IV.
There is but one thing more to describe in the mechanism of the body—the connecting link between the last two sections. In the last we saw how the body receives stimuli from the external world; in the one before, that when these stimuli reach the central nervous canal it in turn stimulates the organs to perform such movements as circumstances require. What, therefore, remains to be described is the working of that canal by which these necessary movements are ordered and controlled.
Now, in speaking of reflex action a few pages back, we said that the nerves which bring in stimuli from the periphery distribute them about the neural canal to those cells whose activity, by sending out fresh stimuli to the muscles, produces the requisite movements. These motor cells, however, are not scattered about the spinal cord anyhow. They are collected into clusters, or nuclei, as they are sometimes called, and each cluster has special duties—i.e., a special organ to control. Thus, we say that there are in the central nervous system centres—a nervous centre to control the leg; another to work the diaphragm; another for the muscles of the ribs; more for the arm, hand, etc. And these centres are in communication with one another, so that they may not pull different ways.
In the first example of reflex action given in [Section II. of this essay], the sensation of a pin-prick was first conveyed to the centres controlling the limb injured, by whose activity it was drawn away from the danger. But the nerve which gave the warning which produced this elementary movement distributed the impression that something was wrong to the higher centres, so that the whole body was involved in protecting, doctoring, and avenging the outraged member; from which it would appear that the lower centres are under control of higher ones. And this is the case. If we may be allowed the metaphor, there are captains of tens, who are under the direction of captains of fifties, and the captains of fifties receive their orders from captains of hundreds. The nerve canal, the manner of whose formation as a simple tube is shown in Diagrams [5] and [42], has therefore different functions in different parts, and this to such an extent that considerable differentiation in bulk and structure is produced.
The neural canal may be roughly divided into two parts—a comparatively simple tube, running the greater part of the animal’s length, containing many centres from which nerves run to the organs they control; and a complicated bulbous enlargement at one end, with thickened walls, in which are the centres controlling those in the cord, and thereby managing not so much organs as the whole animal. The former is called the spinal cord, the latter the brain.
This division, accustomed as we all are to take it for granted, offers plenty of food for reflection. Why should an animal have such a brain placed in its head? Why, indeed, should it have a head, regarding that member as a group composed of eyes, nose, mouth, ears and brain? The mouth gives us the key to the riddle; the mouth is the essential organ, and all the rest are its accessories.
In [the first essay] we saw that the basis of life was chemical, and in the second that the materials necessary for the chemical action, or food, must, in the higher animals, be taken into the digestive tube through the opening which we call the mouth. Therefore, as it is highly important that only the most beneficial substances shall be received into it, and that all which are actively injurious shall be excluded, it is plain that the organs of chemical perception must be placed in its neighbourhood—the organs of smell to enable the mouth to find its food, and the organs of taste to aid in selecting it. As, moreover, our humble ancestors, the fishes, move literally mouth foremost, it is not surprising to find the organs of space perception, the eyes, also situated in its neighbourhood, especially when one considers that their food is often of a lively character, and requires precision of movement to secure it. The inevitable consequence of thus grouping the more important organs of perception under the fore-end of the neural canal is that it grows and develops more highly here than elsewhere along its length, and soon is in a position to dictate to the rest of the body. Another reason why it must develop is that it must contain centres for turning its impressions to practical account, not only by producing complicated movements in the jaws, eyes and gills, but also by ruling the centres in the cord, and instructing the body to carry the mouth whither it needs to go.
Diagram 60.—Showing Primary Division of Nervous Tube.
In the preceding diagram ([60]) the origin of the brain is shown as a dilatation of the end of the neural canal into a bulb with thickened walls, which has already become constricted in places, so that it is subdivided into three. The next diagram ([61]) is intended to give, in no matter how crude and schematic a way, some idea of the lines on which the development continues. We do not show all, or even half, the structures which go to make up the brain. To do so would be out of place in a book like this. Further, we shall endeavour as far as possible to speak of the brain in general terms, avoiding the five-syllable bastard Græco-Latin names with which the early anatomists have endowed almost every square inch of its substance, and confine ourselves to summing up its functions as briefly as can be done with justice.
Diagram 61.—Giving a Rough Idea of how the Brain is developed.
In pursuance of this method, attention must be drawn to the fact that only the foremost of the three original bulbs (marked A in the diagram) and the hindermost (C) continue to grow. The middle one (B) remains comparatively simple. From the foremost lobe buds grow out to form the eyes in the manner which we have already described, and other buds push forwards to meet the nerves from the nose. The latter have, even in the early stages shown in [Diagram 61], reached an extraordinary size; and when we come to trace them further, we shall find that they become very complex, and acquire remarkable and unexpected powers, considering their humble origin. Strange changes also take place in the hindermost bulb. It splits along the top, so that the cavity it contains is open like a saucer, though bridged over by a three-lobed body called the cerebellum.
Following the spinal cord up into the brain, we are conscious of no sudden line of demarcation separating the one from the other, only of an increasing size and complexity. The lower parts of the brain send out and receive nerves much as the cord does; three pairs go to the muscles which turn the eyes; other pairs bring in sensations from the face and throat; others control the muscles of the face, tongue and throat. But the brain differs from the cord in being directly connected by nerves, not only with adjacent parts, but also with the distant and more important organs in the interior of the body—heart, lungs, etc.; in containing groups of cells which have stimuli sent on to them from all over the body viâ the cord; and in possessing centres which control those lower down in the nervous system. It therefore not only receives and balances stimuli from all over the body, but, by governing the centres which preside over the bodily movements, is able to wield and direct the body as a whole.
The hinder divisions of the brain, which we shall consider first, have no connection with consciousness or volition. They only produce reflex movements, which, however, owing to the wealth of material they have to work upon, are wonderfully complex and far-reaching.
Let us take a few examples. In the hindermost division of the brain (C in the diagrams) there is the centre which presides over the oxygen supply, the importance of which we saw in [the essay on vital chemistry]. This centre perceives when the lungs have been filled with a gas, and causes them to be emptied; it perceives when they are empty, and again does not allow them to remain too long in that state, before ordering an inspiration; it notes the quality of the air which is passing through the nose, and it notes the quality of the blood which bathes its own cells. The condition of the blood, indeed, is closely watched. An excessive quantity of carbonic acid gas, poverty of oxygen, even temperature, all produce through it an effect upon the rhythm of the breathing.
Close by the respiratory centre is the centre which controls the circulation. But enough has been said in [the section on reflex action], wherein the process of fainting was described, to give an idea of the part it plays in the body; so it need not detain us here.
We cannot, however, pass over its neighbour, the centre of temperature, so briefly. Its methods not only afford one of the most striking and interesting examples of harmonious regulations by reflex action, but the subject of temperature itself is so important that we must describe in some detail how that of the body is kept level.
As we said when discussing protoplasm generally, life—that is, the change always going on in the protoplasmic substance—is influenced by temperature: the single cell becomes less active at a low temperature, and dies at a high one; so obviously there is a temperature at which its functions are most easily carried on. Inside the body the cells are all kept at the temperature best for them by the circulation of the blood; but the absolute temperature of the whole body depends upon the heat which is generated within it by chemical action, and the heat which it loses to, or receives from, its surroundings. Under normal conditions this temperature in man is 98·4° F., when the production of heat from its own metabolism is balanced by the loss of heat by radiation. If, however, the atmosphere be very hot, less heat is developed in the body, the general metabolism being slower; and more heat is lost, since by reflex action the skin is bathed in sweat and cooled by its evaporation, and the small bloodvessels under the skin are dilated, so that more blood being brought to the surface, its chance of being cooled by radiation is thereby increased. If, on the other hand, the atmosphere is cool, the loss at the surface is minimized by constriction of the cutaneous bloodvessels, and a checking of the perspiration and consequent evaporation; while internally more heat is generated by increased metabolism. The cells which are mainly responsible for the production of heat are those of the muscles; and when much heat is required they increase in activity, not only in their general tone, but even by a visible movement, which we describe as shivering. So, within reasonable limits, whatever the temperature of its surroundings may be, that of the body remains the same, and though we may raise or lower our temperature by lying in a hot or cold bath, reflex adjustment of the sweat glands, bloodvessels and muscles brings it quickly back to normal when we emerge.
With a passing mention of the cerebellum, the three-lobed organ shown in [Diagram 61], and seen again in a more advanced stage in [Diagram 63], we may dismiss the two hinder divisions of the brain.
The cerebellum lies on the upward path of fibres from the cord to the higher centres in the fore-brain. It is a somewhat complicated organ, and its functions are not yet fully known. The older physiologists took a very extreme view of its importance, assigning to it, among other romantic duties, that of providing a habitation for the soul. This opinion on the strength of later research we can hardly endorse. The cerebellum really seems mainly concerned in co-ordinating the action of the muscles, especially in maintaining equilibrium in standing and walking.
Our knowledge of the whole brain is very far from complete. We should like to know the peculiar function of each little group of cells that can be made out under the microscope, and the paths of all the fibres connecting the different parts of the nervous system. As it is, we have to wait with the best patience we can while they are being investigated, and hope. In few departments, however, have the labours of the physiologist proved more fruitful and interesting than in the study of the fore-brain (A in the diagrams).
In the simpler form, as shown in [Diagram 61, A], and [Diagram 62, Fig. 1, A], the fore-brain is remarkable in that it throws out buds for the two most important sense organs—those of sight and smell. So important are these senses, especially in our humble ancestors, as we have already pointed out, that it is not surprising to find the impressions of the other senses brought on up from the hinder parts of the brain to be compared with them. The fore-brain is, in fact, a sort of terminus whither the whole of the afferent or incoming stimuli are brought, and whence, since information is only received in order to be acted upon, the supreme orders to the body issue.
In the fore-brain there are centres for specially governing all the motor organs; but by a strange arrangement the main root of the brain is overwhelmed by its own offshoot, the hemisphere, or lobe which gives rise to the olfactory bud. In fact, so great is the importance of the sense of smell to an animal whose one object in life is to find food, that, instead of the hemisphere being subordinate to its parent, it seems to take over most of the latter’s business, receiving a report of the sensations collected by it, and sending out orders upon its own initiative. Yet, unimposing though the history of this division of the brain may be, it ultimately becomes the seat of consciousness, whereby the mental processes are carried on, and whence all voluntary movements spring.
Diagram 62.—Showing how the Cerebral Hemispheres are developed from A, the Foremost Bulb of the Brain.
Of course, in order to do this, the hemispheres have to grow considerably, and thus we find them enveloping the rest of the fore-brain and swamping it in structure as well as in function. [Diagram 62] indicates how this is done, while [Diagram 63] shows roughly the proportion and position the different parts of the brain ultimately attain. Finally, [Diagram 64], which is rather more realistic, but still much simplified, presents a view of the organ in the head.
The size of the cerebral hemispheres, compared with the rest of the brain, is especially remarkable. So, too, is their endeavour to increase their surface still more by throwing it into deep folds. ([See Diagram 64.]) These two features vary with the position of the animal in the scale of development; in man, who stands highest in intelligence and dexterity, the hemispheres are very large indeed compared with the other organs, and seamed all over with a maze of winding furrows. Another remarkable feature is the extreme degree to which specialization is carried out. Different parts of the body are represented, each by a small area of the cortex, or surface layer, and we know at what spot on the cortex such sensations as sight and hearing are perceived, and from exactly what little patch the impulse to move each limb emanates. In the accompanying diagrams ([65] and [66]) these areas are mapped out, their locality being fixed by the principal folds which act as landmarks on the surface of the hemisphere.
Diagram 63.—Relation of Different Parts of the Brain.
There is another important fact which we must not omit to mention in speaking of this localization: each hemisphere presides over the opposite side of the body. Early in development the nerve fibres from the eye cross over to the opposite side of the brain, and the afferent fibres from the lower parts of the body have accordingly to follow suit. Then, as the efferent fibres—i.e., those which set the muscles in motion—have to bring about the movements in response to information received, they must also cross to get back to the side from which it came. So, if a tumour grows inside the head on the right side, it is the left eye which becomes sightless, or the left hand which grows numb and powerless, according to the part of the cortex which is pressed upon.
Perhaps the most interesting part of the whole body is that little band of the cortex running upwards from behind the temple to the crown of the head, in which (cf. Diagrams [64], [65] and [66]) the motor areas of the limbs, and the perception of those sensations which we have grouped together and called ‘touch,’ are situated. The minute structure of this region is roughly shown in section in [Diagram 67], as it has been made out with the microscope; but only a few of the nerve cells are shown, the connective tissue of feltwork in which they are suspended, and the bloodvessels by which they are nourished, being left out. All the structures represented are of course very, very small; the large black patches which represent cells would really be invisible, and the whole field of the diagram only a mere speck, to the naked eye.
Diagram 64.—Position of the Brain in the Head.
A represents the nerve by which impulses are brought in. It runs straight up to the surface of the cortex, and there its branches end, interlaced with those of a many-branched distributing cell (B). The two cells (C and D) shaped like pyramids, which send up branched processes from their apexes, receive an impulse from the distributing cell, and transmit it along the fibre which runs downwards from the middle of their base. Where the fibre from the smaller one goes to we are not sure—probably to another part of the brain to insure harmonious working—but the large pyramidal cell sends its fibre right away through the lower parts of the brain, passing the cell-stations they contain, on into the spinal cord, till it reaches the centre there, which immediately works some particular limb.
Diagram 65.—Map of the Cerebral Hemisphere, showing the Areas in which Various Functions are localized.
Supposing we anæsthetized somebody, throwing him into deep unconsciousness, and then opened his skull, laying bare the brain as is done in [Diagram 64], only not quite in such a wholesale manner. If we then stimulated the part of the brain we are now considering at different places with electric needles, using a weak induction current, we should see him moving different members according to the different regions touched—now an arm, now a leg, now the whole head. If we were to place the electrodes in the centre for the hand, and then gradually increase the strength of the current, the activity of the hand centre would throw other centres into activity. The arm would move next, raising the hand towards the face. Then the eyes would turn, and the whole head to meet the hand. Lastly the mouth would open. The movements are those of putting something into the mouth—the ruling passion strong in unconsciousness.
Diagram 66.—Map of the Cerebral Hemisphere, showing the Areas in which Various Functions are localized.
Such experiments were, of course, first made upon animals, but they have been fully verified on the human subject. The story of how this was done is not, however, a romance with a martyr or a criminal for the central figure. The corpus vile was not provided by a volunteer, or kidnapped and bound in a dark cellar, but treated as a patient in the airy wards of a hospital. With increasing knowledge of the brain, it was found that epilepsy was cortical in origin. A little piece of the cortex becomes diseased and hyperexcitable. The sufferer suddenly becomes acutely conscious of one of his members—a hand or a foot, say—not because there is anything the matter with it, but because the corresponding area in the brain is morbidly active, and he refers the sensation to the part from which it receives its nerves. The next moment the limb begins to twitch, and the excitement spreading, as in the experiment we described above, to other centres which are not diseased, they, too, become morbidly active, and the whole body is thrown into convulsions. This is a disease which must be checked as soon as possible. The surgeon accordingly lays bare the part of the brain affected, knowing now where to look; finds the exact spot which is diseased by reproducing the first twitchings of a fit by electric stimulation, and removes the source of the trouble.
Diagram 67.—Showing the System of Nerve Cells in the Cortex.
A, Afferent fibre; B, distributing cell; C, small pyramidal cell; D, large pyramidal cell.
Returning to general considerations, an important point is the way in which the different centres are connected by fibres, which put them into relation. The brain may consist of many centres, just as the body consists of many organs, but both body and brain must live as a whole. If the heart and lungs get out of harmony there is trouble, and if the bridge which connects hearing with motion in the brain breaks down, as occasionally happens for a time in an overworked man, he is mentally at a discount. He can hear and understand, but he cannot write or talk sense: he is sane, but quite helpless, and generally very frightened.
Still more important are the intermediate stations and sidings on these lines of communication, for it is here that the most exhaustive weighing and comparing of incoming stimuli is carried on—the final balancing before a voluntary action; in a word, thought.
These courts of inquiry are called association centres. It used to be believed that they were all in the fore-part of the brain, under the forehead; but this is evidently not the case. Several men in war or by accident have had the frontal lobes of their brain damaged beyond repair; and when they have been discharged from hospital, where, thanks to the advance made by surgery since anæsthetics and antiseptics were discovered, they have been successfully treated, they have gone back to their work seemingly in no way different from men whose brains were whole. In some cases they have even been reported as having become quicker and sharper than before, probably owing to there being fewer association centres, and thought being accelerated by simpler machinery: facts are thenceforward shaken through a larger-meshed sieve.
A few general considerations, and we have done. There is no centre for memory in the brain. The facts which we remember are not stored as in a box, nor can one imagine how they could be, considering that the physical basis of an idea is molecular change. The whole nervous system is probably concerned in memory, a particular change, which has momentarily occurred in its tissues, being more likely to occur again under certain circumstances than a fresh one, and certain tracks becoming well beaten and more permeable than others. Pleasure and pain are other general phenomena: they are not to be localized in the brain like vision or hearing. Pleasure is the consciousness that the whole body is under favourable conditions; and pain, the knowledge that the protoplasm of certain cells of the body is being acted upon by injurious agents, chemical or physical. There seems to be good evidence that separate nerves convey impressions of injury, distinct from those of touch and temperature; but it is the revolt of the whole body against conditions affecting a part which constitutes pain.
Amidst the maze of perplexities which lies between physiology and psychology, there is, however, one fact which stands out clear and bold: the brain can create nothing. We have seen how matter is taken into the body and matter is cast off from the body. We have seen how energy is released in the body from chemical compounds, and made use of by the body. So now, after a moment’s thought, it must be plain that every stimulus which goes to the brain must have its effect there, and that a man’s thoughts and conduct are entirely dependent on what has, at some time or other, come in from the external world. The association centres can evolve wonderful thoughts, but they are structurally derived from the grosser sense organs, and must get all the material they work upon from them.
The nervous system puts the body into relation with the external world as a whole, but for convenience it is subdivided into the afferent system, by which impressions come in, and the efferent system, by which the muscles are set in motion. Of the two halves, the afferent system has a just right to priority, for the efferent system is merely its consequence. Sights, sounds, smells, etc., reach the brain by afferent paths from the external world, and are there moulded into thoughts. Their effects we see in poetry, architecture, sculpture, or laundry work, according to the method of the brain in treating the raw material it receives, and of a quality corresponding to the fineness with which the brain examines them, and can control the motor organs of the body.
Whatever goes in at the afferent door, and some people’s sensory apparatus is much more easily affected than others, produces its effect within. Sometimes the energy is expended in thought, sometimes in action; sometimes it trickles away as laughter. But all these phenomena have a material basis: matter producing changes in matter. ‘Those delicate tissues wherein the soul transacts its earthly business,’ as Stevenson so picturesquely describes the brain, stick to their earthly business. There is no astral department opened yet. A man may evolve a great idea from the data he receives, but he must give it a material coefficient if he does not wish it to be lost to his earth-bound brothers. He may write it in a book, or he may sculpture it in marble; but the most convenient means of communicating with his fellows is by sound, which he can command by expelling the air in his lungs over vibrating cords in his throat. These cords are adjusted at the position and tension to give a desired note; and the cavities of the chest, throat and mouth acting as resonators, a noise is produced, which is shaped by the tongue, lips and teeth into words.
By means of language the human body is enabled to co-operate with others of its kind for the development of the resources of the earth, the shaping of society, and the forming of individual character. But here physiology ends and other sciences begin.