Chapter XI: The Nervous System (2)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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