GANGLIA AND CENTRES,
usually spoken of as if they were convertible terms. That this is inexact may be readily shown, and that it is misleading appears in its causing physiologists to credit every ganglion, wherever found, with central functions; and, by an almost inevitable extension of the error, has led to the assignment of central functions to a single ganglionic cell! This is but part of that “superstition of the cell” against which I shall have to protest. I will not here raise the doubt which presses from various sides respecting the central functions of the ganglia in the heart and intestines, because the reader perhaps shares the general opinion on that point; but let me simply ask what central function can possibly be assigned to the ganglia on each of the spinal sensory nerves? above all to those grouped and scattered ganglionic cells which are found at the peripheral termination of some nerves, and in the very trunks of others? There may, indeed, be imagined a central function for the ganglia in the mesentery, and even in the choroid coat of the retina, on the hypothesis (quite gratuitous, I think) of their regulating the circulation; but even this explanation cannot be adopted with respect to the ganglionic cells which appear in the course of the nerve.[87]
The meaning of a physiological centre is, that it is a point to which stimulations proceed, and from which they are reflected. The meaning of a ganglion is, that it is a group of nerve cells dispersed among, or in continuation with, nerve fibres: it may be a centre of reflection, or it may not; and in the latter case its physiological office is at present undetermined. A ganglion is no more a centre in virtue of its cell-group than a muscle is a limb. All function depends on connection, and central function demands a connection of afferent and efferent parts.
19. The ganglia found in the ventral cord of the Invertebrate (see [Fig. 1]) are centres, each of which has considerable independence, each regulating a single segment of the body, or a group of similar segments. As the scale of animal complexity ascends, these separated centres tend more and more to coalesce, and with this coalescence comes an increasing combination of movements.[88] Observe the caterpillar slowly crawling over a leaf; each segment of its body moves in succession; but when this caterpillar becomes a butterfly the body moves rapidly, and all at once. Open the caterpillar, and you find its nervous centres are thirteen separate ganglia, each presiding over a distinct part of the body, and each capable of independent action. Open the butterfly, and you find the thirteen ganglia greatly changed: the second and third are fused into one; the fourth, fifth, and sixth into another; the eleventh and twelfth into another; the only trace of the original separation is in a slight constriction of the surface. The movements of the caterpillar were few, simple, slow, and those of the butterfly are many, varied, and rapid.
20. In the Vertebrates the coalescence of ganglia is such that the spinal axis is one great centre. We do indeed anatomically and physiologically subdivide it into several centres, because several portions directly innervate separate organs; but its importance lies in the intimate blending of all parts, so that fluctuating combinations of its elements may arise, and varied movements result. Each centre combines various muscles; the axis is a combination of centres. The brainless frog, for instance, has still the spinal cord, and therefore the power not only of moving either of his limbs, but also of combining their separate movements: if grasped, he struggles and escapes; if pricked, he hops away. But these actions, although complex, are much less complex and varied than the actions of the normal frog.
There is not only a coalescence of ganglia, but a greater and greater concentration of the substance in the upper portions of the axis. In the inferior vertebrates, and in the mammalian embryo, the spinal cord occupies the whole length of the vertebral canal from the head to the tip of the tail; and here the centres of reflexion correspond with the several segments. But as the cranial mass develops there is a withdrawal of neural substance from the lower parts, and the centres of reflexion are then some way removed from the segments they innervate. In the animal development there is even a greater and greater predominance of the upper portions, so that the brain and medulla oblongata are of infinitely more importance than the spinal cord.
21. Besides the central group of elements which belong to fixed and definite actions, we must conceive these elements capable of variable combinations, like the pieces of colored glass in a kaleidoscope, which fall into new groups, each group having its definite though temporary form. The elements constitute really a continuous network of variable forms. It is to such combinations, and not to fixed circumscribed ganglia, that we must refer the subordinate centres of the axis. We speak of a centre for Respiration, a centre for Laughing, a centre for Crying, a centre for Coughing, and so on, with as much propriety as we speak of a centre for Swallowing or for Walking. Not that in these cases there is a circumscribed mass of central substance set apart for the innervation of the several muscles employed in these actions, and for no other purpose. Each action demands a definite group of neural elements, as each geometric form in the kaleidoscope demands a definite group of pieces of glass; but these same pieces of glass will readily enter into other combinations; and in like manner the muscles active in Respiration are also active in Laughing, Coughing, etc., though differently innervated and co-ordinated.
22. The physiological rank of a centre is therefore the expression of its power of fluctuating combination. The medulla oblongata is higher than the medulla spinalis, because of its more varied combinations; the cerebrum is higher than all, because it has no fixed and limited combinations. It is the centre of centres, and as such the supreme organ.
CHAPTER II.
THE FUNCTIONAL RELATIONS OF THE NERVOUS SYSTEM.
23. The distinguishable parts of this system are the central axis, the cranial nerves, and the spinal nerves, with the chain of ganglia and nerves composing the Sympathetic. Let us briefly set down what is known of their special offices.
Men very early discovered that the nerves were in some way ministrant to Sensation and Movement; a divided nerve always being accompanied by insensibility and immobility in the limb. Galen, observing that paralysis of movement sometimes occurred without insensibility, suggested that there were two kinds of nerve; but no one was able to furnish satisfactory evidence in support of this suggestion until early in the present century, when the experiments of Charles Bell, perfected by those of Majendie and Müller, placed the suggestion beyond dispute.
Fig. 12.—Transverse sections of spinal cord (dorsal region).
24. [Fig. 12] is a diagram (not a drawing of the actual aspect, which would be hardly intelligible to readers unversed in such matters) representing two transverse sections of the spinal cord just where the nerve-roots issue. The gray substance is somewhat in the form of a rude H, in the dorsal region, and of the expanded wings of a butterfly in the lumbar enlargements (Figs. 4–6); the extremities of this gray substance are the anterior and posterior horns. We have already said that from the anterior horns of each half issue the roots of the motor nerves, which pass to the muscles. From the posterior horns issue the sensory nerves, which, soon after leaving the cord, enter the ganglia before joining the motor nerves, and then pass to the skin, in the same sheath with their companions, separating again as they reach the muscles and surfaces where they are to be distributed. When this mixed nerve is cut through, or tied, all sensation and movement disappear from the parts innervated. But if only one of the roots be cut through, above the ganglion, there will then be only a loss of movement or a loss of sensation. Thus suppose the section be made at a, b, A: we have then divided a sensory nerve, and no pinching or pricking of the part innervated by that nerve will be felt; but movement will take place if the under nerve be irritated, or if a sensation elsewhere be excited. Now reverse the experiment, as at B, c, d. Then, pricking of the skin will be felt, but no movement will respond. The nerve which enters the cord at the upper (posterior) part is therefore a sensory nerve; that which enters at the under (anterior) part is motor. The direction is in each case indicated by the arrow. The central end b, if irritated, will produce sensation; whereas the peripheral end a produces neither sensation nor movement. The central end d produces neither sensation nor movement; the peripheral end c produces movement.
25. Two facts are proved by these experiments. First, that the co-operation of the centre is necessary for Sensation, but not for Movement. Although normally all the muscles of the trunk are moved only when their centre has been excited, yet any irritation applied directly to the muscle nerve, even when separated from its centre, produces a movement. And to this we may add that a slighter stimulus will move the muscle by direct irritation of the nerve, than by indirect irritation through the centre; a slighter stimulus also will suffice when applied to the nerve than when applied to the muscle itself.
26. The second fact proved is known as Bell’s Law, that the sensory and motor channels are respectively the posterior and anterior nerves. The fact is indisputable, but its theoretic interpretation can no longer be accepted in its original form. Bell supposed the two nerves to be different in kind, endowed with different specific energies, the one sensitive, the other motor. The majority of writers still express themselves as if they adopted this view. We shall, however, presently see reason for replacing it by the more consistent interpretation which assigns one and the same property to both nerves, marking their distinction by the terms afferent and efferent; the one set being anatomically so disposed that it conveys stimuli from the surfaces to the centre, and the other set conveying stimuli from the centre to the muscles, glands, and other cells.[89]
27. Bell’s discovery was rapidly generalized. The principle of localization was extended to all nerves, and of course to the posterior and anterior columns of the spinal cord, which indeed were assumed to be continuations of the nerves. Bell, who was greater as an anatomist than as a philosopher, always maintained that anatomical deduction was superior to experiment. But this was to misunderstand the reach of deduction, which is only valid to the extent of its premises.[90] In the present case, the premises assumed that the posterior columns were continuations of the posterior roots, and carried impressions to the brain, the anterior columns carrying back from the brain the “mandates of the will.” Experiment has, however, decisively shown that it is not through the posterior columns that sensory impressions travel to the brain, but through the central gray substance.
28. The spinal cord with its central gray substance is at each point a centre of reflexion. Connected as it is with different organs, we artificially consider it as a chain of different centres, and try to detect the functional relations of its parts. The inquiry is important, but we must bear in mind the cardinal principle that diversity of Function depends on the organs innervated, and not on a diversity of Property in the nervous tissue. Although all nerves have a common structure and common property, yet we distinguish them as sensory and motor; and the sensory we subdivide into those of Special Sensation and those of Systemic Sensation. The motor we divide into muscular, vasomotor, and glandular. The hypothesis of specific energies must be relinquished (§ [63]).
In like manner all centres have a common structure and a common property, with a great diversity of functional relations. Here also the hypothesis of specific energies has been generally adopted, owing to a mistaken conception of the biological principle just mentioned. The cerebral hemispheres are credited with the properties of sensation, thought, and volition; the cerebellum with the property of muscular co-ordination; the spinal cord with the property of reflexion.
29. No attempt to assign the true functional relations of the centres will be made at the present stage of our exposition. We must learn more of the processes in Sensation, Thought, and Volition, before we can unravel the complex physiological web on which they depend. But here, provisionally, may be set down what observation and experiment have disclosed respecting the part played by certain centres. We know, for example, that when the cerebral hemispheres are carefully removed from a reptile or a bird, all the essentially vital functions go on pretty much as before, but a great disturbance in some of the psychical functions is observed. The brainless bird eats, drinks, sleeps, moves its limbs separately and in combination, manifests sensibility to light, sound, and touch, performs such instinctive actions as preening its feathers, or thrusting the head under the wing while roosting. Throw it into the air and it will fly. In its flight it will avoid obstacles, and will alight upon a ledge, or your shoulder. But it will not fly unless thrown into the air; it will not escape through the open door or window; it will avoid objects, but will show no fear of them,—alighting on your head, for example, without hesitation. It is sensitive to light, and may in a certain sense be said to see; but it fails to perceive what is seen. It will eat and drink, if food and water be administered, but it will starve near a heap of grain and never peck it, not even if the beak be thrust into the heap. A grain, or strip of meat, may be thrust inside the beak; there it will remain unswallowed, unless it touches the back of the mouth, then swallowing at once follows the stimulus. The bird with its brain will fly away if you turn the finger, or stick, on which it is perching; without its brain, it makes no attempt to fly, but flutters its wings, and balances itself. If you open the mouth of a cat, or rabbit, and drop in some bitter fluid, the animal closes its mouth firmly, and resists your efforts to repeat the act; without its brain, the animal shows the same disgust at the taste, but never resists the preliminaries of the repetition.
30. These, and analogous facts, have been noted by various experimenters. They are very far from proving what is usually concluded; but they prove the important negative position that the cerebrum is not the centre of innervation for any of the organs on which the observed actions depend. Thus, the cerebrum is not necessary to sight: ergo it does not innervate the eye. It is not necessary to hearing: ergo it does not innervate the ear.[91] It is not necessary to breathing, swallowing, flying, etc.: ergo it does not innervate the organs of these functions.
What then is lost? We have only to remember that the cerebrum is continuous with the thalami and corpora striata, and, through its crura, with the medulla oblongata and medulla spinalis, to foresee that its removal must more or less affect the whole neural axis, and consequently disturb the actions of the whole organism; this disturbance will often have the appearances which would be due to the removal of a central apparatus, so that we shall be apt to attribute the cessation of a function to the loss of its organ, when in fact the cessation is due simply to an arrest of the organ by irritation. Thus the cessation of consciousness, or of any particular movements, when the cerebrum is removed, is no decisive proof that the cerebrum is the organ of consciousness, or of the movement in question. This point will be duly considered hereafter. What we have now to consider is the facts observed after removal of the cerebrum.
First, we observe a loss of that power of combining present states with past states, present feelings with feelings formerly excited in conjunction with them, the power which enables the animal to adjust its actions to certain sensations now unfelt but which will be felt in consequence of the adjustment. Secondly, we observe a loss of Spontaneity: the bird, naturally mobile and alert, now sits moveless for hours in a sort of stupor, occasionally preening its feathers, but rarely quitting its resting-place. All the most conspicuous phenomena which we assign to Intelligence and Will seem absent. The sensations are altered and diminished. Many Instincts have disappeared, but some remain. The sexual feeling is preserved, although the bird has lost all power of directing its actions so as to gratify the desire. But these effects are only observed when the whole of both hemispheres have been removed. If a small portion remain the bird retains most of its faculties, though with less energy. In frogs and fishes there is little discernible effect observed when a large portion of the cerebrum is removed.
31. Now take away from this mutilated bird its cerebellum: all the functions continue as before except that some combined movements can no longer be effected; flight is impossible; walking is a mere stagger. Remove only the lateral lobes, and though flight is still possible great incoherence of the wings is observed, whereas walking is not much affected. If only the cerebellum be removed, the cerebrum being intact, the phenomena are very different. All the perceptions and almost all the emotions, all the spontaneity and vivacity are retained; but the sexual instinct, which was manifested when the cerebrum was removed, is now quite gone. What we call Intelligence seems unaffected. The bird hears, and understands the meaning of the sounds, sees and perceives, sees and fears, sees and adjusts its movements with a mental vision of unseen consequences.[92]
32. Are we from these facts to conclude that the cerebrum is the “organ of the mind”; that it is “the seat” of sensation, thought, emotion, volition; and that the cerebellum is the “seat” of the sexual instinct, and muscular co-ordination? Such conclusions have found acceptance, even from physiologists who would have been startled had any one ventured to affirm that the medulla oblongata was the “organ” of Respiration, because Respiration ceases when this centre is destroyed. I shall have to combat this notion at various stages of my exposition. Here let me simply say that it is irreconcilable with any clear conception of organ and function; and is plainly irreconcilable with any survey of psychical phenomena in animals in whom the cerebrum does not exist, and in animals from whom it has been removed.
What the facts indisputably prove is that the cerebrum has an important part in the mechanism by which the most complex psychical combinations are effected, and that the cerebellum has an important part in the mechanism by which the most complex muscular combinations are effected. The supreme importance of the cerebrum may be inferred from its dominating all the other centres, and from its preponderance in size. In man it stands to all the other cranial centres together in the relation of 11 to 3. It is about five times as heavy as the spinal cord—that is to say from 1,100 to 1,400 grammes, compared with 27 to 30 grammes. The quantity of blood circulating through it is immense. Haller estimated the cranial circulation as one fifth of the whole circulation. If, therefore, the Nervous Centres are agents in the production of Sensation and Intelligence, by far the largest share must be allotted to the cranial centres, and of these the largest to the Cerebrum.
33. It is, however, one thing to recognize the Cerebrum as having an important part in the production of psychical phenomena, another thing to localize all the phenomena in it as their organ and seat—a localization which soon becomes even more absurd, when of all the cerebral structure the multipolar cells alone are admitted as the active agents!
As was said just now, we recognize in the Medulla Oblongata the nervous centre of Respiration, but we do not suppose that Respiration has its seat there, nor that this centre is absolutely indispensable for the essential part of the process. We respire by our skin, as well as by our lungs; many animals respire who have nothing like a medulla oblongata; as many animals feel, and manifest will, who have nothing like a cerebrum. The destruction of centres is of course a disturbance of the mechanisms which they regulate. But even the observed results of a destruction require very close examination, and are liable to erroneous interpretations. The disappearance of a function following the destruction, or disease of a particular part, is not to be accepted as a proof that this part is the organ of the lost function; because precisely the same phenomena may often be observed following the destruction of a totally different part.[93] But one result may always be relied on, and that is the persistence of a function after removal of a particular part. Thus there is a certain spot of the cerebral convolutions from which movements of the limbs are excited when the electrodes are applied to it; removal of the substance is immediately followed by paralysis of the limbs. Are we to conclude that this spot is the organ of the function? It is true that the function is called into action by a stimulus applied to this spot: true that the function suddenly vanishes when the substance of this spot is destroyed. Nevertheless, what seems a loss of function is only a disturbance. In two or three days the paralysis begins to disappear, and at the end of a week the limbs are moved nearly in the normal manner. And the same is true when the spot in question is destroyed on both sides. The recovery of the function shows that the absent part was not its organ. There is a paradoxical experiment recorded by M. Paul Bert which may be cited here. He removed the right cerebral hemisphere from a chameleon, and found that the limbs on the left side were paralyzed; but on his then removing the left cerebral hemisphere the limbs of the left side recovered their activity. A similar result was obtained by Lussana and Lemoigne by extirpation of the thalami. When we find combined movements persisting after the cerebellum has been destroyed, we may be sure that the cerebellum is not the organ by which such combinations take place; and when we find sensation and volition manifested after the cerebrum has been removed, we may be sure that the cerebrum is not the organ for these sensations and volitions.
34. And this we do find. Physiologists, indeed, for the most part, deny it; or rather, while they admit the observed facts, they refuse to admit the only consistent interpretation, biassed as they are by the traditional conception of the brain. After having for many years persistently denied Sensibility to any centre except the cerebrum, they are now generally agreed in including the medulla oblongata within the privileged region; but they still exclude the medulla spinalis.
35. If all the cranial centres as far as the medulla oblongata are removed from young rabbits, dogs, or cats, there are unmistakable evidences of Sensibility in their cries when their tails are pinched, their moving jaws (as in mastication) when bitters are placed in their mouths, and their raised paws rubbing their noses, when irritating vapors are applied. It is said indeed that the cries are no signs of pain; and this is probable; but they are assuredly signs of Sensibility.
35. The frog thus mutilated has lost indeed all its special senses, except Touch, but it still breathes, struggles when grasped, thrusts aside the pincers which irritate it, or wipes away acid dropped on its skin. If the eye be lightly touched, the eyelid closes; if the touch be repeated three or four times, the foreleg is raised to push the irritant away; if still repeated, the head is turned aside; but however prolonged the irritation, the frog neither hops, nor crawls away, as he does when the cerebellum remains. Place the brainless frog on his back, and if the medulla oblongata remains he will at once regain the normal position; but if that part is absent he will lie helpless on his back. The power of preserving equilibrium in difficult positions—which of course implies a nice co-ordination of muscles—resides in the so-called optic lobes of the frog (what in mammals are called the corpora quadrigemina).
37. With the destruction of each part of the central mass there will necessarily be some disturbance of the mechanism; but difficult as may be the task of detecting by experiment what is the normal action of any one part, there ought to be no hesitation in recognizing the persistence of functions after certain parts are destroyed. The spinal cord is anatomically known to be the centre from which the limbs, trunk, and genito-urinary organs are innervated. So long as the mechanism of the actions involving such organs is intact, no removal of other parts will prevent this mechanism from exhibiting its normal action. There may indeed arise, and there has arisen, the doubt whether Sensibility is involved in the action of any nerve centre below the medulla oblongata. But this doubt is founded on the traditional hypothesis respecting the seat of Sensation, and is flagrantly at variance with the logical conclusions of Anatomy and Experiment.
38. Anatomy shows that the structure of the spinal cord is in all essential characters the same as that of the medulla oblongata; and indeed that the whole central axis has one continuous tissue, somewhat variously arranged, and in relation with various organs.
Abundant Experiment has shown that the spinal cord, apart from the encephalon, is capable of acting as a sensorial and volitional centre. The striking facts advanced by Pflüger, Auerbach, and myself, have not been impugned;[94] but their interpretation has been generally rejected. We showed that a brainless frog responded to stimulation in actions which bore so close a resemblance to actions admitted to be sensorial and volitional—showed the frog adapting itself to new conditions, and acquiring dexterity in executing actions which at first were impossible or difficult, devising combinations to effect a purpose which never by any possibility could have formed part of its habits—manifesting, in a word, such signs of Sensibility, that no one witnessing the experiments could hesitate as to the interpretation, had he not been biassed by the traditions of the schools.
39. Our opponents argued that in spite of all appearances there were profound differences between the actions of the normal and the brainless animal, and that the latter were due simply to Reflex Action. I also insist on profound differences; but underlying these there are fundamental identities. As to the Reflex Action, two points will hereafter be brought forward: 1°, that all central action is reflex, the cerebral no less than the spinal; 2°, that the hypothesis of Reflex Action being purely mechanical, and distinguished from Voluntary Action in not involving Sensibility, is an hypothesis which must be relinquished.
40. Postponing, however, all discussion of these points, let me here say that the doctrine maintained in these pages is that the whole cerebro-spinal axis is a centre of Reflexion, its various segments taking part in the performance of different kinds of combined action. It has one common property, Sensibility; and different parts of it minister to different functions—the optic centre being different from the auditory, the cerebral from the spinal; and so on. To make this intelligible, however, we must first learn what is known respecting the properties of nerve-tissue.
CHAPTER III.
NEURILITY.
41. Observation having found that the activity of a nerve was always followed by a sensation when the nerve ended in a centre, and by a movement when the nerve ended in a muscle, Theory was called upon to disclose the nature of this peculiar property of nerves. That a peculiar and mysterious power did act in the nerves no one doubted; the only doubt was as to its nature. The ancient hypothesis of Animal Spirits seemed all that was needed. The spirits coursed along the nerves, and obeyed the mandates of the Soul. When this hypothesis fell into discredit, its place was successively taken by the hypotheses of Nervous Fluid, Electricity, and Nerve Force. The Fluid, though never manifested to Sense, was firmly believed in, even so late as the days of Cuvier;[95] but when the so-called electrical currents were detected in nerves, and the nervous phenomena were shown to resemble electrical phenomena, there was a general agreement in adopting the electrical hypothesis. The brain then took the place of a galvanic battery; the nerves were its electrodes.
42. Closer comparison of the phenomena detected various irreconcilable differences, which, if they proved nothing else, proved that nerve-action took place under conditions so special as to demand a special designation. Electricity itself is so little understood, that until its nature is more precisely known, we cannot confidently say more than that nerve-action resembles electrical-action; meanwhile the speciality of neural conditions renders all deduction illusory which is based on electrical-action as observed under other conditions. In presence of these difficulties, cautious physiologists content themselves with assigning the observed phenomena to the observed and inferred conditions, condensing these in the convenient symbol “nerve-force,” without pretending to any specification of the nature of that force. It may be a wave of molecular movement dependent on isometric change or on metamorphic change. It may be the liberation of molecular tension resembling electricity; it may be electricity itself. But whatever the nature of the change, it is an activity of the tissue, and as such comes under the general dynamic conception of Force or Energy.
43. In this sense the term has nothing equivocal or obscure. It is a shorthand expression symbolizing certain well-defined observations. Nevertheless, it is a term which we shall do well to avoid when possible, and to replace by another having less danger of misinterpretation; the reason being that Force has become a sort of shibboleth, and a will-o’-wisp to speculative minds. All that we know of Force is Motion. But this is too meagre for metempirical thinkers, who disdain the familiar experiences expressed in the term Motion, and demand a transcendent cause “to account” for what is observed. They seek an entity to account for the fact. Motion is a very definite conception, expressing precise experiences; we know what it means, and know that the laws of moving bodies admit of the nicest calculation. A similar precision belongs to Force when understood as “mass acceleration,” or M V². But this does not content those metaphysicians who understand by Force “the unknown reality behind the phenomena”—the cause of Motion. This cause they refuse to recognize in some antecedent motion (what I have termed a “differential pressure”), but demand for it a physical or metaphysical agent: the physical agent being a subtle fluid of the nature of Ether, or a nerve atmosphere surrounding the molecules; the metaphysical agent being a Spirit or aggregate of Soul-atoms. The second alternative we may decline here to discuss. The first alternative is not only a pure fiction, but one which is inconsistent with the demonstrable velocity of the neural process, which is not greater than the pace of a greyhound, whereas the velocities of light and electricity are enormously beyond this. It is inconsistent also with the observation that a much feebler current of electricity is requisite for the stimulation of a muscle through its nerve than when directly applied to the muscle: a proof that the nerve does not act solely by transmission of electricity—unless we gratuitously assume that the nerve is a multiplicator.
When it is said that the living nerve is incessantly liberating Force which can be communicated to other tissues, the statement is acceptable only if we reject the metaphysical conceptions it will too generally suggest—the conceptions of Force as an entity, and of its being passed from one object to another like an arrow shot from a bow. The physical interpretation simply says that the molecules of the nerve are incessantly vibrating, and with varying sweep; these vibrations, when of a certain energy, will set going vibrations in another substance by disturbing the tension of its molecules, as the vibrations of heat will disturb the tension of the gunpowder molecules, and set them sweeping with greater energy: this is the communication of the force. Just as we say that a magnet communicates magnetic force to a bit of iron, though all we mean is that the magnet has so altered the molecular condition of the iron as to have given it the movements called magnetism—in short, has excited in the iron the dormant property of becoming magnetic—so we say the nerve communicates its force to the muscle, exciting in the muscle its dormant property of contraction. But in truth nothing has passed from magnet to iron, or from nerve to muscle.
44. Do what we will, however, there is always, in the present condition of philosophical chaos, the danger of being misunderstood when we employ the term Nerve-force; and I have proposed the term Neurility as an escape from the misleading suggestions. It is a symbol expressing the general property of nerve-tissue. For reasons presently to be stated, I restrict Neurility to the peripheral system, employing Sensibility for the central system. The excited muscle manifests its special property of Contractility; the excited nerve manifests its special property of Neurility; the excited centre manifests its special property of Sensibility.[96] The terms are simply descriptive, and carry with them no hypothesis as to what Neurility is in its hidden process, nor how Sensibility arises in a nerve-centre, and not elsewhere. We know that a stimulated muscle contracts, and we express the fact by assigning to muscular tissue the property of Contractility. We know that a stimulated nerve translates an impulse from one point to another, and excites the muscle to contract; and we express the fact by assigning to nerve-tissue the property of transmitting stimulation, which is further specified, as unlike other transmissions, by the term Neurility.
45. What is the meaning attached to the term Property, and how it is distinguished from Function, has been already expounded in Problem 1, §§ [81–6]. There also was laid down the principle of identity of structure implying identity of property. Inasmuch as observation reveals a fundamental similarity in the structure of the nervous tissue throughout the animal kingdom, we must conclude the existence of a fundamental similarity in the property of that tissue: a conclusion confirmed by observation. There is a corresponding agreement in the organs and functions; so that, within certain limits, the experiments performed on an insect may be verified on a mammal. Everywhere nerve-tissue has certain characters in common, accompanied by variations in the degree and mode of manifestation corresponding with variations in structure and connection. Obvious as the fact is, we must emphasize the great variety which accompanies the underlying uniformity, for this is recognizable both in the individual organism and in the animal kingdom at large. Even such seemingly individual terms as nerve-cell and nerve-fibre are in truth generic; and the description which accurately represents one cell or fibre needs modifying for others.
Properties are generalized expressions; they result from the composition, the structure, and the texture of a substance. Thus one bar of iron may differ from another of equal bulk in being more or less crystalline in structure, though having the same composition and the same texture. This difference will modify the mode of manifestation of the iron-properties. Cast-iron pillars, for example, will support, as a roof, a weight which would break them if suspended; wrought-iron pillars of similar bulk will bear a weight suspended which would crush them as a roof. Yet both cast and wrought iron pillars have the same properties, because they have the same composition and similar structure; the variation of structure only producing a difference in the modes. Texture may also vary. The bar of iron may be beaten into a plate, rolled into a cylinder, or split into wire-work, without any change in its properties, but with marked differences in its modes of manifestation, and in the uses to which it may be applied. These uses are of course dependent on the connections established between the iron and other things. In Physiology, uses are called functions.
46. Nerve-tissue must be understood as having everywhere the same general Property. In one animal and in another, in one part and in another, Neurility is the same in kind, but not everywhere manifesting the same degree, nor applied to the same Function. The composition of nerve-tissue varies, but not more than the composition of all other organized substances; the structure is variable, but only within a small range; the texture also; while the connections are very various. Hence, whatever the variations in composition or structure, the nerve-fibre has everywhere one fundamental property, which in connection with a muscle has the functional activity of exciting contraction; in connection with a gland of exciting secretion; and in connection with a centre of exciting reflexion.[97]
47. Had a clear idea of Function as dependent on connexion been present to their minds certain physiologists would hardly have raised the mirage of “Nerve-force,” a mysterious entity endowed with “specific energies,” and capable of producing vital and psychical phenomena by an occult process; nor would others have been led to the monstrous hypothesis of particular nerve-cells being endowed with thought, instinct, and volition. They would have sought an explanation of functions in the combined properties of the co-operant organs and tissues. They would not have endowed one nerve with Sensibility, and another nerve of identical structure with Motility;[98] one nerve with a motor property, and another with the opposite property of inhibition. They would have seen that all nerves have the same property, but different uses when in different connexions.
48. Throughout the animal kingdom we see movement following on stimulation. Stimulation may be defined the change of molecular equilibrium. The stimulation of a muscle is produced indirectly through a change in the nerve, or directly through a change in the muscle itself. In the simplest organisms there is no trace of nerve-tissue; but their substance manifests Irritability (or as it is often called Sensibility); and a stimulus to one part is propagated throughout—the whole body moves when touched. Even in Polypes, where there is the beginning of a differentiation, the motion is slowly propagated from one part to the rest. A single tentacle retracts when touched; but the movement rarely ends there; it is slowly communicated from one tentacle to the other, and from them to the whole mass. Touching the body, however, will not, if the touch be slight, cause the tentacles to move; so that we see here a beginning of that principle of specialization which is so manifest in the higher organisms: the tentacles have become the specially sensitive parts. Ascending higher in the scale of organisms we find those which habitually move particular parts without at the same time necessarily moving the rest; and this independence of parts, accompanying a more perfect consensus, we find to be developed pari passu with a nervous system. An immense variety of part-movements, with varying combinations of such movements, is the physiological expression of the more complex nervous system.
48 a. Deferring what has to be said of Sensibility till the next chapter, we may here touch on its relation to Irritability, which is often used as its synonym. Objectively it cannot be distinguished from Irritability, nor indeed from the most general phenomenon of reaction under stimulation; in this it is an universal property. But subjectively it is distinguishable as a peculiar mode of reaction, only known in nerve-tissues. While all tissues are irritable, and react on being stimulated, each tissue has its special mode of reaction. The secreting-cell reacts differently from the muscle-cell. The reaction of the nerve is the innervation of a centre or a muscle; the reaction of an innervated centre is sensation; of a muscle, contraction. There are three aspects of neural reaction: excitation, propagation of the disturbance, and innervation. The first is expressed by irritability, the second by conductibility, the third by sensibility; but these are only artificial distinctions in the general phenomenon of transmitted excitation. The nerve substance is specially distinguished by its instability of molecular equilibrium; it undergoes chemical change with a readiness comparable to that of explosive substances. Hence its facility of propagation of disturbance. There is irritability and propagation of disturbance in muscular tissue, notably evident in the continuous tissue of the heart, intestines, and ureter; but the propagation is slow and diffusive; whereas in the nerve it is rapid, and restricted along a definite path. By this rapidity and restriction the force of the impact is increased; and thus a slight stimulus applied to the nerve is capable of disturbing the state of the muscle.
49. Thus while molecular movement is a fundamental condition of Vitality, and is incessant throughout organized substance, the massive movements of the organism, and the movements of particular parts, are the directed quantities of this molecular agitation. They are due to stimulation. We distinguish this from mechanical impulsion. It is a vital process involving molecular change; it is not simply the communication of motion from without, but the excitation of motion within. It is not like the blow which merely displaces an object, but like the blow which disturbs its molecular equilibrium. The effect, therefore, depends on this molecular condition: the blow which scatters a heap of gunpowder will explode a fulminating salt, and this, in exploding, will excite the gunpowder to explode. The stimulus which is too feeble to excite contraction in a muscle will be powerful enough to excite the neurility of a nerve, and that will excite the contractility of the muscle. The nerve-force is simply neural stimulus. It acts upon the other tissues as the nitrogenous salt upon the gunpowder.
Although it is now common to speak of nerves as transmitting waves of molecular motion, and to regard nerves as the passive medium for the “transference of force,” whereby the force is thus made an abstract entity, we must always remember that such phrases are metaphors, and that the truer expression will be not “transference of force,” but the “propagation of excitation.” I mean that it is not the force of the impact nor its energy which a nerve transmits, it is the vibratory change produced in the nerve by the impact, which excites another change in the organ to which the nerve goes. We know by accurate measurements that the excitation of a nerve lasts much longer than the stimulus, a momentary impact producing an enduring agitation. We know also that the excitation of a centre lasts longer than the muscular contraction it has initiated. We know, moreover, that a nerve may be totally incapable of conducting an external stimulus, yet quite capable of conducting a central stimulus; were it a passive conductor like a wire this would not be so.[99]
50. The nerve is essentially an exciter of change, and thereby a regulator. A muscle in action does not appreciably determine action in any other (except in the comparatively rare cases of anastomosing muscles); a secreting cell does not propagate its excitation to others. The nerve, on the contrary, not only propagates its excitation, and awakens the activity of the muscle or gland with which it is connected, but through the centre affects the whole organism—
“Ein Schlag tausend Verbindungen schlägt.”
Thus it is that stimulation which in the simpler organisms was diffused throughout the protoplasm, has in the complex organisms become the specialized property of a particular tissue.
51. Two general facts of supreme importance must now be stated: One is the law of stimulation—every excitation pursues the path of least resistance. The second is the condition of stimulation—unlike mechanical impulsion, it acts only at insensible distances.
52. This means that although a nerve may be excited by any stimulus external to it which changes its molecular condition, no propagation of that change (i. e. no stimulation through the nerve) is possible except through continuity of substance. Mere physical contact suffices to excite the nerve; but if there be an interruption of continuity in the nerve itself, no stimulus-wave passes across that line. Cut a nerve, and bring the divided surfaces once more into close contact, there will still be such a solution of continuity as to arrest the stimulus-wave, mere physical contact not sufficing for the propagation. Whereas across the cut ends of a divided nerve, even visibly separated, the electric current easily passes. This necessity for the vital continuity of tissue in the propagation of stimulation must always be borne in mind. The presence of a membrane, however delicate, or of any tissue having a different molecular constitution, suffices to arrest or divert the wave. I conceive, therefore, that it is absolutely indispensable that a nerve should terminate in and blend with a muscle or a centre, otherwise no stimulation of muscle or centre will take place through the nerve.
Fig. 13.
53. The difference between excitation from contact and stimulation from continuity may be thus illustrated. In [Fig. 13] we see the legs of a frog attached to the spine by the lumbar nerves (l), and lying on the muscles (m) of one leg is the nerve (c) of another frog’s leg. Applying the electrodes to (l), the muscles (m) are violently contracted; not only so, but their contraction excites the other nerve (c), and the leg attached to this nerve is thereby thrown into contraction. This “secondary contraction,” as Dubois Reymond calls it, might be supposed to be due to a diffusion of the electrical current; but that it is due to a change in the muscles (m) is proved by delicate experiments showing that the movements in the detached leg are of precisely the same kind as those in the legs directly stimulated. If there is only a muscular shock in the one case, there is only a muscular shock in the other; if there is tetanus in the one, there is tetanus in the other; if the muscles of the first leg are fatigued and respond slowly and feebly, the response of the second is slow and feeble. Moreover, the secondary contraction may be produced by chemical or mechanical stimulus, as well as by the electrical.
54. Although the contraction of a muscle is thus seen to be capable of exciting a nerve in contact with it, the reverse is not true: we can produce no contraction in a muscle by exciting a nerve simply in contact with the muscle, and not penetrating its tissue and terminating there. Accordingly we always find a nerve when about to enter a muscle or a centre losing its protecting envelopes; it gradually becomes identified as a protoplasmic thread with the protoplasm of the muscle or the centre.
55. Neurility, then, is the propagation of molecular change. Two offices are subserved by the nervous system, which may respectively be called Excitation—the disturbance of molecular tension in tissues, and consequent liberation of their energies; and Co-ordination—the direction of these several energies into combined actions. Thus, when the muscle is in a given state of molecular tension, the stimulation of its nerve will change that state, causing it to contract if it be in repose. But this stimulation, which will thus cause a contraction, will be arrested, if at the same time a more powerful stimulation reaches the antagonist muscle, or some distant centre: then the muscle only tends to contract.