Essay Iv.

THE NERVOUS SYSTEM.

Now comes the final problem. Protoplasm forms a structure always changing, always making good its waste by chemical action upon raw material, always capturing raw material or in search of it, always, when it exists in large quantities, and the labour is therefore divided between many cells, economically apportioning the work and the spoils. How is it that all the actions, chemical as well as physical, of a vast number of cells composing a large body are, no matter how complicated, always harmonious, and always with purpose directed to the advantage of the whole animal?

In the first essay in this book we discussed the phenomenon of life, and described briefly the chemical and physical peculiarities of protoplasm. These in the two succeeding essays we have gone into more fully; but there is one characteristic of that interesting substance which yet remains for us to examine in specialized cells, viz., its extreme readiness to respond to changes in its environment.

In Essay I. we saw that chemical agents, light, heat, electricity, etc.—had a definite effect upon protoplasm, and that, though they might influence different kinds in different ways, the effect was nevertheless invariable; in a word, the response of protoplasm to circumstances is automatic. But the most remarkable thing about this is that the response is not confined to the protoplasm actually affected, but is transmitted to that nearest to the part stimulated, and again passed on to that beyond, so that a wave of excitation passes through the whole mass, not stopping till it has reached the extreme confines of the cell. It may even pass beyond these and set up activity in neighbouring cells. The power of conductivity once grasped, it may easily be seen that certain cells, by specializing in this direction and adapting their shape to the needs of the body, might by throwing out long threads to reach distant parts set up an organic system of telegraphy.

The organs developed for the control of the body owe their origin to the outer layer. (See Diagram 5.) This was only to be expected. In the second essay, in which we treated of the chemistry of the body, we, of course, touched upon all three layers from which the body is built up; but the one which chiefly occupied our attention was the innermost layer, which is so admirably arranged as a chemical laboratory. In the third essay we dealt chiefly with the middle layer, which both by its position and its bulk might have been guessed to be the foundation of most of the motor organs. Now that we have come to the organs of perception and transmission of impressions, it is only natural to expect that they should be specialized from the cells already in contact with the external world, and which, since they form the envelope of the animal, must allow all such stimuli as reach the subjacent motor layer to pass through them.

Hitherto we have not dealt at great length with the development of the organs whose functions we have been describing, either from the point of view of the embryologist or the evolutionist. Nor have we spent much time upon their gross anatomy. With the nervous system we must proceed rather differently; for to understand how its higher functions can be performed they must be traced from their origin step by step, while their complexity is largely vested in the structure of special organs.

The way in which the nervous system was evolved is shown in Diagram 5. Originally, no doubt, the cells of the outer layer, when the latter was in its simplest form—that is to say, only one cell thick, not several, as it is in our skin—would, when influenced in any way directly call forth the activity of the motor cells lying beneath them. (See Diagram 40, Fig. 1.) In Fig. 2, however, we see one cell of the outer layer becoming specialized. It has thrown out a process above the surface of the skin the more readily to catch impressions, and has sent another down into the body the better to distribute them. Diagram 41, Fig. 1 shows the nerve cell at a further stage. The principle is the same, but the cell is removed to a safer place. In Fig. 2 it is not exposed to the outside world at all, but by receiving its impulses second-hand from several cells the same work is done with greater economy and uniformity. Some of the special sense organs are still developed in this way.

Once the nerve cell is developed and safely shifted into the interior of the body, it is clothed with a protecting feltwork of connective tissue, and the nerve fibres are also surrounded by connective-tissue cells which secrete around them the fatty substance which makes nerves look white.

Such is the nerve cell or intermediary between the world and the muscles; but thence to harmonious movement in a body with complex organs capable of varied actions is a long step. To obtain precision and uniformity throughout the body, all the impressions received must be collected and balanced, and stimuli, the correct outcome of this balancing, must be transmitted to the muscles, glands, etc., whose activity circumstances require. The way in which cells of the outer layer become enclosed to form a central nervous system is shown in Diagram 5; but its development will be better seen in the figures of Diagram 42.

This diagram shows how certain cells of the outer layer are budded off and transferred to a safe place within the body. In this position the cells are further developed, throwing out one long fibre, which goes to some distant organ of the body, and short fibres, which, though they do not join those of other cells and become continuous, closely interlace and put them into communication. They are also separated from one another by connective tissue, which supports them, holding them suspended with only their fibres approaching one another (Diagram 43). Diagram 44 shows how the bone which replaces the supporting rod (see Diagram 6) throws an arch round the feltwork of connective tissue in which the nerve cells are suspended, giving them still further protection.

It will be noticed in the figures of Diagram 42, which is fuller than Diagram 5, that there are three of these buds—one central and two lateral. The central one becomes a tube running the whole length of the animal, while the lateral buds form solid clusters or ganglia, arranged in pairs at intervals beside it (see Diagram 45). Fibres from these ganglia go to the skin, and bring to the nerve cells information from the outside world, which they duly pass on to the cells of the central column. The cells of the central column, when set in motion by the ganglion cells, send out impulses to the muscles, whose contraction is necessary to perform the movement which circumstances indicate. A movement brought about in this way is called reflex.

The reflex movements are, however, not quite the simplest. For instance, the food is moved along the alimentary canal by the contraction of two sets of muscle fibres—an outer longitudinal coat and an inner circular one. Between these two coats are some nerve cells, which are thrown into activity by the presence of food and the iron compounds of the bile secreted by the liver in the tube. These sympathetic cells do not send their impulses to any centre for examination, but at once stimulate the muscle fibres between which they lie, thereby producing the peristaltic movements we have already described. Yet it should be remembered that, though these cells act independently of the central nervous system, they are under its control, and can, if need be, have their action modified for the benefit of the body as a whole.

For convenience’ sake, we had better here specify the chief kinds of nervous action. First there is what we may call the immediate nerve action, such as that we have just been describing; secondly there is reflex action, the centres for which are in the spinal cord and the base of the brain; and thirdly there is voluntary movement, which arises out of the interaction of centres in the hemisphere of the brain, where the most complex machinery of all is kept.

II.

Of the first kind we need say no more. The instance of peristaltic movement illustrates it sufficiently; so we can at once begin a more careful examination of reflex action.

The simplest instance of reflex action may be taken from the schoolroom. If a boy suddenly sticks a pin into an unsuspecting schoolfellow, the latter invariably starts, and frequently lets fall an exclamation also. In this case the presence of an injurious agency is reported to the nearest motor centre, which is in the spinal cord, and this automatically convulses the body, jerking the limb out of danger.

This is reflex movement; the nerve fibre, which conveys an intimation of the injurious influence, is a prolongation, or really two prolongations, of a spinal ganglion cell. (See Diagram 46.) The near end of this fibre, which enters the cord, has several branches. Some run a little way up the cord, and some a little way down, so as to communicate with several motor cells; but one branch runs right up the cord, and sends the message on to the brain. Our outraged schoolboy starts a fraction of a second before he is conscious of the pain of being pricked, and this first response is involuntary and unvarying; the sensation, however, is reported to his brain, and the workings of that wonderful organ are less easy to predict. It leads to his taking stock of the aggressor, on the strength of which he decides whether it is safe to attempt a reprisal, and, if so, in what form it will be most effective and least likely to attract the master’s attention. This knotty point settled, the motor cells of the brain send down messages to the motor cells of different parts of the spinal cord, and these in turn set the necessary muscles in motion for delivering a surreptitious kick or aiming a splash of ink, as the case may be. This is voluntary movement.

→ shows the path taken by an impulse in reflex action.

↣ shows the path for a voluntary action.]

The difference between reflex and voluntary movement is, as may be seen from the above instances, very much a matter of degree; but we had better leave a comparison between them, and any discussion as to the extent to which the manifestations of consciousness are automatic, until we have finished describing reflex movement, and set forth the little we know about voluntary movement.

Time and space forbid a complete list of reflex movements. The following are, however, a few typical examples of how the body is automatically made to perform such acts as are necessary, and of how such as do not require deliberation are brought about without taxing the intellect.

A reflex action which is unpleasantly familiar is the cough, also the somewhat similar phenomenon of the sneeze. In this case, a foreign body which obstructs the windpipe, or causes irritation to the membrane lining the nose, is, on being reported at the spinal cord, incontinently blown out by an explosive blast of air from the lungs.

An organ which is very important, and at the same time very sensitive—viz., the eye—has many protective reflexes. The external surface of the eye is covered by a very delicate membrane, which must be kept moist and scrupulously clean. Whenever this membrane gets in the least dry, or any dust falls on it, the eyelids are closed for a moment, thereby bathing it with the secretion of the tear glands. Few people are aware, I think, that they blink their eyes on an average twice every minute. The eyes are also closed quite involuntarily by a reflex when any danger threatens them—for instance, a sudden dazzling light, a strong wind, or a blow aimed at the face; and if any foreign substance—say a fly—does get into one of them, the secretion of the tear glands is enormously increased to wash it out.

The size of the pupil, again, is quite involuntarily, i.e., reflexly, altered in proportion to the strength of the light.

Reflex actions are, however, by no means only protective. The act of swallowing is reflex. So is the secretion of the digestive glands when the lining membranes of the stomach are stimulated by the presence of food. The very act of standing depends on the reflex principle, the tendency of the body to collapse and fall being unconsciously perceived and corrected by the spinal cord. Walking is also a reflex action. It may be objected that we think about walking, and do so with intention; but it is of common experience that we can walk along ‘thinking of something else,’ and the way in which an intellectual though absent-minded man will run into people, charge lamp-posts, trip over steps, and tread upon dogs, is sufficient to absolve the organ of thought and intention from any share in the performance.

The blood-pressure is also automatically regulated, both the diameter of the bloodvessels and the frequency of the heart-beat being under reflex control; and we may, as a final instance of reflex action, describe one of Nature’s most perfect and merciful contrivances—fainting. Suppose a man receives a severe wound—say, has his hand struck off by a sword—the shock to his system causes an immediate dilatation of the large bloodvessels of the abdomen; this results in a great fall of blood-pressure, and the heart, finding that it has much less resistance to overcome, slackens its beats so that soon the flow of blood is very slow indeed. Hence, it has time to clot over the wound, and the man does not bleed to death. Incidentally, the feeble current of blood is insufficient to keep the most delicate organ of the body, the brain, in its normal state of activity, and the man is relieved from his pain by unconsciousness, which passes off when the heart again quickens its beat. It is perhaps needless to remark that fainting fits are not always and only caused by flesh wounds; they may be due to weakness or other causes.

Now, if we consider the instances quoted above, we are able to deduce a few general principles from them. In the first place, it may be noticed that reflex action compels us to perform the movements necessary to our existence whether we like it or no. It is not for us to decide whether we will breathe or not. We must. The strongest-willed man who ever lived, no matter how much a philosopher, could not commit suicide by holding his breath, as Cato boasted he could. Directly he lost consciousness, supposing he managed to hold out till then, the tainted blood bathing the respiratory centre would awake it to activity, and he would start breathing afresh. Again, it is noticeable that many of these actions could not possibly be performed by a voluntary effort. We can, to a certain extent, regulate the depth and frequency of our breathing, and we can blink our eyes voluntarily; but an average man would be quite at a loss what to do if asked to make the pupil of his eye dilate and contract, the glands of his stomach secrete, or his heart alter its rhythm.

It is a familiar fact that some reflex actions can be altered by an effort of the will; in other words, an impulse from a brain cell will prevent a nerve cell in the spinal cord from discharging. But it is an equally familiar fact that with continuous stimulation the impulses accumulate and ultimately overcome this resistance. Most people have at some time or other striven to resist the inclination to cough, consequent upon a tickling sensation in the throat, and know that there comes a time when they can restrain themselves no longer. This is because the accumulated stimuli from the throat, having reached a greater strength than the prohibitive impulse from the brain, succeed in compelling the cells in the cord to discharge.

Lastly, reflexes can be learnt. When a young child first endeavours to stand upright, the sensation of falling is doubtless conveyed to the brain, and thought taken of how the erect position can be maintained. But it is not until after many experiments and failures that the brain-cells can send messages to the right cells in the cord, and these set the necessary muscles in motion. Experience teaches what must be done, and constant practice eventually enables the spinal cord to act for itself without referring for orders to the brain. It is on the same principle that we learn to ride the bicycle. At first we have to devote our whole attention to keeping our balance, but in a short time we find we are doing it with our mind free to contemplate the scenery.

What can be done by reflex action can only be appreciated by observing an animal from which the brain has been removed. A frog which has been treated in this way—the operation, it should be said, if performed under an anæsthetic can cause no pain, either at the moment or afterwards—will live for weeks—in fact, almost indefinitely—if proper precautions be taken. But it is an automaton pure and simple. Unless touched it sits absolutely still. If touched it hops once or twice straight ahead regardless of obstacles. If placed in water it swims, equally regardless of obstacles. If turned on its back it immediately resumes its normal position. If small chips of wood are placed on its back it kicks them off. If the table on which it is sitting be tilted it will crawl up the incline until it reaches a level. But it will starve in the midst of plenty, having lost all power of thought, memory and perception. If diligently fed by hand a frog, a fish, or a bird will live for a long time without any brain, since their repertoire of movements is small and mostly reflex, and their occasions for deliberated action comparatively few. But the higher we get in the scale of life the more the brain takes over the duties of the cord, the less automatic become the greater number of the actions, and hence the more open does the animal’s conduct lie to moral criticism.

III.

We have now seen how protoplasm exists in a large body, sharing the work of living amongst specialized cells, and how it responds as a whole to the influences exerted upon it by its surroundings. The next thing to consider is how it is situated with regard to matter which does not form part of its own body; how protected from, and how put into communication with, the rest of the universe.

With regard to the former, we have seen that in the single cells, constituting unicellular organisms, there is always a bounding membrane of denser texture than the rest of the protoplasm. As the cell develops its capabilities, we have a shell or case of non-living matter secreted around it, with apertures for communication with the outside world, and increasingly effective protection is provided as protoplasm, whether in the single cell or the body, leaves the water, and has to face the inclemencies of terrestrial life.

In the schematic embryo (Diagram 6) and other diagrams contained in this volume, the skin has so far been represented as consisting of a single layer of living cells; but we must now admit that the skin of man is quite different. Such a covering would be no protection from heat, cold, or irritating chemicals, while, in order to prevent its drying up, it would have to be kept moist with slime, and we should look very like frogs. In order that an adequate defence may be provided for the body, this layer of cells divides tangentially, forming two layers. The inner of these two then divides tangentially again, and a second layer is interposed between the innermost and that first formed. The skin now consists of three layers, and so the process is repeated until it is several layers thick. (See Diagram 48.) It is the innermost and best-nourished layer which keeps dividing; the other layers, as they get pushed outwards, are only reached by a little lymph which filters between the cells, and are eventually starved even of that. As they get pushed away from the dividing layer, however, they set to work to surround themselves with a horny wall, which thickens and thickens, until eventually there is hardly any cell left. (See Diagram 49.) Finally the cells die and the horny envelopes form a dead cuticle, protecting the living layers beneath, and are ultimately sloughed off when their successors are ready to replace them.

Not even a horny layer of dead cells is, however, always sufficient protection, and the growing layer has sometimes to supplement it by hair or feathers. How hair is developed is shown in the accompanying diagram (50). The growing layer sends a strand straight downwards into the connective tissue, which forms the basement of the skin. The cells in the middle of this strand, which behaves like ordinary skin, are the least well nourished, and accordingly die and leave a tube. This tube, if no further development took place, might become a sweat gland; but if it is to give rise to a hair it becomes cup-shaped at the base, enclosing a small loop of bloodvessel. The cells just above the capillary, being better nourished than the rest, grow more rapidly than their neighbours, and the result is that a column of cells which we know as a hair pushes its way up through the tube. (See Diagram 50.)

This outer layer comes everywhere between the main bulk of the body and the outer world. Hair and sweat glands do not by any means represent its only modifications. Teeth are formed from it in somewhat the same manner as hair, while we have already seen that it gives rise to the whole nervous system.

The next thing which we have to consider is how knowledge of the external world reaches the central nervous system. Sensations of touch, temperature, and pain are fairly easy to understand, since the nerves which convey such impressions have numerous endings in the skin. End organs of nerves in the joints and muscles doubtless enable the animal to perceive and estimate strain and resistance in moving or lifting things. But the power of perceiving the chemical peculiarities of things; light, involving the formation of visual images, which we call seeing; sound; and position and equilibrium, it is not possible for the whole surface of the body to possess. The principle of division of labour is extended to the task of perception as well as to that of motion; and cells, with their property of responding to light, vibration, chemical stimulation, etc., are grouped together to form special organs, connected with the central nervous system by special nerves.

Perhaps the most important factor which can influence protoplasm is the chemical nature of its surroundings; and in the first essay, on the general nature of protoplasm, we touched upon the way in which it is drawn towards some substances, and repelled by others.

In the body there are two sets of cells deputed to act for the rest in this particular. One set is situated in the membrane lining the nose, over which the air we breathe passes; and these cells examine our gaseous surroundings, and warn us, by what we term ‘smell,’ whether the atmosphere is fit for us or we had better seek a purer. The other set is for the examination of liquids. Against these we are protected by our skin, and, as we do not absorb anything through it, it is devoid of the power of examining the things it touches. But with our food it is different; we must have the power of testing that. Accordingly, there are Customs officers in our mouth in the form of little groups of cells, which report upon the liquids and solids moistened by saliva, and enable the animal to reject pernicious imports. Thus, the stimulation of a small portion of the protoplasm composing a body is transmitted over the whole, and is able to awake in it the necessary response.

So much for the chemical sense organs; they are comparatively simple. But between a single cell, which always makes towards or always hurries out of a ray of light passing through the water in which it swims, and an animal with eyes capable of recognising the colour, shape, size, and distance of objects in space, there really does seem to be a wide gulf. It is not, however, too wide to be bridged.

After the single-cell stage has been passed, and we have beasts consisting of an inner layer of cells which is digestive in function, and an outer layer which is protective, motor, and sensory, the power of perceiving light is doubtless vested in the outer layer. When we get beasts consisting of three layers progressing along the straight path of development which leads to man, we find the outer layer becoming too opaque for this purpose, and the torch is handed on to the sensory tube derived from it. (See Diagram 5.) As more and more protection is required, the skin thickens, and the neural tube comes to lie deeper, as in Diagram 51. In order not to lose the light altogether, it has to throw out buds, which concentrate in themselves the peculiar faculty of perceiving it, and at the same time little pits are formed in the skin just over them to help the light to reach them. (See Diagram 52.) In Diagram 53 both the nervous elements and the integumentary are developing their possibilities; and in Diagram 54 a large surface has been prepared for the reception of light, and a lens formed to focus the rays upon it. Diagrams 55 and 56 give the concluding stages in the development of the eye: the formation of the cornea and its protecting eyelids. The two cavities are filled with clear liquids, and the whole eyeball supported by connective tissue.

So fascinating is everything connected with the eye that the temptation to describe it in detail is great; but in a book of rough outlines, and in consideration of the many important matters yet awaiting their turn, we must confine ourselves to briefly mentioning a few of the more important points concerning it. The light is focussed by the lens upon the nervous curtain at the back, and produces there a picture, as in the photographic camera. Thus we perceive the shape of objects. The different rays of the spectrum affect different elements in this curtain or retina, whereby we get sensations of colour. Finally, the clearness of the picture, its size, the degree of convergence of the two eyes, and the effort of focussing—for the curvature of the surface of the lens can be altered—enable us to estimate the size and distance of an object. And now, though it would take volumes to do justice to the physiology of vision, we must pass on to deal equally briefly with the functions of that no less important organ, the ear.

The essential part of the ear is a membranous bag, formed by the pouching in of the outer layer of cells—as shown in Figs. 1, 2, and 3 of Diagram 57—which comes to lie in a bony chamber beneath the skull, and assumes the somewhat complicated shape depicted in Fig. 4. We have not time, nor is it for our purpose necessary, to trace all the steps in the development of the ear, either external or internal, nor need we spend much time upon its structure, beyond indicating its position. But its position, which is shown in Diagram 58, must be grasped in order to understand how it is influenced by sound.

U, Utricle; C, cochlea; S, saccule; S.C., semicircular canals.]

It will be seen that the membranous bag, which is fitly termed the labyrinth, is situated in a bony cavity which fits so closely as to be termed the bony labyrinth (C). The membranous labyrinth is filled with a liquid, called endolymph, and the bony labyrinth (C) is also filled with a liquid, called perilymph, in which the membranous bag swims. All this is called the inner ear. The inner ear communicates with a second cavity—the middle ear (B)—by two apertures in the bony wall, which are closed by membranes. The middle ear is full, not of liquid, but of air, and is separated from the external ear, the cavity marked A, which is open to the external world, by another membrane called the tympanum, or drum, of the ear. The middle ear is connected by a tube with the throat, so that the pressure of the air on both sides of the drum may be the same.

A., Outer ear; B., middle ear; C., inner ear.]

Now, the object of this arrangement is that the ear may be able to fulfil one of its principal duties, namely, the perception of sound. Sound, as the reader is doubtless aware, is transmitted through the air as waves of condensation and rarefaction, due to the swinging backwards and forwards of its particles; it resembles the passing on of a bump along a line of trucks on the railway when the engine runs up against the end one preparatory to coupling. The magnitude of this oscillation we perceive as the loudness, the frequency as the pitch of a note. Now, when the waves of sound strike against the drum of the ear, they cause it to vibrate backwards and forwards also. Supposing there was no middle ear, and the sound waves beat directly upon the membranous windows of the inner ear, these could not be made to vibrate, as there is liquid behind them, and liquids are incompressible; so, in order that the movements of the drum may be transmitted to the liquids of the inner ear, they are carried across the middle ear by a chain of small bones, by which their extent is curtailed, but their force increased, and brought to bear upon one only of the two openings. The consequence of this is that the membrane closing it is able to vibrate and pass on the vibrations to the liquid within, since when it is pushed in, the membrane covering the other hole is pushed out.

Exactly how the different parts of the membranous labyrinth contribute to our perception of sound we do not quite know. It appears as though the difference of pressure in saccule and utricle originally conveyed to the brain a sensation of noise without any idea of quality, while the cochlea was developed later to analyze sounds and give information as to pitch and tone. Whether the rest of the labyrinth has any longer a part to play in the perception of sound, we cannot say with certainty; but it seems pretty certain that the cochlea is the organ for receiving musical impressions. Here, again, though, we are at a loss, for we do not know with certainty how the cochlea acts. In shape it is a long tube, and in the head is coiled spirally—like a snail’s shell to look at. Along its whole length is a ridge of cells with short hairs projecting from their inner surface into the liquid it contains; and to the cells along this ridge a branch of the auditory nerve is distributed. But as to whether one of the cells along this keyboard responds to each of the notes we can distinguish, or whether they are affected as a whole, physiologists are not yet agreed.

At least one other important duty the ear performs; it tells us in what position we are, and how our whole head moves or is moved. On the top of the saccule, in Diagram 57, Fig. 4, there are shown three little loops which are called the semicircular canals. They are shown again more clearly by themselves in Diagram 59.

Fig. 1 shows their position with regard to each other. It will be seen that two of them are vertical, with their loops forming a right angle with one another, and that the other is horizontal—in fact, that they lie in the three planes of space. Fig. 2 shows the structure of one of them; it has a swelling at one end (a), and a knob projecting into it where the nerve joins it (b). In Fig. 3 is shown a section through this knob, which gives the key to the use of these structures. A little head of cells projects from the wall of the canal into its lumen, and from these cells hairs bristle out into a dome-like covering of jelly, weighted, to prevent its moving too easily, with small particles of lime. Now, if you take up a round vessel full of liquid—say a bowl of gold-fish—and give it a twist round, you will notice that, though the bowl turns, the water inside does not; the fish remain in their old position. If there were a rod projecting from the side of the bowl, it would, of course, move with it, and if a fish came in its way would strike against it. This is the principle of the semicircular canal. For if we turn our head, the tube of the canal turns, passing over the liquid in it, which of course does not move, though it appears to flow in the opposite direction. The consequence is that the hairs on the side of the knob in the direction in which the head is being moved are pressed upon by the dome of jelly, which, as it floats in the liquid, tends to remain where it is. The nerves, stimulated in this way, inform the animal generally of the movement.

These little organs are very important to us, though we have our eyes to correct our ideas of position, and they are still more so to the fish, which dart and turn in the wide expanse of the ocean, and the birds and bats, which wheel about in the air. There are, however, some occasions when we do not feel inclined to bless them; for, inasmuch as they faithfully report every roll and plunge of a ship to a person on board, it is they which are mainly responsible for sea-sickness.

* * * * *

And now that we have seen how the body lies with regard to the external world; how it is efficiently protected from its surroundings; how it is placed in communication with them; and have briefly examined the organs by which it makes its chemical and physical investigations, looks out into space, and is kept aware of what is going on therein, we may return to the means whereby it responds as a whole to the stimuli thus reported—the central nervous system—and try to learn how the right response is brought about.

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.

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.

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.

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.

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.

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.

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.

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.

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.