161. The foregoing remarks have had the object of showing how little substantial aid Psychology can at present derive from what is known of the elementary structure of the nervous system, indispensable as an accurate knowledge of that structure must be to a complete analysis of its functions. This caution has been specially addressed to those medical and psychological students whose researches leave them insufficient leisure to pursue microscopical investigations for themselves, and who are therefore forced to rely on second-hand knowledge, which is usually defective in the many qualifying considerations which keep scepticism vigilant. Relying on positive statements, and delusive diagrams which only display what the observer imagines, not what he actually sees, they construct on such data theories of disease, or of mental processes; or else they translate observed facts into the terms of this imaginary anatomy, and offer the translation as a new contribution to Science.

162. But little aid as can at present be derived from the teaching of the microscope, some aid Psychology may even now derive from it. The teaching will often serve, for instance, to correct the precipitate conclusions of subjective analysis, which present artificial distinctions as real distinctions, separating what Nature has united. It will show certain organic connections not previously suspected; and since whatever is organically connected cannot functionally be separated, such sharply marked analytical distinctions as those of periphery and centre, or of sensation and motion, must be only regarded as artificial aids. The demonstration of the indissoluble union of the tissues is a demonstration of their functional co-operation. So also the anatomical demonstration of the similarity and continuity of all parts of the central system sets aside the analytical separation of one centre from another, except as a convenient artifice; proving that cerebral substance is one with spinal substance, having the same properties, the same laws of action.

For the present, Psychology must seek objective aid from Physiology and Pathology rather than from elementary Anatomy. In the paragraphs which are to follow I shall endeavor to select the chief laws of nervous activity which the researches of physiologists and pathologists disclose. By these laws we may direct and control psychological research.

THE ENERGY OF NEURILITY.

163. Vitality is characterized by incessant molecular movement, both of composition and decomposition, in the building up of structure and the liberation of energy. The life of every organism is a complex of changes, each of which directly or indirectly affects the statical and dynamical relations, each being the resultant of many co-operant forces. In the nourishment of every organite there is an accumulation of molecular tension, that is to say, stored-up energy in a latent state, ready to be expended in the activity of that organite; and this expenditure may take place in a steady flow, or in a sudden gush. The molecular movements under one aspect may be called convergent, or formative: they build the structure, and tend to the state of equilibrium which we call the statical condition of the organite, i.e. the condition in which it is not active, but ready to act. Perfect equilibrium is of course never attained, owing to the incessant molecular change: indeed Life is inconsistent with complete repose. Under another aspect the molecular movements may be called discharging: they constitute the dynamic condition of the organite, in which its functional activity appears. The energy is now diverted, liberated, and the surplus, over and above that which is absorbed in formation, instead of slowly dribbling off, gushes forth in a directed stream. The slow formation of a secretion in a gland-cell, and the discharge of that secretion, will illustrate this; or (if muscular tone be admitted) the incipient contraction of the chronic state, and the complete contraction of the dynamic state, may also be cited.

164. The discharge which follows excitation may thus be viewed as a directed quantity of molecular movement. Because it is always strictly relative to the energy of tension, and is inevitable when that tension attains a certain surplus over what is required in construction, there is a limit, 1°, to the growth and evolution of every organite, and every organism (comp. Problem I. §118), and, 2°, to its dynamical effect. When there is no surplus, the organite is incapable of discharge: it is then exhausted, i.e. will not respond to stimulus.

165. The speciality of nerve-tissue is its pre-eminence in directive energy. Like all other tissues, it grows, develops, and dies; but above all others it has what we call excitability, or readiness in discharging its energy in a directed stream. By its topographical distribution it plays the functional part of exciting the activity of other tissues: it transmits molecular disturbance from periphery to centre, from centre to centre, and from centre to muscles, vessels, and glands. When a muscle is excited it moves, and when a gland is excited it secretes; but these actions end, so to speak, with themselves; the muscle does not directly move any other muscle;184 the gland does not directly excite any other gland. The nerve, on the contrary, has always a wide-spreading effect; it excites a centre which is continuous with other centres; and in exciting one muscle, usually excites a group. Hence the nervous system is that which binds the different organs into a dynamic unity. And Comparative Anatomy teaches that there is a parallelism between the development of this system and the efficient complexity of the organism. As the tissues become more and more specialized, and the organs more and more individualized, they would become more and more unsuited to the general service of the organism, were it not that a corresponding development of the nervous system brought a unifying mechanism.

The great instability of neurine, in other words, its high degree of tension, renders it especially apt to disturb the tension of other tissues. It is very variable; and this variability will have to be taken into account in explaining the restriction of discharges to particular centres. A good example of exaggerated tension is furnished by strychnine poisoning. The centres are then so readily excitable that a touch, or a puff of cold air on the skin, will determine convulsions. And it is worthy of remark that for some hours after this convulsive discharge the centres return to something like their normal state; and the animal may then be stroked, pinched, or blown upon without abnormal reactions. But during this interval the centres are slowly accumulating excess of tension from the poisoned blood; and at the close, convulsions will again follow the slightest stimulus. This alternation of exhaustion and recrudescence is noticed by SchrÖder van der Kolk in the periodicity of the phenomena exhibited in spinal disease.185

THE PROPAGATION OF EXCITATION.

166. Understanding, then, that the propagation of an excitation depends on the state of tension of the tissue, and always follows the line of least resistance, whichever that may be at the moment, we have to inquire whether the transmission takes place only in one direction, from periphery to centre in sensory nerves, and from centre to periphery in motor nerves? By most physiologists this is answered affirmatively. Indeed a special property has been assigned to each nerve, in virtue of this imaginary limitation of centripetal and centrifugal conduction. The “nerve-current” (accepted as a physical fact, and not simply a metaphor) is supposed to “flow” from the central cells along the motor nerve to the muscles; but by a strange oversight the current is also made to “flow” towards the central cells which are said to produce it! Now although the fact may be, and probably is, that normally the sensory nerve, being stimulated at its peripheral end, propagates the stimulation towards the centre, and the motor nerve propagates its central stimulation towards the periphery, the question whether each nerve is not capable of transmission in both directions is not thus answered. A priori it is irrational to assert that nerves fundamentally alike in composition and structure are unlike in properties; and we might as well suppose that a train of gunpowder could only be fired at one end, as to suppose that a nerve could only be excited at one end. And how does the evidence support this a priori conclusion? Dubois Reymond proved that each nerve conducted electricity in both directions; but as Neurility has not been satisfactorily shown to be identical with the electric current, this may not be considered decisive. Such a doubt does not hang over the following facts. M. Paul Bert, pursuing John Hunter’s curious experiments on animal grafting, has grafted the tail of a rat under the skin of the rat’s back, the tip of the tail being inserted under the skin, its base rising into the air, so that there is here an inversion of the normal position. In the course of time Sensibility gradually reappears in this grafted tail; and at the end of about twelve months the rat not only feels when the tail is pinched, but knows where the irritation lies, and turns round to bite the pincers.186 Here we have a case of a sensory nerve reversed, yet transmitting stimulation from the base to the tip of the tail, instead of from the tip to the base, as in a normal organ. Vulpian and Philippeaux having divided two nerves, united the central end of the sensory nerve with the peripheral end of the motor nerve; when the organic union was complete, and each nerve was formed out of the halves of two different nerves, the effect of pinching one of these was to produce simultaneously pain and movement, showing that the excitation was transmitted upwards to the centre, and downwards to the muscles.187 It may be compared with a train of gunpowder having a loaded cannon at one end and a bundle of straw at the other, when if a spark be dropped anywhere on this train, the flame runs along in both directions, explodes the cannon, and sets alight the straw.

167. Indeed we have only to remember the semi-liquid nature of the axis cylinder to see at once that it must conduct a wave of motion as readily in one direction as in another. A liquid transmits waves in any direction according to the initial impulse. There is consequently no reason for asserting that because the usual direction is centripetal in a sensory nerve, and centrifugal in a motor nerve, each nerve is incapable of transmitting excitations in both directions. And I think many phenomena are more intelligible on the assumption that neural transmission is in both directions. If the eye is fixed steadfastly on a particular color during some minutes, the retina becomes exhausted, and no longer responds to the stimulus of that color: here the stimulation is of course centripetal. But if instead of looking intently on the color, the mind (in complete absence of light) pictures it intently, this cerebral image is equally capable of exhausting the retina; and unless we believe that color is a cerebral, not a retinal phenomenon (which is my private opinion), we must accept this as proof of a centrifugal excitation of a sensory tract. Another illustration may be drawn from the muscular sense. There may be a few sensory fibres distributed to muscles; but even if the observations of Sachs188 should be confirmed, I do not think that all muscle sensations can be assigned to these fibres, but that the so-called motor fibres must also co-operate. When a nerve acts upon a muscle, the muscle reacts on the nerve; and when a nerve acts on a centre, the centre reacts on the nerve. The agitation of the central tissue cannot leave the nerve which blends with it unaffected; the agitation of the muscular tissue must also by a reversal of the “current” affect its nerve. Laplace points out how the movement of the hand which holds a suspended chain is propagated along the chain to its terminus, and if when the chain is at rest we once more set that terminus in motion, the vibration will remount to the hand.189 The contraction of a muscle will not only stimulate the sensory fibres distributed through it, but also, I conceive, stimulate the very motor fibres which caused the contraction, since these fibres blend with the muscle.190

168. To understand this, it is necessary to remember that the stimulation of a nerve does not arise191 in the changed state of that nerve, but in the process of change, i.e. the disturbance of the tension. The duration of the stimulation is that of the changing process, and the intensity increases with the differential of the velocity of change. So that when a nerve which has been excited by a change of state returns to its former state, this return—being another change—is a new excitation. That it is not the changed state, but the change, which is operative, explains the fact noted by Brown SÉquard: a frog poisoned by strychnine, when decapitated and all respiration destroyed, will remain motionless for days together, if carefully protected from all external excitation; but its nervous system is in such a state of tension all this time that the first touch produces general convulsions. Freusberg also notes that if a brainless frog be suspended by the lower jaw, and one foot be pinched, the other leg is moved at first, then quickly droops again, and remains at rest until the pincers are removed from the pinched foot, when suddenly all four legs are violently moved by the stimulation which the simple removal produces. Let us also add the well-known and significant fact that if a nerve be divided rapidly by a sharp razor, neither sensation nor motion is produced, because the intensity of a stimulus being, to speak mathematically, the function of the changing process, the duration of the process is in this case too brief. On the same ground the application of a stimulus will excite no movement, if the force be very slowly increased from zero to an intensity which will destroy the nerve; but at any stage a sudden increase will excite a movement.

169. We may group all the foregoing considerations in this formula:

Law I. Every neural process is due to a sudden disturbance of the molecular tension. The liberated energy is discharged along the lines of least resistance.

The conditions which determine the lines of least resistance are manifold and variable. The nervous system is a continuous whole, each part of which is connected with diverse organs; but in spite of this anatomical diversity, the deeper uniformity causes the activity of each part to depend on and involve the activity of every other, more or less. By “more or less” is meant, that although the excitation of one part necessarily affects the state of all the others, because of their structural community, so that each sensation and each motion really represents a change in the whole organism, yet the responsive discharge determined in each organ by this change, depends on the tension of the organ and its centre at that moment. A bad harvest really affects the whole nation; but its effect is conspicuous on the welfare of the poor rather than of the rich, although the price of bread is the same to rich and poor. Nervous centres, and muscular or glandular organs, differ in their excitability; one condition of this greater excitability being the greater frequency with which they are called into activity. The medulla oblongata is normally more excitable than the medulla spinalis; the heart more than the limbs. Hence a stimulus which will increase the respiration and the pulse may have no appreciable effect on the limbs; but some effect it must have.

170. Imagine all the nerve-centres to be a connected group of bells varying in size. Every agitation of the connecting wire will more or less agitate all the bells; but since some are heavier than others, and some of the cranks less movable, there will be many vibrations of the wire which will cause some bells to sound, others simply to oscillate without sounding, and others not sensibly to oscillate. Even some of the lighter bells will not ring if any external pressure arrests them; or if they are already ringing, the added impulses, not being rhythmically timed, will arrest the ringing. So the stimulus of a sensory nerve agitates its centre, and through it the whole system; usually the stimulation is mainly reflected on the group of muscles innervated from that centre, because this is the readiest path of discharge; but it sometimes does not mainly discharge along this path, the line of least resistance lying in another direction; and the discharge never takes this path without also irradiating upwards and downwards through the central tissue. Thus irradiated, it falls into the general stream of neural processes; and according to the state in which the various centres are at the moment it modifies their activity. A nervous shock—physical or mental—sensibly affects all the organs. A severe wound paralyzes, for a time, parts far removed from the wounded spot. A blow on the stomach will arrest the heart; a fright will do the same. Terror relaxes the limbs, or sets them trembling; so does a concussion: if a frog be thrown violently on the ground, all its muscles are convulsed; but if the nerves of one limb be divided before the shock, the muscles of that limb will not be convulsed.

171. We are apt to regard the discharge on the moving organs as if that were the sole response of a stimulation; but although the most conspicuous, it is by no means the most important effect. Besides exciting the muscles, more or less, every neural process has its influence on the organic processes of secretion, and effects thermal and electrical changes. Schiff has demonstrated that every sensation raises the temperature of the brain; Nothnagel, that irritation of a sensory nerve causes constriction of the cerebral arteries, and hence cerebral anÆmia. Brown SÉquard and Lombard find the temperature of a limb raised when its skin is pinched, and lowered when the skin elsewhere is pinched. Georges Pouchet has shown that fishes change color according to the brightness or darkness of the ground over which they remain; and these changes are dependent on nervous stimulation, mainly through the eye, division of the optic nerves preventing the change. These are so many a posteriori confirmations of what a priori may be foreseen. They are cited here merely to enforce the consideration, seldom adequately kept before the mind, that every neural process is a change which causes other changes in the whole organism.

STIMULI.

172. Stimuli are classed as external and internal, or physical and physiological. The one class comprises all the agencies in the External Medium which appreciably affect the organism; the other class all the changes in the organism which appreciably disturb the equilibrium of any organ. Although the pressure of the atmosphere, for example, unquestionably affects the organism, and determines organic processes, it is not reckoned as a stimulus unless the effect become appreciable under sudden variations of the pressure. In like manner the blood is not reckoned among the internal stimuli, except when sudden variations in its composition, or its circulation, determine appreciable changes. Because the external stimuli, and the so-called Senses which respond to them, are more conspicuous than the internal stimuli and the Systemic Senses, they have unfortunately usurped too much attention. The massive influence of the Systemic Sensations in determining the desires, volitions, and conceptions of mankind has not been adequately recognized. Yet every one knows the effect of impure air, or a congested liver, in swaying the mental mood; and how a heavy meal interferes with muscular and mental exertion.192 What is conspicuous in such marked effects, is less conspicuously, but not less necessarily, present in slighter stimuli.

173. A constant pressure on the tympanum excites no sound; only a rhythmic alternation of pressures will excite the sensation. A constant temperature is not felt; only changes in temperature. If Light and Sound were as uniform as the circulation of the blood, or the pressure of the atmosphere, we should be seldom conscious of the existence of these stimuli. But because the changes are varied and marked, our attention is necessarily arrested by them. The changes going on within the tissues are too graduated to fix the attention; it is only by considering their cumulative effects that we become impressed with their importance. For example, the development of the sexual glands determines conspicuous physical and moral results—we note consequent effects on voice, hair, horns, structure of the skull and size of the muscles, no less than the rise of new feelings, desires, instincts, ideas. Any organic interference with the activity of the ovaries will alter the moral disposition of the animal: suppression of this organic process means non-development of the feelings of maternity; the moral superstructure is absent because its physical basis is wanting.

174. Blood supplies the tissues with their plasmodes; a constant supply of oxygenated blood is therefore necessary to the vitality of the tissues. But it is an error to suppose that oxygen is the special stimulus of nerve-centres, or that their activity depends on their oxidation; on the contrary, the deficiency of oxygen or surplus of carbonic acid is that which stimulates. When saturated with oxygen, the blood paralyzes respiration; when some of the oxygen is withdrawn, respiration revives. Here—as in all other cases—we have to remember that differences in degree readily pass into differences in kind, so that an excess of a stimulus produces a reversal of the effect; thus although surplus of carbonic acid excites respiratory movements, excess of carbonic acid causes Asphyxia. Abundance of blood is requisite for the continuous activity of nerve-centres; but while a temporary deficiency of blood renders them more excitable, too great a deficiency paralyzes them. AnÆmia, which causes great excitability, and convulsions (so that nerves when dying are most irritable), may easily become the cause of the death of the tissue. There are substances which can only be dissolved by a given quantity of liquid; if this quantity be in excess, they are precipitated from the solution. There are vibrations of a given order which cause each string to respond; change the special order, and the string returns to its repose.

In the stillness and darkness of the night we are excluded from most of the external stimuli, yet a massive stream of systemic sensations keeps the sensitive mechanism active, and in sleep directs the dreams. The cramps and epileptiform attacks which occur during sleep are most probably due to the over-excitability produced by surplus carbonic acid. To temporary anÆmia may be assigned the strange exaggeration of our sensations during the moments which precede awakening; and the greater vividness of dream-images.

It is only needful to mention in passing the varied stimuli by which cerebral changes act upon the organism. The mention of a name will cause a blush, a brightening of the eye, a quickening of the pulse. The thought of her absent infant will cause a flow of milk in the mother’s breast.

175. We may formulate the foregoing considerations in another law:

Law II. The neural excitation, which is itself a change, directly causes a change in the organ innervated, and indirectly in the whole organism.

The significance of this law is, that although for the convenience of research and exposition we isolate one organ from the rest of the organism, and one process from all the co-operant processes, we have to remember that this is an artifice, and that in reality there is no such separation.

STIMULATION.

176. Passing now from these general considerations to their special application, we may formulate the law of stimulation:

Law III. A faint or moderate stimulation increases the activity of the organ; but beyond a certain limit, increase of stimulation diminishes, and finally arrests, the activity. Duration of stimulation is equivalent to increase.

A muscle stimulated contracts; if the stimulation be repeated, the muscle becomes tetanized, and in this state has reached its limit; a fresh stimulation then relaxes the muscle. A very faint stimulation of the vagus quickens the pulsation of the heart, but a slight increase, or duration of the stimulation, slackens and arrests the heart.193 Every one knows how a moderate feeling of surprise, pleasure, or pain quickens the heart and the respiration; and how a shock of surprise, joy, grief, or great physical pain depresses, and even arrests them. Excess of light is blinding; excess of sound deafening.

177. The nervous system is incessantly stimulated, and variably. Hence a great variation in the excitability of different parts. While the regular and moderate activity of one part is accompanied by a regular flow of blood to it, so that there is a tolerably constant rhythm of nutrition and discharge, any irregular or excessive activity exhausts it, until there has been a nutritive restoration. We can thus understand how one centre may be temporarily exhausted while a neighboring centre is vigorous. Cayrade decapitated a frog, and suspended light weights to each of its hind legs; when either leg was stimulated, the weight attached to it was raised. After each repetition the weight was raised less and less, until finally the weight ceased to be raised: the centre had been exhausted. But now when the other leg, which had been in repose, was stimulated, it energetically contracted, and raised its attached weight; showing that its centre was not exhausted by the action of the other.194

178. This seems in contradiction with the principle that the excitation of one centre is an excitation of all. It also seems in contradiction with the principle urged by Herzen, that irritation of one sciatic nerve diminishes the excitability of the opposite leg; and this again seems contradicted by the principle urged by Setschenow, that although moderate excitation of one sciatic nerve will diminish the excitability of the other, a powerful excitation will increase it.

179. All three principles are, I believe, exact expressions of experimental evidence; and their seeming contradictions may be reconciled on a wider survey of the laws of neural activity, interpreted according to the special conditions of each case. These laws may be conveniently classified as laws of Discharge, and Laws of Arrest; the second being only a particular aspect of the first.

THE LAW OF DISCHARGE.

180. The physiological independence of organs, together with their intimate dependence in the organism, and the fact that this organism is incessantly stimulated from many sides at once, assure us a priori that the “waves” of molecular movement due to each stimulus must sometimes interfere and sometimes blend with others, thus diverting or neutralizing the final discharge in the one case, and in the other case swelling the current and increasing the energy of the discharge. We are accustomed to speak of one part “playing on another,” sympathizing with another, and so on; but what is the process expressed in these metaphors? When an idea, or a painful sensation, quickens the pulse, or increases the flow of a secretion, we are not to imagine that from a spot in the cerebrum, or the surface, there is a nerve-fibre going directly to the heart, or the gland, transmitting an impulse; in each case the central tissue has been agitated by a sudden change at the stimulated point, and the discharge on heart and gland is the resultant of this agitation along the lines of least resistance. The nerves of the great toe, for example, pass into the spinal cord at a considerable distance from the spot where the nerves of the arm enter it; when, therefore, the great toe is pinched, the arm does not move by direct stimulation of its nerves, but by the indirect stimulation which has traversed the whole central substance.

181. This is intelligible when we know that the whole central substance is continuous throughout; but the difficulty arises when we have to explain why, if this central substance is stimulated throughout, only arms and legs respond; in other words, why the toe-centre “plays upon” the arm-centre, and not on the others? When a frog is decapitated, if we gently touch one leg with the point of the scalpel, the leg will move, but only this leg. Prick more forcibly, and both legs will move. Keep on pricking, and all four legs are drawn up, and the frog hops away. Each excitation was propagated along the cord; but the discharge was restricted in the first case to one limb, in the second to two, in the third it involved all the muscles of the trunk. At the sight of a friend a dog wags his tail gently: as there is no direct connection between the optic nerves and the tail, this playing of one centre on another must be by the agency of intermediate centres; and we know that if the dog’s spinal cord be divided, this excitation from the optic centre is no longer possible, yet the tail will wag if the abdomen be tickled, or the leg pinched. Now compare the effect on the dog produced by the sight of his master, or of a friend accustomed to take him out. There is no longer a gentle wagging of the tail, but an agitation of the whole body: he barks, leaps, and runs about; the central stimulation is discharged through many outlets; and could we test the effect, we should find an appreciable alteration in the thermal and electrical condition of the whole organism, with corresponding changes in circulation, secretion, etc. So different are the consequences of two slightly different retinal impressions mingling their stimulations with the same mass of central substance!

182. The discharge is determined by two conditions: the state of tension, and the energy of the stimulation. The state of tension is increased by every stimulation which falls short of a discharge; that is to say, faint and frequent stimulation augments the excitability, whereas powerful stimulation exhausts it. When, therefore, one wave succeeds another in the same direction, it reaches a centre more disposed to discharge; or, as Cayrade expresses it, “a certain agitation of the cells is necessary for the manifestation of their property of reaction, in the same way that the concentric circles produced on the surface of water by a falling stone are more rapid and more numerous if a stone has already agitated the surface.”

183. So much for the tension. What has been called the energy of the stimulation is more complicated. It is not measurable as a simple physical process; we cannot say that a given quantity of any external force will determine a given discharge. It is mostly complicated by psychical processes, and these so modify the result that instead of the predicted discharge there is arrest, or discharge from another centre. Press a dog’s skin with increasing violence, and the effect increases from pleasurable to painful irritation; but whether the dog will cry out and bite, or cry out and struggle to escape, depends upon whether the pincher is a stranger or a friend. If you hurt a dog while removing a thorn from its foot it will cry out, but although the pain causes it to initiate a biting movement, by the time your hand is reached that movement will have been changed, and the dog will lick the hand which he knows is hurting him in the endeavor to relieve him of the thorn. The co-operation of the mind is here evident enough. A purely psychical process has interfered with the purely physiological process. And I shall hereafter endeavor to show that psychical processes analogous in kind though simpler in degree are really co-operant in actions of the spinal cord. The dog would be said to discriminate between the pain inflicted by a friend, and the same pain inflicted by a stranger. In other words, the sensitive mechanism would be differently determined in the direction of discharge, although the initial stimulation was the same in each case. If we admit that the resulting action is in each case the consequence of the particular group of elements co-operating, there will be no ground for denying that analogous discrimination is manifested by the brainless animal, who also responds differently to different external stimuli, and differently to the same stimulus under different central conditions. The brainless frog croaks if its back be gently stroked with the handle of a scalpel; but if the point be used, or if the handle be roughly pressed, instead of croaking, the frog raises his leg in defence. Here the difference in the peripheral irritation has excited a different reaction in the centre; and this might be interpreted as purely physical; if now the leg be fastened, and the movement of defence be thus prevented, the frog will employ the other leg; or adopt some other means of relieving itself from the irritation. It was a mass of registered experiences which determined the dog not to bite his master. An analogous registration of experiences determines the changed reactions of the brainless frog. But this is a point which can only be touched on in passing here, and it is touched on merely to facilitate our exposition of the complicated conditions of neural discharge. These may be formulated in

184. Law IV. The simultaneous influence of several stimuli, each of which separately excites the same centre, is cumulative: stimuli then assist each other, and their resultant is their arithmetical sum.

Simultaneous stimuli, each of which excites a different centre, interfere with each other’s energy, and their resultant is their algebraical sum.

In this law there is a condensed expression of that composition of forces which may either result in Discharge or Arrest. By simultaneity is not to be understood merely the coincidence of impressions, but also the reverberations of impressions not yet neutralized by others. Thus when Sensibility is tested by the now common method,195 it is found that if one leg is withdrawn after a lapse of, say, ten pendulum beats, the other leg, which has not been irritated, will nevertheless, on irritation, be withdrawn in less than ten beats, provided the central agitation caused by the first stimulation has not yet subsided. But, on the contrary, the withdrawal will be considerably deferred, or even prevented altogether, if at the same time that the leg is acted on by the acid, a more powerful excitation takes place in some other part of the body. In the one experiment we see simultaneous excitation in the same centre and the same direction. In the other simultaneous excitation in different centres. The more powerful excitation suppresses the discharge from the less powerful; but although it prevails, it loses just as much force as it arrests.196

185. There is another very interesting experiment by Freusberg, which must be cited here.197 When the sciatic nerve is divided, the frog’s leg is of course not withdrawn from the acidulated water, because in that case no sensory excitation is propagated from the skin to the centre; but although there is no stimulation from the skin, there is one from the muscles, as appears in the fact that if a small weight be suspended on this leg, the other leg is more rapidly withdrawn from the acidulated water—the action of the muscles having affected the centre and increased its excitability.

186. When the motor group of one leg is moderately stimulated, the discharge is confined to the muscles of that one leg; and according to Herzen the excitability of the motor group of the other leg is thereby somewhat diminished. But if the stimulation be increased, there is an irradiation to the other group, which irradiation, although not sufficient to excite a discharge, renders it much more ready to discharge, so that a feeble stimulus suffices. This accords with Setschenow’s observations, and is confirmed by Freusberg’s experiment, in which, when one leg was stimulated by acid, if the acid were not wiped off but allowed to keep up the irritation, the other leg moved without being irritated; and this other leg having come to rest, when in its turn dipped in the acid, was more rapidly withdrawn than the first leg had been on first being stimulated; showing that the central groups had become more excitable by the stimulation of either leg.

187. While it is intelligible that an excitation of one group should increase the activity of neighboring groups, by an increase of the vascular activity of the region, it is not so readily intelligible why the feebler excitation of one group should diminish the excitability of its neighbor; yet the facts seem to warrant both statements.

188. The conditions which determine Discharge are obscure. We may, however, say that anatomical and physiological data force the conclusion that whenever the central tissue is powerfully stimulated in any one part, there is either a discharge, or a greater tension (tendency to discharge) in every other part; in consequence of which, every fresh stimulus in the same direction finds the parts more prepared to react; while every fresh stimulus in a contrary direction meets with a proportional resistance. Stated thus generally, the principle is clear enough; but the immense complication of stimulations, and the statical variableness of the organs, renders its application to particular cases extremely obscure. Why does the ticking of a clock arrest the attention, even with unpleasant obtrusiveness, at one time, and presently afterwards cease to be heard at all? Why does the cut of a knife cause intense pain, and a far greater cut received during the heat and agitation of a quarrel pass unfelt? Why will the same external force excite convulsions in all the muscles, and at another time scarcely be distinguishable? These are consequences of the temporary condition of the centres; but there are permanent conditions which in some organisms determine equally variable results. Thus the shock of terror which will simply agitate one person, will develop an epileptic attack in another, and insanity in a third; just as exposure to cold will in one person congest the liver, in another the lungs. A loud and sudden sound causes winking in most persons, and in many a sort of convulsive shock. The harsh noise of a file causes a shiver in some persons, and in others “sets the teeth on edge,” while in others it causes an increased flow of saliva.

189. Nerves and centres have different degrees of excitability. The nerve-terminals in the skin are more sensitive to impressions than those in the mucous membrane; those in the alimentary canal are more sensitive than those in the peritoneum; and all nerve-terminals are more sensitive than nerve-trunks. A touch on the surface of the larynx will produce a cough, but the nerve-trunk itself may be pinched or galvanized without producing any such reflex. Moreover, there is the difference of grouping. If the skin of the abdomen be tickled, there is a reflex on the adductor and extensor muscles of the leg; but these movements are reversed if the skin of the back be tickled. Nor indeed are these movements invariable in either case; the one series will sometimes quite suddenly change to the other, if the irritation is kept up. That one and the same stimulus applied to the same spot should now excite this group and now the other, shows that both motor groups are affected, and that the discharge takes place from the one which at the time being is in the highest tension. The alternation of tension explains rhythmical discharge.

THE LAW OF ARREST.

190. The Law of Arrest is only another aspect of the Law of Discharge, and may be regarded as the conflict of excitations. If a stranger enters the room where a woman lies in labor, there will often be caused a sudden cessation of the uterine contractions.198 Again, every one knows how the breathing and the beating of the heart are arrested by the idea of danger. The arrest is in each of the three cases only temporary, because when the shock of the new stimulus has caused its discharge (arrest), the peripheral irritation which caused the former discharges resumes its influence, and uterus, heart, and diaphragm begin to move again, even more energetically. Note, moreover, that not only will the cerebral excitation arrest the spinal discharge—an idea check the contractions of the uterus or the heart—but the reverse also takes place. The brain of the woman may be intently occupied with some scheme for the education or welfare of her expected child, but no sooner do the labor pains set in, than all these cerebral combinations are arrested.

191. One sensation arrests another; one idea displaces another. If the foreleg of a headless frog be irritated, the hind-leg will also be moved by the stimulation; or vice versa. Here there has been a propagation of the excitation in either direction. But if while the legs are thus irritated, and the centres are ready to discharge, another and more powerful irritation reach the centre—say by pinching the skin of the back—there will be no discharge on the legs. If the vagus be irritated, the heart is arrested; but this does not take place if at the same time, or immediately before, the foot has been sharply pinched. A few gentle taps on the abdomen suffice to stop the heart; but if a drop of acid be previously placed on the skin, we tap in vain, the heart continues to beat. Brown SÉquard cites several cases in which convulsions were arrested by irritation of sensitive surfaces;199 and Dr. Crichton Browne records a case of a patient in whom there was abolition of spinal reflex, due to cerebral irritation: tickling the soles of the feet, or pricking the toes, which normally excites reflex movements, in this case excited none whatever. “This seems to prove that nerve currents, set in motion by irritation of the brain, or some of its convolutions, transmitted down the cord, may inhibit reflex action.”200 Examples might indefinitely be multiplied. Pinch the skin of a rabbit between the eyes, and you will observe that pulse and respiration are slackened; but if the tail, which is very sensitive, be pinched, this slackening is only momentary, and is succeeded by a quickening—unless the pain be great. Even the effect of intense pain may be neutralized by stimulating the vagus—just as the effect of stimulating the vagus may be neutralized by pain. Claude Bernard found that having dropped ammonia on the eyelid of a dog, the pain caused a convulsive closure of the lid; but on galvanizing the vagus, the lid opened again, to be closed when the galvanism ceased.201 When the heart is beating faintly (as in syncope), any irritating vapor applied to the nostrils will cause a more energetic pulsation; yet a very irritating vapor lowers the action of the heart beating normally, and will even arrest that of a rabbit. Over-stimulation has almost always the opposite effect of moderate stimulation.

192. While there seems every reason to believe that an excitation necessarily affects the whole cerebro-spinal axis, there is no doubt that there is a certain restriction of this irradiation to definite paths, i.e. the responsive discharge is confined to definite groups. Some of these restrictions are connate pathways: we bring them with us at birth; but most of them are pathways acquired after birth. The boy who sheds tears at parting from his mother when he goes to school, will shed no tears when he parts from her to go to college, nay, perhaps will shed none when he parts from her forever: not that his love has lessened, but that the idea of such expression of it as “unmanly” has become an organized tendency and arrests the tears. A youth of southern race, who has not learned to be ashamed of tears, weeps freely under such circumstances.

193. The pathways organized at birth are not many. Examples are the inspiration which follows expiration; the movements of coughing when the larynx is tickled; the movements of swallowing, sneezing, etc. Even these may be arrested for a brief time by what is called “the will”; but when once the discharge begins in any part of the mechanism, the whole group is necessarily involved and the action is then inevitable. Many of the reflex actions which are universal are nevertheless acquired. Winking, for instance, when an object approaches the eye, is universal among us, but is never seen in infants, nor in animals. It is even doubtful whether the drawing up of the leg when the toes are pinched is not an acquired reflex. Doubtful, I mean, in this sense, that although the fact of non-withdrawal is observable in infants, who cannot localize their sensations, this may be due to the imperfect development of their nervous system. Mr. Spalding has proved that although the callow bird cannot fly, the mechanism of flight is no sooner developed than the action follows at once, without any previous tentative experiences.

194. By experience we learn to restrict the paths of irradiation, so as to wink with one eye while the other is unmoved, to bend one finger while the rest are extended, to move one limb, or one group of muscles, while the others are at rest; in short, to execute any one particular action, and not at the same time agitate superfluously many other organs. The boy when first learning to write is unable to prevent the simultaneous motions of tongue and legs, which are ludicrously irrelevant to the purpose of writing; but he learns to keep all his organs in subjection, and only the eyes and hands active.202 An analogous restriction takes place in thinking. A train of thought is kept up by the exclusion of all suggestions which are not pertinent; and the power of the thinker is precisely this power of concentration.

THE HYPOTHESIS OF INHIBITORY CENTRES.

195. The facts and their formulated laws which have just been adduced furnish a sufficient explanation of all the phenomena of arrest which of late years have been detached and assigned to a special mechanism of inhibitory nerves and centres. In spite of the eminent authorities countenancing the hypothesis of a particular set of inhibitory nerves, and particular centres of inhibition, I must confess that the hypothesis appears to me inadmissible; and that I side with those physiologists who hold that each nerve and each centre has its inhibitory action. Indeed, if the action of arrest be, as I maintain, only another aspect of the action of discharge, the result of the conflict of forces, to say that all centres have the property of excitation, is to say that all have the properties of discharge and arrest: the discharge is only the resultant of the conflict along the line of least resistance; the arrest is the effect of the conflict along the line of greatest resistance. The observed phenomena of arrest are so varied and numerous that the upholders of the inhibitory hypothesis have been forced to invent not only arresting centres, but centres which arrest these arresting centres! Dr. Lauder Brunton candidly remarks: “At present our notions of nervous action seem to be getting as involved as the Ptolemaic system of astronomy, and just as epicycles became heaped upon cycles, so nerve-centres are being added to nerve-centres. And yet, clumsy though the system may be, it serves at present a useful purpose, and may give real aid until a better is discovered.” I do not think a Copernicus is needed to discover a better. The Law of Arrest as a general neural law suffices, when the right conception of a centre as a physiological rather than an anatomical designation is admitted. (See p. 173.)

196. It would be out of place here to consider the conflicting evidence which at present renders the question of the movements of the heart one of the most unsatisfactory in the whole range of experimental physiology. After devoting much time to it, and after writing a long chapter on it, I suppress what I had written, and content myself with the statement that no advantage whatever is derived from the hypothesis of a special mechanism of arrest, unless perhaps in giving a temporary precision to the direction of research. I mean that the search for special centres may lead to the discovery of the particular paths to which an impulse is restricted in any one action: as, for instance, the vagus in retarding the pulsation of the heart. If the cerebrum can determine a movement, and combine various movements, it is a centre of arrest; if the cerebellum can determine and regulate movements, it is a centre of arrest; if the medulla oblongata can determine and regulate movements, it is a centre of arrest; if the medulla spinalis can determine and combine movements, it is a centre of arrest; if a nerve can dilate a constricted blood-vessel, or constrict a dilated one, it is a nerve of arrest. In other words, every centre exerts its action either in discharging, or in arresting the discharge of some other centre.

The physiological process of Arrest may be physically interpreted as Interference;203 not that the process in nerve-tissue is to be understood as the same as that observed in fluids, or that the metaphor of neural waves is to be taken for more than an intelligible picturing of the process; the difference in the two agents forbids our admitting the resemblance to be more than analogical. Thus interpreted, however, we see that not only will one centre arrest the action of another, but one nerve may be made to arrest itself! I mean that, under similar conditions of interference, the stimulation which normally follows on external stimulus may be inhibited by a previous, or a counter stimulation. Thus the nerve which will be stimulated by a chemical or mechanical stimulus, wholly fails to react if a constant current is passing through it, although this constant current does not itself cause a constant contraction. Remove the electrodes, and then the chemical or mechanical stimulus takes effect. Or the experiment may be reversed: let the nerve be placed in a saline solution, and the muscles will be at once thrown into violent contraction; if the electrodes are now applied to the nerve, the contractions suddenly cease, to begin again directly the electrodes are removed.

ANATOMICAL INTERPRETATION OF THE LAWS.

197. The problem for the anatomist is twofold: First, given the organ, he has to determine its function, or vice versa, given the part of an organ, to determine its functional relation; secondly, given the function, he has to determine its organ. The structural and functional relations of nerves and centres have been ascertained in a general way; we are quite sure that the posterior nerves carry excitations from sensitive surfaces, that the anterior nerves carry excitations to muscles and glands; and that the central gray substance not only reflects a sensory excitation as a motor excitation, but propagates an excitation along the whole cerebro-spinal axis. But when we come to a more minute analysis of the functional activities, and endeavor to assign their respective values to each part of the organic mechanism, the excessive complexity and delicacy of the mechanism baffles research. We are forced to grope our way; and the light of the hypothetic lamps which we hold aloft as often misdirects as helps us. The imaginary anatomy which at present gains acceptance, no doubt seems to simplify explanations; but this seeming turns out to be illusory when closely examined. The imagined arrangement of fibres and cells we have seen to be not in agreement with observation; and were it demonstrable, it would not account for the laws of propagation. Suppose sensory fibres to terminate in cells, and fibres from these to pass upwards to other sensory cells and transversely to motor cells, how in such a connected system could irradiations take place, if the law of isolated conduction were true? And how could isolated conduction take place, if the excitation of a part were necessarily the excitation of the whole? Why, for example, is pain not always irradiated? Why is it even localized in particular spots, determining movements in particular muscles; and when irradiation takes place, why is it circumscribed, or—and this is very noteworthy—manifested in two widely different places, the intercostal and trigeminal nerves? Why does the irritation of intestinal worms manifest itself now by troubles of vision, now by noises in the ear, and now by convulsions?

198. Answers to such questions must be sought elsewhere. Our first search should be directed to the anatomical data, which have hitherto been so imprudently disregarded. Under the guidance of the laws formulated in this chapter, let us accept the anatomical fact of a vast network forming the ground-substance in which cells and fibres are embedded, and with which they are continuous; let us accept the physiological principle Of similarity of property with similarity of composition and structure; let us accept the hypothesis that the discharge of neural energy is dependent on the degree of stimulus and the degree of tension at the time being—and we shall have at least a general theory of the process, though there will still remain great obscurities in particular applications. We shall have before us a vast network of pathways, all equally capable of conducting an excitation, but not all equally and at all moments open. It will always be difficult to determine what are the conditions which at any moment favor or obstruct particular openings. Paths that have been frequently traversed will of course be more readily traversed again; but this very facility will sometimes be an obstacle, since it will have caused that path to be preoccupied, or have fatigued the organ to which it leads.

199. Since the escape of an excitation must always be along the lines of least resistance, an obvious explanation of the restriction to certain paths has been to assume that some fibres and cells have naturally greater resistance than others. But this explanation is simply a restatement of the fact in other words. What is this greater resistance? Why is it present in one fibre rather than in another? We should first have to settle whether the resistance was in the nervous pathway itself, or in the centre, or in the organ innervated; an excitation might pass along the nervous tract, yet fail to change the state of the centre, or the organ, sufficiently to produce an appreciable response; and only those parts where an appreciable response was produced would then be considered as having had the pathways of propagation open.

200. When we reflect on the innumerable stimulations to which the organism is subjected from so many various points, and remember further that each stimulation leaves behind it a tremor which does not immediately subside, we shall conceive something of the excessive complexity of the mechanism, and marvel how any order is established in the chaos. What we must firmly establish in our minds is that the mechanism is essentially a fluctuating one, its elements being combined, recombined, and resolved under infinite variations of stimulation. If it were a mechanism of fixed relations, such as we find in machines, or in the “mechanism of the heavens,” we might accept the notion of certain organites having greater resistance as a consequence of their structure, just as one muscle resists being moved by the impulse which will move another. Nor is it doubtful that such differences exist in nervous organites; but the laws of central excitation are not interpretable by any such hypothesis, since we know that the paths which were closed against an impulse of considerable energy may be all open to an impulse of feebler energy, and that a slight variation in the stimulus will be followed by a wide irradiation. For example, a grain or two of snuff will excite the violent and complex act of sneezing, but the nerves of the nasal cavity may be pinched, cut, or rubbed, without producing any such result. One group of nervous organites will fail to involve the activity of neighboring groups; and the simple movement of a single organ is then all that appreciably follows the stimulation; yet by a slight change in the stimulation, the organites are somewhat differently grouped, and the result is a complex movement of many organs. It is this fluctuation of combination in the organites which renders education and progress possible. Those combinations which have very frequently been repeated acquire at last an automatic certainty.

We are now in a position to examine with more precision the extremely important laws of nervous action which are involved in the phenomena designated by the terms Reflex Action, Automatic Action, and Voluntary Action.