Experimental evidence points to the following conclusions: (1) Resistance is offered at a synapse. This resistance must be overcome before an impulse can get through from neurone 1 to neurone 2. (2) The impulse does not, properly speaking, pass from 1, through 2. It infects 2, causing it to discharge a fresh impulse. (3) Time is of the essence of this process. Either the impulses head up at the synapse, or, passing through into the neurone, they produce a cumulative effect within it, which provokes it to discharge. (The latter hypothesis, which is the less likely of the two, transfers the resistance from the synapse to the neurone to be infected.) These conclusions are based upon experiments of the following kind: The minimal stimulus which will evoke a reflex action is determined. A stronger stimulus is then applied. The reflex occurs more promptly, and is more pronounced. But on further increasing the stimulus, it is found that the limit of effectiveness is soon reached. The proportional relation of response to stimulus is much less evident than it is when the experiment is tried with a nerve-muscle. Choosing a reflex action easily provoked, the afferent path is stimulated with an electric current interrupted fifty times a second. The impulses which flow down the efferent path to the muscle follow one another at the rate of about ten a second. A column of nerve-fibres within the spinal cord is stimulated fifty times a second. Again, the discharge into anterior roots has the natural rhythm of about ten. The cortex of the “motor area” of the great brain is stimulated with a rapidly interrupted current. The muscles which it governs contract with their natural rhythm. The cortex is sliced away, and the stimulus applied to the white matter beneath. A similar result is obtained. Evidence such as this points to an independence of action on the part of the neurones which one can express only in terms of resistance and explosion. But there is another line of thought which leads to the development of a picture of the working nervous system which seems at first sight incompatible with the one that we have sketched. The phenomenon of the knee-jerk ([p. 274]) reveals a nervous system so intimately linked together, so homogeneous, so mobile, that no event, however trivial, occurs in any part without sending a vibration throughout the rest. Instead of a multitude of batteries enveloped in a labyrinth of wires interrupted by myriads of switches which are crackling on and off, the image of a sheet of water better figures our conception—a material so frictionless that it is a-ripple from side to side and end to end, from the most distant rivulet which feeds it to the farthest trickle in which it drains away. It is a fluid in a state of infinite commotion, the movements of its particles varying in amplitude from tremulous quiverings which scarcely frost the silver of its surface to waves which, breaking on the muscular system, throw it up in heaps. The vinegar experiment seems to demand a scheme of batteries and wires. The knee-jerk points to a continuous conducting medium. Other phenomena suggest the superposition of the two pictures; the conception of a nervous system consisting of a uniform medium conducting, not indifferently in all directions, but with such freedom that from our point of view the paths are infinite in number; and within this conducting medium nerve-cell bodies and their processes which collect and distribute groups of vibrations sufficiently strong in combination to produce visible effects. In order that one of these neurones may be stimulated to discharging-point, the medium by which it is surrounded must be thrown into such a state of agitation as suffices to infect it. The considerations which point to the formulation of this double or superposed scheme are such as follow: The passage of tone-impulses does not appear compatible with the ideas we have formed on other evidence of synaptic resistance and neuronic discharge. They are too feeble for such a mechanism. The short “reflex time” of the knee-jerk points to the passage of the agitation up a sensory root to the spinal cord, and through a non-resistant medium to the environment of the motor cells which it discharges, missing the neurone or neurones which intervene in the case of ordinary reflex actions. This is an illustration of the way in which tone-impulses, which we imagine as conducted by the non-resistant medium, pass over into discharges which produce visible effects. Again, the phenomena of inhibition appear to require the supposition of extra-neuronic conduction. Whenever a reflex path is in use, all other paths in its neighbourhood are closed. The passage of impulses leading to a particular reflex action is favoured by the suppression of conduction in its vicinity. When A is talking to D through the nerve-telephone, B and C are compelled to hold their peace. Inhibition is a phenomenon of universal occurrence. In relation to various actions, it is sufficiently pronounced to be visible in the effects which it produces. A simple experiment will illustrate this. Holding water in the mouth has no effect upon respiration, but during the act of swallowing respiratory movements are suspended. Whilst the swallowing reflex is occurring the respiratory reflex is inhibited. This might be attributed to the volitional control of respiration, and certainly when attention is being directed to the process volition plays a large part. But if a finger is placed on the pulse, it is possible to detect that, during the act of swallowing, the pulse quickens, owing to the suppression of the slowing action of the vagus upon the heart. Here is a case in which inhibition is in no degree a voluntary action. Nor is it of any value as an adjunct to the particular reflex with which it is associated. It is an illustration of the universal rule that activity of any one spot in the nervous system is the cause of the quieting of the surrounding area. Impulses which reflexly check the heart cannot get through the medulla oblongata whilst the swallowing impulses are traversing it. Inhibition has been described as a drainage of nerve-force into the active area. On the structural side it seems to require the conception of an extra-neuronic substance which, agitated in the vicinity of the cells which are to be discharged, is brought to rest around neighbouring cells. The promulgation through the nervous system of the state which, when it reaches the centres of consciousness, produces pain also seems to call for an hypothesis of extra-neuronic conduction.
Any reference to pain in a work on physiology needs a few words of preface, since popularly the term “pain” is used in various senses. When I see pink geranium and nasturtiums growing in the same flower-bed, I may exclaim: “It is positively painful.” The want of harmony, and at the same time the insufficiency of contrast, of chalky pink and translucent orange, jars my æsthetic sense. Dislikes, however well founded, are ruled out in thinking of the physiology of pain. Further, in defining pain, we must be careful to isolate the real thing, and not to confuse it with sensations which seem to lead up to it. If, putting my finger in a pair of pincers, I touch it as lightly as possible, the first sensation is one of contact; a little harder, and it becomes a sense of pressure; harder still, and all sense of contact or pressure is lost in pain. It is usual to regard pain as sensation carried to excess. But neither is this physiological. An excessively bright light or an excessively loud sound is disagreeable. It causes a sudden movement for the purpose of avoiding it—just such a movement as one would make if one touched a red-hot poker—but it is not, strictly speaking, painful. Not uncommonly in cases of accident or disease of the spinal cord a sharp distinction is drawn between the sense of touch and the capacity for experiencing pain. Below the injury the patient retains his sense of touch undiminished in acuteness, but no blow, or cut, or burn, causes him any pain. The pain caused by squeezing the finger in a pair of pincers is not, therefore, an excess of touch sensation. Pain begins to be experienced in the skin just when the object applied to it is affecting it to an extent which might do harm. If the point of a needle touches it, it causes pain as soon as the pressure is a trifle less than that needed to pierce its surface. A hot object begins to hurt when the temperature reaches 48° C.—almost enough to coagulate the tissue fluids. Pain is not a discriminative sensation. If I hold my arm out at right angles, I am conscious for the first few minutes of its weight, and have, besides, some sense of the traction exerted by the muscle of the shoulder. At the end of ten minutes these sensations are merged in pain, and for some time after lowering the arm the shoulder-muscle aches, much as it does in rheumatism. Pain is an effect upon consciousness, which absorbs, engulfs, and therefore obliterates sensation. To use an ancient phrase, “It is less that I feel pain than that I am pain.” If we speak of the capacity for pain as a sense, we may call it for the purpose of our present argument the “sense of damage.” The nerves of the skin are acutely affected by any agent which is likely to do harm. It is their business to convey to the central nervous system an influence which so affects it as to set up in consciousness the condition of pain. Sensations of damage evoke reflex movements by means of which the part of the body likely to be injured, or the whole body, is removed to a safe distance. It being the duty of the skin to give this warning, a service of nerves sensitive to noxious agents has been developed which scouts in co-operation with the services devoted to the recognition of physical contact and heat and cold ([cf. p. 425]). If, imagining that the fire has not been lighted, I touch an almost red-hot stove, I acquire quite a considerable amount of information of which I am able to make use. I gain an accurate notion of the situation of the stove, and I put the right part of my finger in my mouth. The skin sends to the brain the ordinary sensations of touch and pressure before the condition of pain is established. In seeking for a definition of pain, we must eliminate the two attributes which have characterized all the forms of stimulation which we have considered up to the present time: (1) The tendency to provoke movement; (2) the supply of information. If I am suffering from a whitlow, the last thing that I am disposed to do is to jerk my finger about. Although it enhances the urgency of skin-reflexes, pain, in general, inhibits movement instead of provoking it. This is well illustrated in pleurisy. So long as a man is healthy he is quite unconscious of the fact that at each respiration the lower part of the lung slides on the lining of the chest-wall; but commencing inflammation on the surface of one of the lungs causes intense susceptibility to friction, and the pain produces an effect which the man is quite unable to produce by an effort of will; it stops the movements of the chest on the damaged side. Pain is inhibitory, not stimulant. It is not, properly speaking, a sensation. Frequently being mixed with sensational elements, it conveys topographical information; but pure pain approaches in quality the nebulous sense of distress of a patient who, when asked where he felt it, replied: “Nowhere; but there is a deal of it in the room.”
Sufferers describe pain in figurative language, as “burning,” “stabbing,” “throbbing,” “aching,” and so forth. Two persons afflicted with the same lesion, the same source of pain, use approximately the same terms. Hence we cannot say that pains do not differ in character. But this is not a sufficient reason for assigning any specific quality to pain. It varies in severity, in continuity or intermittence, in suddenness of onset, in the sensations which accompany it, in the emotional tone to which the disturbance of the organ from which it proceeds gives rise, in the tenseness of the part affected and its consequent sensitiveness to a throbbing pulse. All these things make a complex of pain plus sensation, which causes toothache to differ from headache, and both from the pain of burned skin. But they do not give specific qualities to different varieties of pain. This being the case, there is no need to presume the existence of special nerve-endings for the reception of pain, or of a special region of the cortex of the brain for its reception. On the contrary, the evidence is conclusive that the nerve-fibres which serve the more highly specialized senses, which have well-defined connections in the cortex of the brain, do not convey the influence which enters consciousness as pain. It is the innumerable nerves which have no specialized receptors that take up pain. The afferent nerves of the viscera—the vagus and sympathetic—convey no impulses which enter consciousness, so long as the tissues which they supply are healthy. They have no representation in the cortex. The organs with which they are connected (with trivial exceptions, easily accounted for) are absolutely insensitive to injury. Before the virtues of chloroform were known—in the days when, however severe the operation, the patient had to nerve himself to bear it without an anæsthetic—surgeons proved that the liver or the intestines, or practically any other viscus, may be cut or cauterized without the patient being aware that it is being touched. The same is equally true of the brain itself. But if damage in a viscus is set up gradually, its nerves convey to the central system an agitation which has the most pronounced results upon consciousness, and on the way profoundly affects the reflex actions which the spinal cord can carry out, and also its capacity as a conductor. Once in his life, perhaps, a man passes a gall-stone; for generations such a thing may not have happened in his family. Yet the man finds that he is provided with a nervous apparatus which conveys to consciousness intensest pain.
It is difficult to think of pain as travelling along nerves in the form of rhythmic impulses, similar to those which produce in consciousness the effects which we have distinguished as sensations. A few lines above we stated that no impulses which affect consciousness normally travel up the vagus or the sympathetic nerve, limiting the term “impulse,” perhaps unjustifiably. The vagus conveys an influence which enters our experience, as hunger. Probably other states of feeling for which we have no names, which resemble pain and hunger and their opposites, are set up through the agency of visceral nerves.
Fifty years ago attention was called to the difficulty of finding pain-paths amongst the white tracts (nerve-fibres) of the spinal cord. It is as difficult to point them out now as it was then; but the inference that pain travels up the grey matter has given way to the “neurone theory”; under a misapprehension as the writer holds. Pain travels slowly. If one happens to notice a person who unsuspiciously touches a hot surface, one observes that an interval elapses between contact of his finger with the iron and the exclamation with which he “relieves his feelings.” It amounts to more than a second—if the iron is not very hot, to several seconds—whereas the “reaction-time” for touch is only one-seventh of a second. The slowness of movement of pain through the nervous system can on the neurone theory be explained only on the hypothesis that it travels from link to link along a very long chain of very short neurones. That pain is a state of the grey matter rather than a succession of impulses, and that (within the cerebro-spinal axis) the state is transmitted through an extra-neuronic medium, seems a simpler explanation.
The state set up in the segment of the cord in which afferent fibres, conveying pain from viscera, embouch affects its conductivity. It subdues reflex action through the segment, and at the same time facilitates or reinforces the transmission of sensory impulses towards the seat of consciousness. This shows itself in the apparent increased sensitiveness of the skin of the area of the surface supplied by the posterior root which joins the segment of the spinal cord into which the pain influence is also being poured. For example, afferent sympathetic nerves from the cardiac end of the stomach join the sixth and seventh thoracic spinal nerves. Other afferent fibres run up the vagus to the medulla oblongata. When the cardiac end of the stomach is diseased, pain is referred to the skin area supplied by the sixth and seventh dorsal roots. The ordinary inevitable stimuli acting upon this area cause pain. Experimental stimuli which elsewhere would be felt as touch or warmth are painful. The impulses to which they give rise pass through pain-agitated segments of the spinal cord. The vagus nerve carries its pain influence to the medulla oblongata. Now, it happens that the sensory nerve of the face—the fifth—spreads for a considerable distance up and down the axis of the brain. The fifth nerve in consequence pours its sensory impulses into a region which is pain-agitated by those fibres of the vagus which come from the cardiac end of the stomach. Hence disease of that organ gives rise also to an “illusion” of pain—pains and illusions of pain are philosophically indistinguishable—on the surface of the head. The viscera, having no direct access to consciousness, appear by deputy. When the stomach is distressed, it makes its appeal to the whole body politic for considerate treatment through certain nerves which have the privilege of appearing at Court. The message is misread as coming from the front of the chest—“heart-burn”—or from the shoulder, or from the scalp, or from the other skin areas which these nerves serve. When the liver is in trouble, consciousness, having no knowledge of its whereabouts—is it the business of hand and eye to explore another man’s liver, or incumbent upon the mind to accept their findings?—infers that the cry comes from the shoulder. Nor have the tissues beneath the root of the nail, or the muscle of the shoulder, or the pulp of a tooth, any direct representation in consciousness; but since the pain-condition in the grey matter converts it into a microphone, messages from neighbouring structures which otherwise would fail to arouse attention, after traversing the pain-segments of the nervous system, ring out clearly, and hence the mind locates approximately the “pain” of the whitlow, the muscle-ache, the decayed tooth. Sufferers from toothache are familiar with the phenomenon of the spread of pain from a definite spot to the whole jaw or the whole side of the head, dependent upon the spread of the pain-agitation from the segment of the axis of the brain in which the dental nerve ends to neighbouring segments. Our ability or inability to localize a pain does not depend upon the presence or absence of pain-nerves, but upon the existence or non-existence of nerves coming from the same organ, or from its neighbourhood, and capable of conveying impulses to the seat of consciousness. In passing through the part of the spinal cord or of the axis of the brain which is disturbed by the influence exercised by a damaged organ, silent impulses acquire force sufficient to render them audible, and combine with the pain to produce a feeling which consciousness can analyse, to a certain extent. Informed as to its whereabouts by these accentuated sensations, consciousness recognizes a sense of pain limited in its topographical extension.
Sneezing when a bright light falls upon the eye is a curious illustration of the exaggeration of the effectiveness of sensory impulses when they happen to be poured into an agitated segment of grey matter. About one person in every three is affected in this way. A friend of the writer, who was particularly sensitive, rising in the night because he heard his child cry, three times lighted a candle and three times sneezed it out before he could watch the application of match to wick without suffering from a nerve-storm. Some nervous dogs—especially fox-terriers—are very liable to this neurosis. Many persons who do not sneeze feel, when the sunshine stimulates their retinæ, a tickling in the nose. Again the illusion is to be traced to the door of the fifth nerve—the sensory nerve of the whole of the face. The nose is the true tip of the body. Morphologically it is anterior to the eyes. Just as the fifth nerve extends its distribution to the nose, so also its root-fibres extend their connection within the axis of the brain forwards, until they traverse the mid-brain, the primary centre of the optic nerve. A bright light, by stimulating the optic nerve, sets up a commotion in the mid-brain. The ordinary every-moment impulses from the nose, carried by the fifth nerve to this region, ought not to appear in consciousness at all; but owing to the excited condition in which they find the grey matter they assume an importance which does not belong to them, and discharge the reflex action of sneezing, just as they would do had one taken snuff. Several lessons are to be learned from this phenomenon—as, for example, one which cannot be too often impressed, that the impulses which appear in consciousness (or, more accurately, the impulses to which attention is directed) are but a most insignificant fraction of those delivered by sense-organs to the central nervous system. The impulses which give rise to the sensation of tickling in the nose are not exceptional impulses which happened to be started when the light fell on the eye. They were reaching the brain in a steady flow before the agitation of the mid-brain gave to them exceptional force. No consideration regarding the working of the nervous system has a more important bearing than this. We cannot picture to ourselves the activity of the sensory nervous system. Our experience is limited to the scattered sensations which we perceive. Are the sensory nerve-endings incessantly responding to external forces, throwing an almost continuous procession of impulses up each of the millions of nerve-fibres which connect them with the central system? Such a conception is probably nearer to the truth than the conception which we should develop if we trusted to experience. Yet even experience tells us that an infinity of messages is delivered to the brain, of which consciousness takes no account. Changing trains at a roadside station in France, my attention was attracted by an electric bell on the platform, which was ringing continuously. “Why does the bell ring?” I asked the station-master. “To make known that everything goes well,” was the response. “If it stops, something is wrong.” “But do you not become so accustomed to it that you cease to hear it?” “Yes, truly; it rings day and night. One does not pay attention to it until it has stopped.” Sensory impulses generated by the contact of my skin with the chair that I am sitting on are incessantly ringing the bell of consciousness. I should notice them immediately if they stopped. As it is, they do not attract my attention until they ring a little louder than usual, or until some particular group, owing to unrelieved pressure, produces a cumulative effect. Another lesson; that the condition of the nervous system, and therefore its conductivity, is determined at any given moment by the sensory impulses which are reaching it. We cannot describe the effect of a bright light as pain, yet it agitates the grey matter, altering its state, in the same way as the nerve-inflow which we recognize as pain. A wet rag on the forehead does not assuage a headache by cooling the brain ([cf. p. 106]). The headache is “in the scalp.” The cool wet rag diminishes the dilation of the bloodvessels of the forehead, and quiets the impulses from the skin which are pouring into a tract of grey matter pain-agitated by the influences ascending a visceral nerve—usually the vagus.
It is necessary to warn the reader that a reversion to the old idea of “conduction through grey matter”—i.e., otherwise than by a chain of neurones—is unorthodox. It is set forth here because it seems to the writer that the various phenomena which have to be accounted for fit in best with the hypothesis of a double path. If evidence of the anatomical possibility of extra-neuronic conduction is asked for, it may be pointed out that the chrome-silver and methylene-blue methods, upon which our knowledge of neurones is based, do not, in the very nature of the case, show that grey matter consists only of neurones and their obvious branches. As they select particular elements of structure, we can never by their use alone know what they fail to show. Attention may also be called to the fact that the same staining process which reveals pericellular nets ([p. 301]) shows also a structure resembling a network in the substance which intervenes between them. Truly the method is a rough one. It may well be thought that the nitric acid used to fix the tissue may cause strange coagulations with solution of uncoagulated substance; but, as was remarked with regard to the pericellular nets, regular patterns indicate architectural differentiation. But whether these nets do or do not give hints as to the nature of the conducting medium, there is no difficulty in finding sufficient material, after all the substance entering into the formation of the conducting neurones, as we imagine them, has been accounted for. Ex hypothesi, the conducting material is provided by the fibrils of the sensory nerves in their extensions beyond the limits to which the deposit of subchromate of silver extends, when the chrome-silver method of displaying neurones has been used. Sensation-impulses enter neuronic chains. The condition which, when it affects the seat of consciousness, is known as pain, progresses up the vertebrate neuropil.
Energy is developed within the nervous system. The force of impulses is adjusted to the resistance which they have to overcome. Stimulation of the millions of twigs of the vagus nerve in the lungs brings about the gentle movements of ribs and diaphragm which constitute peaceful respiration. A crumb of bread touching the mucous membrane of the larynx stimulates a few of the endings of the same vagus nerve. Like an avalanche, the impulses gather head as they advance, causing, not the diaphragm and intercostal muscles alone to do their utmost, but calling into action half a dozen accessory muscles of respiration. It is difficult to account for this reverberation of the messages which clamour for the ejection of the crumb of bread without figuring them as spreading from neurone to neurone, urging each in turn to deliver its maximal discharge.
Neurones are provided with material which serves as a store of energy. In their cell-bodies, including their dendrites, are to be seen coarse granules of nucleo-protein, which, being fitted in between groups of neuro-fibrillæ, assume an angular form. They are known as Nissl’s corpuscles, or are termed “tigroids,” owing to the spotted appearance which they give to the substance of a cell. If the nerve-cells of birds be examined just after they have alighted from a migratory flight, the granules are found to be few and small. In a bee returning to the hive at evening with its last load of pollen, they are smaller than they were when it commenced its morning’s work. They disappear in certain pathological conditions, and under the influence of various drugs; and since their presence is revealed by staining, their disappearance is spoken of as “chromatolysis.”