminates and analyses musical sounds. To these three sense-organs, which are situate in the inner ear, certain structures are accessory.

The concha, which enables a horse or a cat to collect sound and to localize its source, is in ourselves merely an ornament to the side of the head.

Fig. 38.—The External, Middle, and Internal Ear of the Left Side.

From right to left, the figure shows the concha and lobule of the ear in profile; the external meatus (abbreviated); the drum, divided vertically, its posterior half visible; the hammer-bone, with the tip of its long arm attached to the drum, an arrow indicating the point of attachment and line of action of the tensor tympani muscle; the anvil attached by a ligament to the bony wall of the middle ear; the stirrup, with its foot-plate almost filling the oval window; the labyrinth, with the three semicircular canals above, and the scala vestibuli below. The curled black line shows the situation of the scala media, or ductus cochleæ (which contains the organ of Corti). Pulsations of sound which move the membrana tympani are transmitted by the three bones to the oval window. They shake the perilymph, producing waves which travel along the scala vestibuli to the apex of the cochlea, whence they return by the scala tympani to the round window (if they do not take a shorter course through the ductus cochleæ). The Eustachian tube opens out of the lower part of the middle ear.

The external meatus is a curved tube, about an inch long. Frequently a tuft of hairs guards its entrance. The wax secreted by its wall serves to attach particles of dust, and to deter insects from entering the tube. The air at the end of it is at a uniform temperature. It is closed by the membrana tympani, or drum. This membrane receives the vibrations of sound; and, in order that it may collect them with absolute impartiality, it is in every respect the opposite in shape and structure to the top of a drum. The stretched parchment which covers a drum is flat. Its tension is uniform in all its parts. Movements have the greatest amplitude at the centre. Every precaution is taken to insure its emitting, with as little confusion as may be, the particular note to which it is tuned. The drum of the ear is shaped like the mouth of a trumpet, depressed to a point, but convex from this point outwards. Its elastic fibres, which are partly radial, partly circular, are at many different tensions. Its deepest part, to which the long arm of the hammer-bone is attached, is not its centre.

The “middle ear” is an irregular cavity communicating with the pharynx by the Eustachian tube. It is filled with air at the same pressure as the atmosphere. Except during the act of swallowing, when it is at first shut tightly and then opened, the pharyngeal end of the Eustachian tube is gently closed. When one is dropped in a lift rapidly down the shaft of a mine, the difference in pressure between the external air and the air in the middle ear stretches the drum to such an extent that deafness to low tones is produced. Conversation becomes inaudible. The deafness is remedied by swallowing saliva, and thus opening the end of the Eustachian tube. The commonest cause of permanent deafness is inflammation followed by thickening of the mucous membrane of the lower end of the Eustachian tube, with its consequent closure, due to frequent sore throats. The air in the middle ear is slowly absorbed. It needs to be constantly renewed through the Eustachian tube.

On the inner wall of the middle ear are two small apertures—the oval window and the round window. Both are closed with membrane. Into the oval window is fitted the sole-plate of the stirrup-bone. Three bones—hammer, anvil, and stirrup—combine in transferring the movements of the membrana tympani to the oval window. They constitute a jointed lever, which swings about an axis passing through the ligament of the anvil ([Fig. 38]), the excursions of the long arm of the hammer being reduced in amplitude by one-third at the stirrup-plate. As the oval window has only one-twentieth of the area of the drum, the movements of the latter are transmitted with concentrated force. Two points in the mechanism of these bones may be specially noticed: (1) The head of the hammer is free to rotate in the cavity of the anvil, checked by a cog. Every inward movement of the drum is faithfully transmitted to the oval window; but when the drum moves outwards, the hammer does not necessarily carry the anvil with it. (2) A muscle—tensor tympani—is inserted near the elbow of the long arm of the hammer. When high notes are listened to its contraction tightens the drum, rendering it more responsive to rapid vibrations. It has a tonic action, but it does not make any special contraction for low notes.

Behind the two windows, within the solid bone, is the inner ear, which our ancestors very aptly termed a “labyrinth.” It is filled with fluid—perilymph—which is shaken by every movement of the stirrup-plate. Since water is incompressible, no waves could be raised in the perilymph were there no second aperture. Every vibration conveyed by the stirrup-plate after passing through the labyrinth ends as a vibration of the membrane which closes the round window.

Nowhere does perilymph come in contact with auditory cells. All the endings of the nerve of hearing are contained within a membranous labyrinth which lies within the bony cavities. The way in which the waves of the perilymph are dispersed over the surface of this closed sac can be inferred from the diagram ([Fig. 38]). They sweep round the utricle and saccule, are lost in the narrow spaces which surround the semicircular canals, run up the scala vestibuli of the cochlea. The course of the waves which traverse the cochlea is of especial interest in connection with the physiology of hearing.

The cochlea—snail-shell—is a spiral tunnel of three turns, in hard bone, about an inch in length. A shelf of bone—lamina spiralis—projects into the tunnel on its convex side. From the free margin of this spiral lamina two membranes extend to the outer wall of the tunnel—one firm, containing straight, stiff, and probably elastic fibres which radiate outwards (the basilar membrane); the other an extremely delicate film of connective tissue. The tunnel is thus divided into three compartments, known as the scala vestibuli, scala media, scala tympani. The scala media belongs to the membranous labyrinth. Waves transmitted through perilymph pass, as we have already explained, up the scala vestibuli. At the apex of the cochlea the two scalæ are in communication; but the aperture is small, and it is unlikely that waves reach the lower passage from the upper through this opening. They pass through the thin membrane which roofs the scala media, shake its endolymph, and reach the lower passage through the basilar membrane. It is noteworthy that, since the round window at the lower end of the scala tympani is, with the exception of the oval window, the only opening of the bony labyrinth, all waves transmitted through the oval window must travel part of the way or all the way up and down the cochlea.

Fig. 39.—A Section through the Axis of the Column of the Cochlea.

The spiral sheet of nerve-fibres which supplies the organ of Corti is cut in eight places. If the bundle to the lowest coil of the shell (on the left side of the diagram) is followed, it will be seen to bear ganglion-cells where it enters the bony spiral lamina. This lamina divides the tube into two canals—scala vestibuli above, scala tympani below. From the edge of the lamina the membrane of Corti stretches to the outer wall. Above the organ of Corti is the membrana tectoria, and above this a very thin membrane which cuts off the ductus cochleæ from the scala vestibuli.

The organ of Corti is spread out on the basilar membrane. It is an epithelial structure of extreme regularity and uniformity. Near to the edge by which the basilar membrane is attached to the spiral lamina rests a double row of rods of Corti, stiff pillars which lean one towards the other, over the tunnel of Corti, the convex head of the outer rod fitting into a concavity in the head of the inner one; in some places one outer rod fits against two inner rods, as the latter are rather the more numerous. On the inner side of the inner rod is seen, in transverse sections a single plump cell filled with cloudy protoplasm, and bearing on its free surface a tuft of very short hairs. On the outer side of the outer rod are three or four hair-cells, each with a cloudy outer segment containing the nucleus, a granular middle segment, and a stiffish stalk, which attaches it to the basilar membrane. Between the hair-cells are supporting cells, thicker below, tapering above, containing in their substance a firm fibre. Still farther to the outer side are epithelial cells, of no special interest. The purpose of the rods of Corti and the supporting cells is to give attachment and support to a reticulated membrane of exquisite delicacy, through the oblong apertures of which the hairs of the hair-cells project into the endolymph. The spiral lamina is traversed by a vast number of fibres of the auditory nerve, which, losing their medullary sheaths, pass across the tunnel of Corti as naked axons, to end amongst the hair-cells. Above the organ of Corti, attached by its edge to the spiral lamina, is a thick, gelatinous, fibrillated structure—membrana tectoria—which rests as a coverlet on the surface of the organ. It has been supposed that it serves to damp the vibrations of the hairs after they have been set in motion by the waves passing across the scala media; but it not impossibly plays a more active part in hearing than this.

Fig. 40.—Organ of Corti.

The spiral lamina, on the left of the drawing, gives attachment to the membrane of Corti, which stretches to the opposite wall. Below the membrane is a bloodvessel which runs its whole length beneath the tunnel of Corti. The tunnel is formed by pillars—the inner on the left, the outer on the right—which meet above it. On the left of the inner pillar is a hair-cell; to the left of this a nerve-cell with two nuclei. To the right of the outer pillar is a space; to the right of this four hair-cells alternating with four supporting cells, which hold up the reticulated membrane through apertures in which the tufts of hairs project. Three nerve-fibres are seen in the spiral lamina; they cross the tunnel to ramify between the rows of outer hair-cells. The lamina tectoria rests upon the tufts of hairs.

The organ of Corti is, beyond doubt, the apparatus which analyses sounds; but the problem of the way in which it responds to tones of different pitch, or analyses compound tones, is not as yet even approximately solved. To escape the acoustic difficulties which have to be faced by anyone who endeavours to expound the theory of the cochlea as a piece of analytical apparatus, various suggestions as to the possibility of an action en masse have been advanced. For example, the basilar membrane has been compared to a telephone-plate which takes up vibrations and transmits them through the auditory nerve to the brain. But if the organ of Corti be the transmitter, there is no ear in the brain to analyse the vibrations given out by a receiving telephone-plate; and without a receiving plate and a listening ear a telephone is purposeless. According to this hypothesis, the basilar membrane vibrates as a whole, moving the hair-cells in various “patterns”; the pressure of the hairs against the tectorial membrane causing irritation of the cells which bear them, and hence producing stimulation of various groups of nerves. Other pattern theories are somewhat similar. But it is obvious that all hypotheses of the vibration of the whole of the basilar membrane, or of large parts of it, simultaneously, leave to the mind the responsibility of reading the pattern which the impulses generated in the organ of Corti make in the brain. It is conceivable that every fraction of a semitone which a musician can discriminate, and every combination of tones which he can analyse, is transmitted to the brain by a large number of co-operating nerve-impulses; but such a theory involves a complexity of mental associations difficult to contemplate.

According to the general principles enunciated in this book, analysis of stimuli is the function of sense-organs. It cannot in all cases be compared with the analysis effected in a physical laboratory; nor is this necessary; but it must be carried so far that nerve-impulses which have no specific qualities apart from their source shall give rise to effects in consciousness which have no basis other than the topographical distribution of the said impulses in the brain. There may be sensory impulses of different orders; there may be in the brain psycho-physical substances which react to impulses of various orders in various ways; but until we have some hint of the existence of specific impulses and specific psycho-physical substances, we are not justified in postulating their existence simply in order that we may escape from physiological embarrassments.

The organ of Corti has in the highest degree the appearance of a piece of apparatus for the analysis of sound. If the basilar membrane, with the cells which rest upon it, be cut out and laid flat, the suggestion of some kind of instrument is very strong. It is a long narrow ribbon, narrowest at the bottom of the spiral, increasing to about twice the width at the apex. It is crossed by radiating fibres, presumably elastic. The cells which rest upon it carry vibrating hairs, and are supplied with nerves. The rods of Corti hold up the reticulated membrane, which keeps the hair-cells in place. It is not to be wondered at that when its structure was first discovered it was thought that the problem of the analysis of musical tones was solved. If two pianos in perfect tune are in the same room, when one is played the corresponding wires of the other twang. Anyone who sings into a piano, whilst the loud pedal raises the dampers, feels an increased fulness in his voice. This is the familiar phenomenon of resonance. Why should not the fibres of the basilar membrane resonate to the tones conveyed to the ear—the shorter ones at the base of the cochlea to high tones, the longer ones at the apex to low tones? This is the order in which we should expect the pulsations of sound which ascend the scala vestibuli to be taken up—the more rapid, near its commencement, the less rapid farther up it. But an explanation of the physics of the selection of vibrations of different frequencies by different sets of the elements which make up the organ of Corti, if such selection occurs, is still to seek. In the first place, the fibres of the basilar membrane are so exceedingly short. What could a fibre less than 0·5 millimetre in length make of the vibrations of a 36-foot organ-pipe? Even if this objection be waived, as certain eminent physicists hold that it may be, there is not a sufficient difference in length between the longest and the shortest fibres to account for the great range of tones which we are able to discriminate; nor is there any evidence that some fibres are more tightly stretched than others.

A further consideration which tempts physiologists to look upon the organ of Corti (including the basilar membrane) as a series of resonators is the somewhat remarkable agreement between the number of separate pieces of apparatus of which it appears to be composed and the number of different musical sounds which, if it were a series of resonators, it might be called upon to discriminate.

The squeak given by a bat at each turn in its flight has a pitch of about 11,000 vibrations to the second—the sixth E above the middle C (Tyndall). In a group of persons listening for the squeak there are usually some who cannot hear it. Above this the range of hearing is very variable. The suddenness of transition from perfect hearing to total want of perception makes experiments with small pipes or with a siren somewhat amusing, when a number of persons are tested at the same time. One complains that the note is intolerably loud and shrill, whilst others assert that there is perfect silence. Thirty-three thousand vibrations is usually regarded as the upper limit for the human ear, but certain physiologists place it at 40,000, or even higher. The upper limit is of little consequence, since there is very little power of discriminating rapidities above the highest note used in music—the piccolo stop of the organ, with a pitch of 4,096. It is possible that a sound with a lower frequency than 27 (the contra-bassoon) may be heard as a tone—16 according to certain writers; but again our power of discriminating very low notes is small. Over a certain range a skilled musician can tell that a note is out of tune when it is one sixty-fourth of a semitone higher or lower than it ought to be. If we assume that by allowing equal sensitiveness for a range of seven octaves, the excess of the allowance over the actual sensitiveness towards either end of this stretch would compensate for the comparatively few distinctions which the ear can make either below or above it—64 × 12 × 7 = 5,376. A much higher estimate, based upon observations which seem to show that the ear can distinguish sounds less than one sixty-fourth of a semitone apart, places the total number at 11,000.

On the assumption that one piece of apparatus is tuned to resonate for every distinguishable sound, between 5,000 and 11,000 pieces of apparatus would be required. Taking one of Corti’s arches as the centre-piece of the resonator, although the rods are certainly not vibratile structures, we find the number to be 3,848 (the number of the outer rods); if either rod with a hair-cell, or hair-cells, is the analytical element, 9,438. Counting gives 3,487 inner, 11,700 outer, hair-cells. The fibres of the basilar membrane are estimated at 24,000; the fibres of the cochlear nerve at 14,000. It will be understood that the counting of structures as minute as these yields results which cannot be more than approximately accurate. Helmholtz, assuming that each arc of Corti indicates an analytical element, accounted for the apparent deficiency in their number by assuming that a tone of which the pitch fell between two arches set both in sympathetic vibration, the arch which was nearest in pitch to the tone vibrating the more strongly. In this way he anticipated an objection which has often been brought against his theory of a long series of resonators.

In opposition to Helmholtz’s theory it is pointed out that when a violinist runs his finger up a bowed string, the pitch rises with perfect smoothness; it does not bump along from resonator to resonator. Especially in the case of very high tones given out by a siren, it is urged that at the rare intervals at which a resonator in the ear is tuned for the tone which the siren is emitting it should sound much louder than when the tone falls midway between two resonators. But the whole question of the nature of the response of the analytical elements is too obscure at present for the discussion of points so nice as this.

Many who think that Helmholtz’s theory of resonators is based upon principles of physics and of physiology which must be regarded as the starting-points of any explanation of the analysis of sounds by the ear and the mind, hold that it goes too far in searching for a separate resonator for every distinguishable tone. The cochlea, as we have already said, does not offer anything like so extensive a choice as this, if regard be had to the tension or length of its elements, and not to their numbers. Those who accept it as an axiom that the cochlea contains a series of responding instruments—but a series far more limited in range than the gamut of our sound-perceptions—seek to discover in musical tones qualities which unite them in groups. Just as in the case of colour-sensations they recognize four (or six) elementary qualities which excite four (or six) pieces of responding apparatus, so also in the case of hearing they seek for a limited number of tone-qualities and a correspondingly limited number of elementary sensations. The ideal of those who take this view is an octave of qualities and of elementary sensations sounded in the middle of the scale when x nerve-endings are stimulated, as the octave above when 2x nerves respond, the octave below with x/2. Such a conception seems to guide thought round insurmountable barriers. There is, however, a risk of making too much of the periodic intervals, because they take so important a place in music. At one side of the gap which sound bridges between the individual and his environment is an elastic body shaking at any possible rate within the range of hearing. At the other side of the gap is the ear. If, having arranged several thousands of stones along the side of the road in order of size, I were to state, picking up No. 512, “This is the fundamental of which No. 1,024 is the octave,” answer would be made to me: “It may be that the larger could be broken into halves, each as heavy as the smaller stone; but I recognize no difference between the stones in shape, colour, or hardness.” A vibrating string divides into equal segments, each of which vibrates within the vibrations of the whole string, sounding the octave. We recognize a similarity in quality between tones and their octaves because we are accustomed to hear the octave, the most prominent of overtones, in all musical sounds. Hence, from association, it has become more difficult to distinguish a note from its octave than it is to distinguish it from its fifth; but it does not follow that the effect of 1,024 vibrations upon the sensory cells more nearly resembles the effect of 512 than does that of 768. But at this point we are compelled to construct some hypothesis as to the way in which the vibrations affect the sensory cells. The protoplasm of the cells is not directly sensitive to them. We can account for the generation of impulses in the nerve connected with a particular cell, or group of cells, only on the supposition that a resonating mechanism which responds to vibrations of a certain frequency shakes the cell. Even then it seems necessary to suppose that there is an accessory mechanism which disturbs the cell-protoplasm sufficiently to render the shake effective, probably the hairs rubbing against the tectorial membrane. Anatomical study gives us no confidence in the theory of the existence of several thousands of resonators tuned to as many notes of different pitch. It remains for the physicists to say whether or not we may picture one of these minute resonators as responding to a given note in 10 separate octaves, another in 9 ... another in only 1. The physicists, on their part, may very properly ask the anatomists to point out the resonators, and even to reproduce them in models of dimensions which allow of experimental investigation.

It is generally agreed that the sensation of a chord is compounded of the sensations to which each of its constituent tones gives rise, and that our power of analysing the compound is a question of attention. A musician can direct his attention to either sensation at will. It is not equally certain that a person who has no knowledge of music can do the same. Familiarity with musical instruments gives us so exact a knowledge of the way in which compound tones are produced that it becomes a difficult matter to decide whether, when we say that we can pick out the E or the G of the common chord, it means that we can hear it as distinct from C and C′, or whether it means that, knowing the constitution of the chord, we think about the E or the G when we hear the compound tone, to the exclusion of its other constituents. Then, again, the several strings which we try to strike simultaneously do not actually “toe the line.” Their vibrations are not in the same phase, even though the strings be in absolute tune. Discrepancy of phase may favour the singling out of the several constituents of the chord. There we touch upon a problem which we passed over in silence when attempting to give an idea of the nature of the pulsations which reach the ear. We then ([p. 405]) described the partial pulsations which are superimposed upon the main pulsation as if they necessarily started simultaneously with it. We assumed that the phase difference of the partials was zero. But it is clear that differences of phase of its constituent tones may produce an almost infinite number of variations in the form of a compound “wave” of sound. Is the ear variously affected by different forms of wave? Does difference of phase result in difference of sensation? In broad terms, the answer to this question must be in the negative; although it can be shown that in certain cases a change in phase of the several constituents of a compound tone, without any alteration in their number or their loudness, makes a change in its acoustic quality. Any attempt to correlate physical changes—the movements of air in the outer ear—with the effects which they may be supposed to have upon the organ of Corti must take into account this wide range of variation of wave-form. We have called attention to the difficulties which it introduces; but have no hope of indicating the way in which they may be overcome.

Nothing connected with the physiology of the sense of hearing is more remarkable than its capacity for education. The cochlea of one human being is as extensive and as elaborate in structure as that of another, yet some men can make an infinitely more refined use of it as an analytical apparatus than can others. A native of the Torres Straits cannot distinguish as two separate notes sounds which are less than a semitone apart. Sir Michael Costa could distinguish sounds into the sixty-fourth parts of semitones. The cochlea of a cat is not less elaborate than that of a man, yet Man’s mental life is based upon the analysis of auditory sensations. His supreme advance in the animal scale has depended upon the invention of language, by means of which he communicates and receives information, thus rendering experience eternal, notwithstanding the transience of the individuals who acquire and transmit it. An animal is born, finds out, dies. A man starts with the wisdom of the race beneath his feet.

Hearing has a nebulous origin in sensations of movement or displacement. The connection between the two special senses—the sense of orientation and the sense of hearing, properly so-called—remains always intimate. David danced before the Ark of the Lord. All people, savage and civilized, associate music with movement. High in the animal scale appears the sense-organ which enables its possessor to discriminate musical tones. By its use Man has developed with great rapidity—as secular time is reckoned—an intelligence which removes him from all other animals a planet’s space. The sounding of his organ of Corti by pure tones and combinations of pure tones gives him extreme pleasure, although it in no way ministers to his intelligence. Yet there is in the enjoyment of music a quality of pleasure which makes it near akin to the satisfaction which we experience in exercising the intellect.

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CHAPTER XV
SKIN-SENSATIONS

The senses, according to a time-honoured classification, are five in number—smell, sight, taste, hearing, and common sensation, or touch; but such a classification of our sensations and of the organs which originate them is too crude for modern needs. Already we have shown that, whereas the nose and the tongue afford the same kind of information, the ear affords information of two, perhaps of three, different kinds. Within the realm of common sensation we pick out three special senses served by specialized sense-organs—touch, cold and heat—and, possibly, a fourth, served by non-specialized nerves, to which alone the epithet “common” properly applies.

The skin is supplied with nerves—naked fibrils—in the richest abundance. They are most easily demonstrated in the layer which covers the cornea, thanks to its transparency; in this, as shown in [Fig. 41], having branched on the front of the fibrous tissue of which the cornea is composed, the nerves pass towards the surface, forming connections with every one of its cells, or, at any rate, with every cell of the more superficial of the three or four layers of which the epithelium is made up. Ramified nerve-twigs of this type do not, under ordinary conditions, convey any sensations to consciousness. So long as the skin-cells with which they are connected are healthy, the nerve-twigs establish for them connections with the central nervous system by which their nutrition is regulated; but they carry no impulses to which we can direct attention. The movement of blinking is accompanied by no sensation until the edges of the eyelids come in contact. A pencil pressed against the lid evokes touch-sensations from the skin, but none from the cornea which underlies it. When a tiny beetle injures the surface of the cornea by scratching the epithelial cells with its horny wings and legs, the ruptured nerve-filaments convey to consciousness impulses, or, as we prefer to express it, an influence which is felt as pain. But even the pain caused by injury to the cornea is trifling as compared with that which originates in the under-sides of the lids, where not only is the epithelium supplied with branching nerve-twigs, but specialized organs of touch are present to localize the seat of injury. Everywhere the epithelium covering the surface of the body is so abundantly supplied that a successful staining of nerve-filaments induces one to think that every epithelial cell has its nervous affiliation. These are the nerves of common sensation, if we retain the term; but sensation so common, so obscure, so little differentiated that we know no more about it than we know about the air which envelops our hands and faces on a warm, windless day. Yet the air, when it moves, gives rise to a dim, broad, generalized sensation, which may be focussed into definiteness by a sensitive nerve.

Fig. 41.—Vertical Section of the Epithelium which covers the Surface of the Cornea,
and of a Small Portion of the Corneal Substance, highly Magnified.

The black lines are naked nerve-fibres (stained with chloride of gold), which are distributed amongst the cells of the more superficial strata of the epithelium in very great abundance. The corneal substance is composed of sheets of transparent fibres with intervening cells. As the fibres of the several sheets cross one another at various angles, they are cut, some transversely, others in the direction of their length.

An observer who has devoted himself for many years to the investigation of skin-sensations, and especially of the “referred pains” which are due to diseases of the viscera, recently caused the large cutaneous nerve which supplies the thumb side of the forearm and hand to be cut in his own arm, in order that he might study carefully the revival of sensations. He found that he never lost his ability to recognize displacements of the tissues beneath the skin. Pacinian bodies and other end-organs of deep-lying nerves recorded pressure and tension caused by pushing or rubbing with a blunt instrument. Seven weeks after the injury he began to recognize stimuli that do harm—hot things, cold things, pricking with a pin—although his power of localizing the spot injured was extremely vague. In seven weeks, that is to say, the protopathic nerves, which do not follow the same definite lines as the nerves of the special senses, but form open networks with many alternative paths, had re-established their skin connections. Only gradually and very slowly did critical sensations return—the ability to distinguish degrees of warmth, to recognize as separate two points of a pair of compasses, to feel a touch with cotton-wool.

According to a theory set forth in this book ([p. 312]), pain is not a set of sensations, but a condition of the central nervous system which renders it unduly excitable, or excitable in a particular manner, to impulses which have the same local origin as the nerve-current which sets up the condition of pain. When a nerve of the skin has been cut, the epithelial ramifications are renewed before any specialized tactile or other sense-organs have regained their nervous connections. When the area which has regained its surface ramifications, but has not regained its sense-organs, is injured, no localization of pain results. Indeed, the obscure sensations which are then experienced if the skin be injured can hardly be described as painful. The ramified nerves pour their agitation into the grey matter of the spinal cord; but it is not the agitation per se which causes pain. It is the passage of impulses through the agitated area that gives to them, when they reach consciousness, not only a topographical meaning, but also a distressful feeling. Until the specialized organs of the skin have been restored to working order, there are no impulses to pass through the agitated grey matter, and therefore no feelings of pain. According to this view there are two systems of afferent nerves, the protopathic and the specialized or critical. The former is very widely and very abundantly distributed to the surface of the body, the lungs, the alimentary canal, and other viscera. It has no end-organs, no defined tracts in the central nervous system, no definite connections with the cortex of the great brain. The currents which it conducts, if they originate in the visceral part of this system, have no direct effect in consciousness; but if they originate on the surface of the body, or in the alimentary canal at the lower end of the œsophagus, or in certain other situations, they co-operate with stimuli of heat, cold, or traction. The critical system works in a more definite way. Its impulses originate in sense-organs. Starting with a certain potential, they are transmitted by the discharge of a succession of linked neurones. When they reach the cortex their potential is sufficiently high to evoke consciousness. Their distribution in the cortex is as definite as their origin.

Specialized sense-organs are necessary for the origin of all sensations. Within the epithelium are certain cells which look as if they were specialized for sensory purposes. The deeper sheet, or derma, of the skin is abundantly provided with structures in which nerves end in the most elaborate and complicated ways ([Fig. 42]). They are found especially in the papillæ of connective tissue, which, set in rows, form the ridges that one can see at the finger-tips and in various other situations. All of these organs are made up of groups of epithelial cells which, displaced from the epidermis, have sunk into the derma, with the nerves connected with them. In their further development the nervous part of the apparatus is complicated by branching, the branches being thickened and usually flattened into ribbons, which lie on the external surfaces of the cells or between them. A more or less marked capsule is provided for the organ by condensation of connective tissue.

Anyone can convince himself that the skin is not uniformly sensitive. He may test it first for the minimal stimulus which excites a sensation of touch. With a hair of the head—it must not be a very fine one—cut across with scissors, and held between finger and thumb at the right distance from the cut end, the skin of the palm of the hand is prodded. Every here and there a spot is found which is insensitive to so slight a pressure. These spots are neither large nor very close together. If the hairless skin of the arm between the elbow and the armpit be investigated in the same way, much larger blank areas are met with—oval patches more than ¼ inch in diameter. When a hairy surface is tested, it is found that contact with a hair can always be felt; and when the hairs are shaved, the touch-spots are found to extend around or from the points at which hairs pierce the epidermis. Touchless areas lie between them. Hair-follicles receive tufts of nerve-filaments, and it appears that they are the chief organs of touch. “Touch-corpuscles,” which are found in great numbers in the papillæ of the skin of the fingers and elsewhere, may probably be regarded as, genetically, hair-follicles which have not developed hairs.

Fig. 42.— Sense-Organs susceptible to Pressure.

All are formed on essentially the same plan; a fibrous capsule invests a group of epithelial cells amongst which a nerve ramifies. The simplest form is known as a Grandry’s corpuscle-a nerve ending in one or two plates between two or three epithelial cells. These organs are found in great numbers in the bills of aquatic birds. If a duck is watched whilst it is gobbling mud at the margin of a pond, it will be seen to have a remarkable capacity for discriminating between the shells of small snails, which it can crush, and stones, which it needs to drop from its bill. Its bill is also provided with small Pacinian corpuscles ([Fig. 43]). Touch-corpuscles, more elaborate in form than the one figured, are found in the papillæ of the skin of the fingers and elsewhere. They appear to be modified hair-follicles. End-bulbs occur in the conjunctiva and elsewhere, and especially in the peritoneum. Together with Pacinian corpuscles, they are accountable for sensations connected with the distension of the stomach and intestines.

If sensitiveness to pain is investigated by tapping very gently with a needle—or, better, by using a stiff horsehair fixed in a cleft stick, from which it projects about ¼ inch—it will be found that every here and there are spots which are exceedingly sensitive, whilst adjoining them are areas which are moderately sensitive, and between these areas small spots or stretches of skin which do not give the smarting sensation even though the horsehair be pushed until it doubles up.

Fig. 43.— Pacinian Corpuscle.

These organs are especially numerous in the neighbourhood of tendons and ligaments. They are also present beneath the skin of the hands and feet. Their capsules are formed of a great number of concentric lamellæ of connective tissue, enclosing lymph-spaces. Within the capsule is a core of finely granular substance, which also shows a tendency to a lamellar disposition. The structure of these relatively large sense-organs is highly suggestive of sensitiveness to pressure, traction, or rubbing.

Testing now for sensitiveness to cold with a cold blunt metal point, “cold-spots” can be mapped on the skin. If the metal is warmed to about 50° C., “heat-spots” are found. The different kinds of spot are very irregularly distributed. They may coincide, or overlap, or leave blank spaces. Their relative abundance varies. In some regions touch-spots, in others cold-spots, in others heat-spots, are more closely grouped. The tongue and the hand, and especially the tips of the fingers, are most sensitive to touch; but whereas the tongue is also exceedingly sensitive to warmth, the hands are relatively insensitive. Yet, speaking generally, parts especially sensitive to touch are little sensitive to temperature, and vice versa. Sensitiveness to cold is much more widespread than sensitiveness to heat. It is concentrated in the skin covering the abdominal viscera. A cold douche directed between the shoulders is doubtfully felt as cold. There is no doubt whatever about it when it strikes the skin over the stomach.

From these observations it appears that the skin contains three sets of organs sensitive respectively to touch, cold, and heat. Certain investigators hold that it also contains specific organs, or nerve-endings, sensitive to painful stimulants; but in this case there is the obvious difficulty of distinguishing between pain and touch. At no spot can pure pain be evoked free from any consciousness of touch.

To a certain extent the combinations of epithelial cells and nerve-endings in the skin fulfil the negative requirement of sense-organs; each kind, whilst specially sensitive to its own specific stimulant, is insensitive to stimulants of other kinds. But mutual exclusion is not absolute in the case of cold and warmth. If a warmed metal point be applied to a cold spot, it produces a sensation of cold. Our feelings of warmth and cold are to a large degree comparative. Luke-warm water feels cold to hands just taken out of hot water; moderately cold water appears luke-warm to hands that have been in contact with ice. The sensory apparatus for cold and heat soon adapts itself, or, in physiological language, it is soon fatigued. If after a prolonged bath at the body temperature a foot be plunged into very hot water and withdrawn quickly, the feeling which first ensues is one of cold. It is indistinguishable from the feeling provoked by dipping the foot into cold water. The sensation of cold subsequently gives place to one of painful warmth. This does not indicate that the heat-spots have been waked out of their lethargy by excessive stimulation. On the contrary, it is the cold-spots which, when they were first stimulated by the very hot water, answered “Cold,” that now cry out “Hot”; for both cold-spots and heat-spots, when strongly stimulated, yield the same sensation. Indeed, it appears that the mind relies upon the simultaneous stimulation of adjacent heat-spots and cold-spots for the assurance that the thing with which the skin is in contact is really hot. If two metal points, one kept warm and the other cold, are applied simultaneously to two closely adjacent spots of skin, the resulting sensation is “hot.” When the cold point is withdrawn, or replaced by a second warm point, the sensation sinks to “warm.”

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CHAPTER XVI
VOICE AND SPEECH

A cut carried horizontally backwards across the cartilage which projects forwards as Adam’s apple, a quarter of an inch below its notch, would show that it is V-shaped, the point of the V in front. Each limb of the V is a broad plate. In the mid-line is a gap, the rima glottidis, through which the windpipe communicates with the pharynx ([Fig. 45]). It is overhung by the stiff leaf-shaped epiglottis, the edge of which can be felt with the finger behind the tongue. (γλωττίς, the mouthpiece of a reed-pipe, is the term commonly used, for short, for the rima glottidis.) When air is being drawn into the lungs, the glottis is widely open. In speaking or singing it is almost closed. It is tightly shut whilst food is passing down the gullet.

The glottis is bounded, as to its anterior two-thirds, by two membranous folds, the vocal cords. In its posterior third it has a triangular cartilage, the arytenoid, on either side. A distinction is sometimes drawn between the anterior part, bounded by the vocal cords, and the whole glottis, the former being termed “rima vocalis”; but it is scarcely justified, for, although it is true that the anterior part is essentially the organ of voice, and its margins alone vibrate when high notes are sung, the anterior ends of the arytenoid cartilages also vibrate during the production of low notes. (The substance of these processes is not, properly speaking, cartilage; it resembles the epiglottis in containing a great abundance of elastic fibres.) And here we must warn the reader not to picture to himself a vocal “cord” as a kind of fiddle-string. It bears no resemblance to a cord, as we ordinarily understand the word; it is but a fold of mucous membrane, such as one might pinch up between finger and thumb from the inner side of the cheek. Its capacity for vibration depends upon the tenseness which is given to it by the pressure of the lymph with which it is distended, and vast numbers of exceedingly slender elastic fibres which traverse it.

Fig. 44.—The Anterior Half of the Larynx seen from Behind.

The drawing shows the folds of mucous membrane, the vocal cords, which stretch from the tips of the arytenoid cartilages to the recess behind the median portion of the thyroid cartilage. To the outer side of each vocal cord is seen the thyro-arytenoid muscle (cut across), consisting of a broad outer portion, chiefly concerned in closing the glottis during the act of swallowing, and a smaller internal portion, which regulates the length and the thickness of the segment of the cord allowed to vibrate.

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Fig. 45.—The Aperture of the Glottis seen from Above.

The leaf-like structure in front of it is the epiglottis; the two triangular structures at the back, the arytenoid cartilages; the white bands on either side, the vocal cords. A, The glottis is widely open during inspiration. Arrows show the lines of action of the muscles which rotate, and approximate, the cartilages. Attached to their outer angles, and pulling these angles forwards, the lateral crico-arytenoid muscles; pulling them backwards and inwards, the posterior crico-arytenoid muscles. Drawing the cartilages together, the arytenoid muscles. B, The glottis during speaking in a deep chest-voice, or when a low note of the lower register is being sung. C, During the production of a high note of the lower register. D, During the production of a note of the head-register. E, During the act of swallowing; the arytenoid cartilages are drawn towards the epiglottis the aperture is folded into a T; the pharynx (the tube behind the glottis) is distended.

The first cartilage below the thyroid—it may be felt with the finger—is termed “cricoid” (κρίκος, a ring), from its resemblance to a signet-ring. Narrow in front, its large signet projects upwards, within the V of the thyroid, behind, and on the top of the signet rest the two arytenoids. Each arytenoid is a triangular pyramid, its anterior, external, and upper angles prolonged into processes. It is united with the cricoid by a swivel joint, which allows its anterior process to swing inwards or outwards under the influence of two antagonistic muscles attached to its outer angle—the lateral and posterior crico-arytenoids. Another muscle attached only to the arytenoids draws them together. Still another muscle—or two muscles, for it is in two separate bands—unites the anterior process of the arytenoid with the back surface of the thyroid just on the outer side of the attachment into that cartilage of the vocal cord. The internal thyro-arytenoid muscle is a comparatively narrow band; the external thyro-arytenoid muscle is thick and broad.[3] By the simultaneous contraction of the encircling muscles the larynx is closely squeezed together, the anterior portion of the slit forming a T, with the transverse limb in front. This occurs only in swallowing. Under the co-operating contractions of the several muscles, the glottis assumes a variety of shapes. The external crico-arytenoids rotate the anterior angles of the arytenoid cartilages inwards ([Fig. 45, A]). If at the same time the arytenoid muscle draws the cartilages together, the glottis is reduced to a slit ([Fig. 45, C]). The posterior crico-arytenoid muscles rotate the cartilages outwards. If the arytenoid muscle is at the same time relaxed, the glottis gapes to its fullest extent ([Fig. 45, A]). The freer the opening, the less is the resistance to the blast of air, the gentler the vibrations of the cords, the lower the voice. The closer the slit, the greater is the resistance which the air in the windpipe has to overcome in passing through it, and consequently the more ample the vibrations into which it throws the vocal cords.

The vocal cords are the tongues of a reed-pipe, which, commencing in the chest at the point where the great bronchi join to form the windpipe, comprises the larynx, and, above the larynx, the complicated chambers of the throat, mouth, and nasal cavities, including the spaces within the bones of the head which open out of them. The pitch of the voice depends upon (1) the length of the vocal cords, and (2) their tension. The first factor is fixed for every individual. The voice is base, baritone, tenor, in a man; contralto, mezzo-soprano, soprano, in a woman—in proportion as the cords are long, of medium length, or short. A man’s vocal cords measure, on the average, 15 millimetres, a woman’s 11 millimetres. When a boy is from twelve to fifteen years of age his vocal cords double in length, and the “breaking” of the voice occurs as he gives up trying to get high notes out of his longer cords, and allows them to produce manly tones of an octave lower.

The lower posterior angles of the thyroid cartilages articulate with the cricoid. If the four cartilages are freed from all soft tissues without disturbing the thyro-cricoid, or crico-arytenoid joints, and if, while the thyroid is held in one hand, a finger of the other is placed on the front of the cricoid, it will be found that as this is depressed the arytenoid cartilages which rest upon its signet are tilted upwards and forwards within the thyroid; as it is raised, they are tilted away from it. In life this movement is effected by a muscle—the crico-thyroid ([Fig. 46])—attached to the front of the cricoid cartilage and to the under border of the lateral plate of the thyroid. This is the muscle of supreme importance in the production of the voice. The thyroid cartilage is slung in a fixed position by the hyoid bone (to be felt in the neck above it). The crico-thyroid muscle, being unable to depress the thyroid, raises the front of the cricoid cartilage, tilts back the arytenoids, tightens the vocal cords. As the voice ascends the scale, the tension of the cords is progressively increased, and their vibrations rendered proportionately more rapid. The range of the human voice is about three and a half octaves; of individual voices about two octaves; if the shrill cry of a baby, which may reach the third G above the middle C, or even higher (E⁗ or F⁗), be excluded. Exceptional voices have a range far greater than two octaves. Falsetto voice is produced by throwing half of the vocal cord out of vibration (the way in which this is accomplished is not clear), and at the same time raising the back of the tongue to the wall of the throat in such a manner as to cut off all the lower part of the upper resonating chamber, leaving it only the mouth and the cavities of the nose.

Fig. 46.—The Larynx from the Right Side.

From above downwards: the hyoid bone, thyro-hyoid membrane, thyroid cartilage, cricoid cartilage, trachea. The upper and posterior angle of the wing of the thyroid cartilage is suspended from the hyoid bone; its lower and posterior angle articulated with the cricoid cartilage. On the summit of the cricoid cartilage it articulates the arytenoid. Dotted lines indicate the position of the vocal cord. The crico-thyroid muscle, which raises the front of the cricoid, tilting the arytenoid cartilage backwards and tightening the vocal cord, extends, fan-like, from the front of the cricoid to the lower border of the wing of the thyroid.

So far the mechanism of voice is easily understood. As the scale is ascended, the vocal cords are progressively tightened by the contraction of the crico-thyroid muscles. But an analysis of the feelings experienced during singing (and of the quality of the sounds produced) shows that by themselves these muscles are not able to make changes in the tension of the cords sufficient to account for the full range of the voice. Or, put in another way, the tension of the vocal cords is not altered to the extent which would be necessary if upon it alone depended a range of from two to three octaves. It is obvious that by some means the length or thickness, or both, of the portions of the cords vibrating is changed as the scale is ascended. If commencement be made on a low note, a point is reached, after a certain number of notes have been sung, at which a sudden change occurs. There is an alteration in the quality of sound, the more marked, the less well trained the singer. The singer experiences a feeling of relief. If a finger be placed on his crico-thyroid muscle, a relaxation of its anterior fibres can be detected. As he proceeds up the scale, these fibres again tighten. At a certain point there is again a change in the quality of voice, and in the feelings which accompany its production. The two points at which change occurs are said to divide the voice into three “registers”—the lower, or chest-register, the middle, and the upper, or head-register. A great effort is needed to hold either register above its natural range.

The physiology of the registers is a subject far too thorny for handling in this book. The larynx can be watched with the laryngoscope during the production of notes of different pitch, but observers are not in accord regarding the appearances which it presents, or their interpretation. The possibilities of changing the reed which vibrates, the vocal cord, otherwise than by increasing the direct pull upon it exerted by the crico-thyroid muscle, appear to be as follows: (1) During the production of the lowest notes the elastic portion of the arytenoid cartilage may be included with the cord. It may be thrown out of vibration by its rotation inwards (under the action of the lateral crico-arytenoid muscle) until it is pressed against its fellow. (2) Certain portions of the cord may be damped by partial contractions of the internal thyro-arytenoid muscle. It has been frequently stated, although the statement is not accepted by all anatomists, that some of the fibres which take origin from the arytenoid cartilage end in the cord, instead of passing right through to the thyroid. It is supposed that by their contraction they throw the posterior portion of the cord—even, it is asserted, as much as its posterior two-thirds in the higher head-notes—out of vibration. (3) It appears that the width (thickness) of the cord vibrating is also regulated by the contraction of the thyro-arytenoid muscle. Those who regard the diminution in the thickness and width of the vibrating fold of mucous membrane and underlying elastic tissue as the chief factor in the adaptation of the larynx for the middle register lay great stress upon the sense of relief from muscular effort which accompanies the transition. Less force is needed to tighten the thinner cord. They also call attention to the loss in volume of the voice when the lower register is left, and to its greater softness. The lower is spoken of as the thick register, the middle as thin, and the upper (on the hypothesis that part only of the cord vibrates) as the small register.

Singing reveals the possibilities of the larynx as a musical instrument. In speech the larynx plays a part, but the form of the syllabic sounds and the relative prominence of overtones in the vowels is of more importance than pitch. Flexibility of voice is dependent upon ability to increase or diminish at will the size of the resonating chambers of the throat, mouth, and nose, or the freedom of access to them. Conversation is carried on in the lower or chest-register. When a practised speaker mounts a platform, he spends the first few minutes in ascertaining the pitch of the hall—that is to say, the pitch of his voice to which the room resonates most freely. Having found the proper tone, he endeavours to maintain a uniform tension of his vocal cords, and therefore a uniform pitch. He relieves the monotony of speech by suitable variations of its overtones. Nothing is more uncomfortable to listen to than an oration delivered in cadences. The speaking voice should be full, round, and musical, and free from affectation—as guiltless of the intoning or preaching quality as it is of harshness or of vulgar flatness. A flexible voice is capable of producing, as occasion calls for them, tones of any and every quality. With the throat and mouth set for the syllable “haw,” it is impossible to do justice to such words as “king ” and “queen.” The voice-tones of a superior person are as distasteful to the hearer as those of a vulgarian. Unpleasant also is a nasal twang, illogically so called, since it is due, not to the opening of the resonating chambers of the nose, but to the restriction of the entry of air into them. In this it is somewhat similar to the effect produced by a severe cold. Resonance in the nasal chambers produces a clear, ringing voice.

A little consideration of the varying qualities of different voices suffices to show how largely they depend on resonance. When vowel-sounds are analysed, it is found that the distinctive character of each of them is dependent upon the overtones which it contains. For every vowel the overtones are fixed, or very nearly so, no matter what may be the pitch of the note to which the vowel is sounded.

It is much to be regretted that the alphabet was settled before the physiology of speech was understood. Were it based upon reasonable principles, children would be spared the bewilderment which overtakes them when they endeavour to establish in their minds some kind of relation between the names of consonants and their effects upon the blast of air as it passes through throat and mouth, and between tongue and palate, teeth and lips. The vowels, had physiologists defined them, would have been real pure vowel-tones—ōō, o, ah, ēē—sounds which can be sustained for an indefinite time, and allowed to die away without deterioration in their quality. A (é as pronounced in France) is doubtfully pure—it has a tendency to tail off in ēē; ī is frankly a diphthong, ai (ah-ēē). Try to hold a long final note on the syllable “nigh ”! An international standard of vowel-sounds would have been fixed, by giving the vibrating periods of the tuning-forks for which in each several case the resonating chambers are shaped, and defining the relative accentuation of each overtone. Greatest boon of all, the irruption of the Essex dialect would have been dammed. It would not have been allowed to inundate London, or to submerge Australia, debasing our English tongue. In Cockney speech vowels degenerate down the line of greatest indolence. Aw becomes or, or ar; a becomes i. It requires a greater effort to pronounce a full a than a flat a, a definite flat a than i. And worse than a Cockney’s unwillingness to take the trouble necessary for the production of dignifiedvowel-tones is his reluctance to make the effort required for the holding of any tone. In his mouth virile, self-reliant vowels are replaced by emasculated diphthongs, which collapse as they present themselves to the ear. It costs trouble to fix the mouth-chamber before a vowel is sounded and to hold it steady until it is finished. Ah slides down through ai to ēē; i slips into ēē. “Cow” becomes kyow; “you,” ye-u-ow; “cart,” kyart. And just as the effort needed for the filling of the vowels is shirked, so also is grudged the expenditure of an accessory blast for their aspiration.

When a vowel is whispered, although the vocal cords do not vibrate, the blast passing through the resonating chambers produces the overtones characteristic of the vowel. Anyone who feels his own larynx while he sings, to the same note, the various vowels between ōō and ēē—he may please himself as to the number of ai, eu, and ŭ vowels he interposes between these two extremes—will recognize that it is pulled farther and farther upwards by the muscles which surround it. The cavity of the mouth is at the same time made shorter and broader for each succeeding vowel. Singing the several vowels before a piano, and at the same time striking various keys, it is felt in the mouth that the resonance of that chamber is reinforced by certain selected notes. Certain tuning-forks, when sounded in front of the mouth shaped for a vowel, ring out more loudly, because the mouth-cavity resonates to their prime tones. The overtones of the vowels can be analysed in this way. Conversely, by sounding simultaneously an appropriate selection of tuning-forks, each with the right degree of force, the overtones of a vowel can be synthesised. Thus if whilst one tuning-fork is sounding B₁♭ (B♭ above middle C), two others be added giving B₂♭ (loud) and F₃ (soft), the composite sound resembles the vowel o. If to these same three forks, with F₃ sounding more strongly, B₃♭ and a loud D₄ be added, the sound changes to ah.

The organ of voice is a combination of a reed-pipe with resonating chambers, the shape of which can be changed at will. The quality characteristic of a vowel is given to it by adding to the note produced in the larynx sounds due to the resonance of the throat and mouth. On the assumption (not allowed by all authorities) that, since the resonating chambers are not sound-producers, they can only add to the larynx-tone, as “formants” of a vowel, its own harmonics—sounds which they have picked out of it—it follows that, if, when the prime is changed, the resonators were not adapted to the new note, they would be dumb. If this attitude in regard to the question be justified, there must be a certain amount of variation in the quality of a vowel as the scale is ascended. But a vowel is not a musical tone; it is a conventional sound. Its whole value depends upon its retaining, as nearly as may be, the same quality, whatever be the pitch of its prime tone. By adjusting the form of the throat and mouth, we can not only prevent one vowel from passing into another, but we can keep it so nearly true to itself as to convince the ear that its quality is unchanged: ōō remains ōō, and ah ah, although the form of the sound as produced on C♯ is different to its form when sung to C.

Apart from the general distinction that low notes are taken more easily with vowels requiring a large mouth-cavity, and high notes with those providing a small one, there are certain very distinct relations between vowel-sounds and musical tones which need to be borne in mind in setting words to music. A singer changes a word when he feels that its vowel-tone does not allow him to give to the note to which it is set the fullest expression of which he is capable.

An account of the physiology of the production of consonants is to be found in most text-books of grammar.

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