41. Observation having found that the activity of a nerve was always followed by a sensation when the nerve ended in a centre, and by a movement when the nerve ended in a muscle, Theory was called upon to disclose the nature of this peculiar property of nerves. That a peculiar and mysterious power did act in the nerves no one doubted; the only doubt was as to its nature. The ancient hypothesis of Animal Spirits seemed all that was needed. The spirits coursed along the nerves, and obeyed the mandates of the Soul. When this hypothesis fell into discredit, its place was successively taken by the hypotheses of Nervous Fluid, Electricity, and Nerve Force. The Fluid, though never manifested to Sense, was firmly believed in, even so late as the days of Cuvier;95 but when the so-called electrical currents were detected in nerves, and the nervous phenomena were shown to resemble electrical phenomena, there was a general agreement in adopting the electrical hypothesis. The brain then took the place of a galvanic battery; the nerves were its electrodes.

42. Closer comparison of the phenomena detected various irreconcilable differences, which, if they proved nothing else, proved that nerve-action took place under conditions so special as to demand a special designation. Electricity itself is so little understood, that until its nature is more precisely known, we cannot confidently say more than that nerve-action resembles electrical-action; meanwhile the speciality of neural conditions renders all deduction illusory which is based on electrical-action as observed under other conditions. In presence of these difficulties, cautious physiologists content themselves with assigning the observed phenomena to the observed and inferred conditions, condensing these in the convenient symbol “nerve-force,” without pretending to any specification of the nature of that force. It may be a wave of molecular movement dependent on isometric change or on metamorphic change. It may be the liberation of molecular tension resembling electricity; it may be electricity itself. But whatever the nature of the change, it is an activity of the tissue, and as such comes under the general dynamic conception of Force or Energy.

43. In this sense the term has nothing equivocal or obscure. It is a shorthand expression symbolizing certain well-defined observations. Nevertheless, it is a term which we shall do well to avoid when possible, and to replace by another having less danger of misinterpretation; the reason being that Force has become a sort of shibboleth, and a will-o’-wisp to speculative minds. All that we know of Force is Motion. But this is too meagre for metempirical thinkers, who disdain the familiar experiences expressed in the term Motion, and demand a transcendent cause “to account” for what is observed. They seek an entity to account for the fact. Motion is a very definite conception, expressing precise experiences; we know what it means, and know that the laws of moving bodies admit of the nicest calculation. A similar precision belongs to Force when understood as “mass acceleration,” or M V². But this does not content those metaphysicians who understand by Force “the unknown reality behind the phenomena”—the cause of Motion. This cause they refuse to recognize in some antecedent motion (what I have termed a “differential pressure”), but demand for it a physical or metaphysical agent: the physical agent being a subtle fluid of the nature of Ether, or a nerve atmosphere surrounding the molecules; the metaphysical agent being a Spirit or aggregate of Soul-atoms. The second alternative we may decline here to discuss. The first alternative is not only a pure fiction, but one which is inconsistent with the demonstrable velocity of the neural process, which is not greater than the pace of a greyhound, whereas the velocities of light and electricity are enormously beyond this. It is inconsistent also with the observation that a much feebler current of electricity is requisite for the stimulation of a muscle through its nerve than when directly applied to the muscle: a proof that the nerve does not act solely by transmission of electricity—unless we gratuitously assume that the nerve is a multiplicator.

When it is said that the living nerve is incessantly liberating Force which can be communicated to other tissues, the statement is acceptable only if we reject the metaphysical conceptions it will too generally suggest—the conceptions of Force as an entity, and of its being passed from one object to another like an arrow shot from a bow. The physical interpretation simply says that the molecules of the nerve are incessantly vibrating, and with varying sweep; these vibrations, when of a certain energy, will set going vibrations in another substance by disturbing the tension of its molecules, as the vibrations of heat will disturb the tension of the gunpowder molecules, and set them sweeping with greater energy: this is the communication of the force. Just as we say that a magnet communicates magnetic force to a bit of iron, though all we mean is that the magnet has so altered the molecular condition of the iron as to have given it the movements called magnetism—in short, has excited in the iron the dormant property of becoming magnetic—so we say the nerve communicates its force to the muscle, exciting in the muscle its dormant property of contraction. But in truth nothing has passed from magnet to iron, or from nerve to muscle.

44. Do what we will, however, there is always, in the present condition of philosophical chaos, the danger of being misunderstood when we employ the term Nerve-force; and I have proposed the term Neurility as an escape from the misleading suggestions. It is a symbol expressing the general property of nerve-tissue. For reasons presently to be stated, I restrict Neurility to the peripheral system, employing Sensibility for the central system. The excited muscle manifests its special property of Contractility; the excited nerve manifests its special property of Neurility; the excited centre manifests its special property of Sensibility.96 The terms are simply descriptive, and carry with them no hypothesis as to what Neurility is in its hidden process, nor how Sensibility arises in a nerve-centre, and not elsewhere. We know that a stimulated muscle contracts, and we express the fact by assigning to muscular tissue the property of Contractility. We know that a stimulated nerve translates an impulse from one point to another, and excites the muscle to contract; and we express the fact by assigning to nerve-tissue the property of transmitting stimulation, which is further specified, as unlike other transmissions, by the term Neurility.

45. What is the meaning attached to the term Property, and how it is distinguished from Function, has been already expounded in Problem 1, §§81–6. There also was laid down the principle of identity of structure implying identity of property. Inasmuch as observation reveals a fundamental similarity in the structure of the nervous tissue throughout the animal kingdom, we must conclude the existence of a fundamental similarity in the property of that tissue: a conclusion confirmed by observation. There is a corresponding agreement in the organs and functions; so that, within certain limits, the experiments performed on an insect may be verified on a mammal. Everywhere nerve-tissue has certain characters in common, accompanied by variations in the degree and mode of manifestation corresponding with variations in structure and connection. Obvious as the fact is, we must emphasize the great variety which accompanies the underlying uniformity, for this is recognizable both in the individual organism and in the animal kingdom at large. Even such seemingly individual terms as nerve-cell and nerve-fibre are in truth generic; and the description which accurately represents one cell or fibre needs modifying for others.

Properties are generalized expressions; they result from the composition, the structure, and the texture of a substance. Thus one bar of iron may differ from another of equal bulk in being more or less crystalline in structure, though having the same composition and the same texture. This difference will modify the mode of manifestation of the iron-properties. Cast-iron pillars, for example, will support, as a roof, a weight which would break them if suspended; wrought-iron pillars of similar bulk will bear a weight suspended which would crush them as a roof. Yet both cast and wrought iron pillars have the same properties, because they have the same composition and similar structure; the variation of structure only producing a difference in the modes. Texture may also vary. The bar of iron may be beaten into a plate, rolled into a cylinder, or split into wire-work, without any change in its properties, but with marked differences in its modes of manifestation, and in the uses to which it may be applied. These uses are of course dependent on the connections established between the iron and other things. In Physiology, uses are called functions.

46. Nerve-tissue must be understood as having everywhere the same general Property. In one animal and in another, in one part and in another, Neurility is the same in kind, but not everywhere manifesting the same degree, nor applied to the same Function. The composition of nerve-tissue varies, but not more than the composition of all other organized substances; the structure is variable, but only within a small range; the texture also; while the connections are very various. Hence, whatever the variations in composition or structure, the nerve-fibre has everywhere one fundamental property, which in connection with a muscle has the functional activity of exciting contraction; in connection with a gland of exciting secretion; and in connection with a centre of exciting reflexion.97

47. Had a clear idea of Function as dependent on connexion been present to their minds certain physiologists would hardly have raised the mirage of “Nerve-force,” a mysterious entity endowed with “specific energies,” and capable of producing vital and psychical phenomena by an occult process; nor would others have been led to the monstrous hypothesis of particular nerve-cells being endowed with thought, instinct, and volition. They would have sought an explanation of functions in the combined properties of the co-operant organs and tissues. They would not have endowed one nerve with Sensibility, and another nerve of identical structure with Motility;98 one nerve with a motor property, and another with the opposite property of inhibition. They would have seen that all nerves have the same property, but different uses when in different connexions.

48. Throughout the animal kingdom we see movement following on stimulation. Stimulation may be defined the change of molecular equilibrium. The stimulation of a muscle is produced indirectly through a change in the nerve, or directly through a change in the muscle itself. In the simplest organisms there is no trace of nerve-tissue; but their substance manifests Irritability (or as it is often called Sensibility); and a stimulus to one part is propagated throughout—the whole body moves when touched. Even in Polypes, where there is the beginning of a differentiation, the motion is slowly propagated from one part to the rest. A single tentacle retracts when touched; but the movement rarely ends there; it is slowly communicated from one tentacle to the other, and from them to the whole mass. Touching the body, however, will not, if the touch be slight, cause the tentacles to move; so that we see here a beginning of that principle of specialization which is so manifest in the higher organisms: the tentacles have become the specially sensitive parts. Ascending higher in the scale of organisms we find those which habitually move particular parts without at the same time necessarily moving the rest; and this independence of parts, accompanying a more perfect consensus, we find to be developed pari passu with a nervous system. An immense variety of part-movements, with varying combinations of such movements, is the physiological expression of the more complex nervous system.

48 a. Deferring what has to be said of Sensibility till the next chapter, we may here touch on its relation to Irritability, which is often used as its synonym. Objectively it cannot be distinguished from Irritability, nor indeed from the most general phenomenon of reaction under stimulation; in this it is an universal property. But subjectively it is distinguishable as a peculiar mode of reaction, only known in nerve-tissues. While all tissues are irritable, and react on being stimulated, each tissue has its special mode of reaction. The secreting-cell reacts differently from the muscle-cell. The reaction of the nerve is the innervation of a centre or a muscle; the reaction of an innervated centre is sensation; of a muscle, contraction. There are three aspects of neural reaction: excitation, propagation of the disturbance, and innervation. The first is expressed by irritability, the second by conductibility, the third by sensibility; but these are only artificial distinctions in the general phenomenon of transmitted excitation. The nerve substance is specially distinguished by its instability of molecular equilibrium; it undergoes chemical change with a readiness comparable to that of explosive substances. Hence its facility of propagation of disturbance. There is irritability and propagation of disturbance in muscular tissue, notably evident in the continuous tissue of the heart, intestines, and ureter; but the propagation is slow and diffusive; whereas in the nerve it is rapid, and restricted along a definite path. By this rapidity and restriction the force of the impact is increased; and thus a slight stimulus applied to the nerve is capable of disturbing the state of the muscle.

49. Thus while molecular movement is a fundamental condition of Vitality, and is incessant throughout organized substance, the massive movements of the organism, and the movements of particular parts, are the directed quantities of this molecular agitation. They are due to stimulation. We distinguish this from mechanical impulsion. It is a vital process involving molecular change; it is not simply the communication of motion from without, but the excitation of motion within. It is not like the blow which merely displaces an object, but like the blow which disturbs its molecular equilibrium. The effect, therefore, depends on this molecular condition: the blow which scatters a heap of gunpowder will explode a fulminating salt, and this, in exploding, will excite the gunpowder to explode. The stimulus which is too feeble to excite contraction in a muscle will be powerful enough to excite the neurility of a nerve, and that will excite the contractility of the muscle. The nerve-force is simply neural stimulus. It acts upon the other tissues as the nitrogenous salt upon the gunpowder.

Although it is now common to speak of nerves as transmitting waves of molecular motion, and to regard nerves as the passive medium for the “transference of force,” whereby the force is thus made an abstract entity, we must always remember that such phrases are metaphors, and that the truer expression will be not “transference of force,” but the “propagation of excitation.” I mean that it is not the force of the impact nor its energy which a nerve transmits, it is the vibratory change produced in the nerve by the impact, which excites another change in the organ to which the nerve goes. We know by accurate measurements that the excitation of a nerve lasts much longer than the stimulus, a momentary impact producing an enduring agitation. We know also that the excitation of a centre lasts longer than the muscular contraction it has initiated. We know, moreover, that a nerve may be totally incapable of conducting an external stimulus, yet quite capable of conducting a central stimulus; were it a passive conductor like a wire this would not be so.99

50. The nerve is essentially an exciter of change, and thereby a regulator. A muscle in action does not appreciably determine action in any other (except in the comparatively rare cases of anastomosing muscles); a secreting cell does not propagate its excitation to others. The nerve, on the contrary, not only propagates its excitation, and awakens the activity of the muscle or gland with which it is connected, but through the centre affects the whole organism—

Thus it is that stimulation which in the simpler organisms was diffused throughout the protoplasm, has in the complex organisms become the specialized property of a particular tissue.

51. Two general facts of supreme importance must now be stated: One is the law of stimulation—every excitation pursues the path of least resistance. The second is the condition of stimulation—unlike mechanical impulsion, it acts only at insensible distances.

52. This means that although a nerve may be excited by any stimulus external to it which changes its molecular condition, no propagation of that change (i.e. no stimulation through the nerve) is possible except through continuity of substance. Mere physical contact suffices to excite the nerve; but if there be an interruption of continuity in the nerve itself, no stimulus-wave passes across that line. Cut a nerve, and bring the divided surfaces once more into close contact, there will still be such a solution of continuity as to arrest the stimulus-wave, mere physical contact not sufficing for the propagation. Whereas across the cut ends of a divided nerve, even visibly separated, the electric current easily passes. This necessity for the vital continuity of tissue in the propagation of stimulation must always be borne in mind. The presence of a membrane, however delicate, or of any tissue having a different molecular constitution, suffices to arrest or divert the wave. I conceive, therefore, that it is absolutely indispensable that a nerve should terminate in and blend with a muscle or a centre, otherwise no stimulation of muscle or centre will take place through the nerve.

53. The difference between excitation from contact and stimulation from continuity may be thus illustrated. In Fig.13 we see the legs of a frog attached to the spine by the lumbar nerves (l), and lying on the muscles (m) of one leg is the nerve (c) of another frog’s leg. Applying the electrodes to (l), the muscles (m) are violently contracted; not only so, but their contraction excites the other nerve (c), and the leg attached to this nerve is thereby thrown into contraction. This “secondary contraction,” as Dubois Reymond calls it, might be supposed to be due to a diffusion of the electrical current; but that it is due to a change in the muscles (m) is proved by delicate experiments showing that the movements in the detached leg are of precisely the same kind as those in the legs directly stimulated. If there is only a muscular shock in the one case, there is only a muscular shock in the other; if there is tetanus in the one, there is tetanus in the other; if the muscles of the first leg are fatigued and respond slowly and feebly, the response of the second is slow and feeble. Moreover, the secondary contraction may be produced by chemical or mechanical stimulus, as well as by the electrical.

54. Although the contraction of a muscle is thus seen to be capable of exciting a nerve in contact with it, the reverse is not true: we can produce no contraction in a muscle by exciting a nerve simply in contact with the muscle, and not penetrating its tissue and terminating there. Accordingly we always find a nerve when about to enter a muscle or a centre losing its protecting envelopes; it gradually becomes identified as a protoplasmic thread with the protoplasm of the muscle or the centre.

55. Neurility, then, is the propagation of molecular change. Two offices are subserved by the nervous system, which may respectively be called Excitation—the disturbance of molecular tension in tissues, and consequent liberation of their energies; and Co-ordination—the direction of these several energies into combined actions. Thus, when the muscle is in a given state of molecular tension, the stimulation of its nerve will change that state, causing it to contract if it be in repose. But this stimulation, which will thus cause a contraction, will be arrested, if at the same time a more powerful stimulation reaches the antagonist muscle, or some distant centre: then the muscle only tends to contract.

ORIGIN OF NERVE-FORCE.

56. After this brief account of Neurility we may pass to the consideration of its origin. Are we to understand that this property belongs to the nerves themselves in the sense in which Contractility belongs to the muscles? or are we to accept the teaching which assigns the origin of “nerve-force” to the ganglia, and regards the nerves simply as passive conductors of a force developed in the cells?

57. It is now many years since I ventured to criticise the reigning doctrine, and to urge the necessity Of consistently carrying out the distinction between Property and Function. I called attention to the positive evidence which contradicted the idea of passive conduction; and pointed out the illusory nature of the favorite analogy, in which ganglia were likened to batteries, and nerves to the conducting wires. But the old image still exerts its empire; and writers are still found speaking of the brain as a telegraphic bureau, the ganglia as stations, and the nerves as wires. In the cells of the gray substance they place a constantly renewing reservoir of nerve-force. There the force is elaborated, stored up, and from thence directed along the nerves. The sensory nerve “transmits an impression to the brain”—as the wire transmits a message to the bureau. The motor nerve, in turn, “transmits the mandates of the will”—and all is clear! Clear, until we come to translate metaphors into visible facts, or try to conjure up some mental image of the process. For myself, I can only conceive nerve-force as the activity of the nerve, and not of something else. This becomes still more evident when I find that the activity is equally manifest after its imaginary source has been removed. Transmitting impressions, or messages, implies as a preliminary that there should be an impressible agent, or a message-sender, at the periphery. No one supposes that simply touching one end of a wire would send an “impression” or a “message” to the battery; or that without the battery this touch would evolve a current. The battery is indispensable; in it is evolved the current which the wire transmits. Not so the ganglion, or brain. Remove the wire from its connection with the battery, and it is a bit of wire, nothing more. But remove a nerve from its connection with a ganglion, and it is still active as nerve, still displays its Neurility when excited, still moves the muscle as before. The amputated limb will move when its nerves are stimulated, just as when a reflex from its centre moved it. Every one knew the fact; it was staring them in the face, yet they disregarded it. Even the old anatomist, Willis, had recorded experiments which ought to have opened their eyes. He tied the phrenic nerve, and found that, when he irritated it below the ligature, the diaphragm moved; but when he irritated it above the ligature, no movement followed. Since his days, thousands of experiments have shown that the presence of a ganglion is not necessary to the action of a nerve.100

58. Of course an explanation was ready. The nerve was said to have been “endowed with force” from its ganglion during their vital connection; and this force, stored up in the nerve, was disposable for some time after separation from the ganglion. We need not pause to criticise this misty conception of one part “endowing” another with force; the plain facts afford the best answer. There seemed, indeed, a confirmation of the hypothesis in the fact that although the nerve separated from its ganglion was capable of excitation, yet after a few excitations it was exhausted, and ceased to stimulate the muscle. It seemed like the piece of magnetized iron which would act as a temporary magnet, though quickly losing this borrowed power. But the whole fabric fell—or ought to have fallen—when extended observation discovered that this exhausted nerve would, if left in repose, recover its lost power. A nerve preserves its excitability as long as it preserves its structural integrity, and recovers its power in recovering that integrity. The length of time varies.101 Gratiolet found the muscles in the leg of a tortoise, which had been amputated a week before, contract when the nerves were irritated; and Schiff found the divided nerve of a winter frog excitable at the end of three weeks. Even after all excitability has disappeared, it will reappear if arterial blood be injected; just as muscles which have already begun to assume cadaveric rigidity recover their contractility after transfusion. Nor is this all. The separated nerve finally degenerates, and loses all its structural characters and physiological properties; yet under favorable conditions it will regenerate—recover its structures and properties; and this even apart from a centre, as Vulpian showed. Very noticeable is the fact that the force said to be produced in the centre, and only “conveyed” by the nerve, vanishes gradually from the centre to the periphery, and recovers from the periphery to the centre—the part of the nerve which is farthest from the centre being excitable when the part nearest the centre is still inexcitable. Again, when a nerve is pinched, contraction in the muscle follows; but the pinch has for a time so disturbed the structural integrity of the nerve (at that spot) that no irritant applied to the spot, or between it and the centre, will be followed by contraction, whereas below the spot an irritation takes effect. This is another form of the experiment of Willis. Even in its normal state, the nerve has different degrees of excitability in different parts of its course,—a fact discovered by PflÜger which is quite irreconcilable with the hypothesis of passive conduction. Doubts have been thrown on PflÜger’s interpretation,102 namely, that there is an avalanche-like accumulation of energy proportionate to the length of the stimulated portion; but the fact remains, that one and the same irritant applied successively to two different points of a nerve does not irritate the muscle in the same degree. Munk also finds the velocity of transmission in a motor nerve increases as it approaches its termination in the muscle.103

59. Nothing can be more unlike the conduction of an electric current than this excitation of Neurility; nothing more accordant with the idea of it as a vital property of the tissue. The notion of its being derived from a centre is on a par with the notion first successfully combated by Haller,104 that the muscle derived its Contractility from the nerves; or the analogous notion that the electric organ in fishes derived its property from the brain. Indeed, it was in support of the hypothesis that the brain was a battery, and nerves the conductors, that the phenomena observed in electrical fishes were frequently cited. The electric organ was seen to be connected with the brain; its discharges were under the control of the animal, and were destroyed on one side when the brain on the corresponding side was destroyed. But Charles Robin long ago suggested, what indeed ought never to have been doubted, that the brain was not the source of the electricity; but that the tissue of the electric organ itself had this special property, which the nerve merely called into activity. The suggestion has been experimentally verified by M. Moreau, who divided all the nerves supplying the electric organ on one side, and, having thus cut off all communication with the brain, produced electrical discharges by irritating the nerves; precisely as the muscles are made to contract when the divided nerves are irritated. Had the experiment ceased here, it might have been interpreted on the old hypothesis: the electric organ might be supposed to have a certain amount of electric force condensed in it, stored up there, as it is said to be in the nerves, and discharged when the organ is irritated. But experiment has decided this point also. Electric fishes notoriously exhaust their power by a few discharges, and recover it after repose. When M. Moreau had exhausted his mutilated fishes, he replaced them in the water, and allowed them repose. On again irritating the divided nerves, the discharges were again produced.105

60. On all sides the idea of nerves deriving their power from another source than their own substance is seen to be untenable. A priori this might have been concluded. Neurility is the vital property of nerve-tissue. “Nerve-force” is nerve-action—molecular changes in the nerve itself, not in some remote substance. That nerve and centre are vitally connected is true; and what their physiological relations are will hereafter be examined; but we must dismiss the idea of nerves having the relation to centres that electrodes have to batteries.

61. In proposing the term Neurility, I not only wished to get rid of the ambiguities which hovered round “nerve-force” and “nerve-current,” but to recall the physiological principle that properties are dependent on structures; and therefore that the special property of nerve-tissue is conditioned by its structure. Neurility is, of course, an abstraction; but so is the nerve an abstraction. The concrete manifestations are the several nerve-actions. These we classify and specify. One class we call sensory, another class motor; not because the nerve-action itself is different, but because it is in each class in a different functional relation to other parts. In classing men as governors and governed, employers and employed, we do not suppose anthropological distinctions, but only differences in their social functions.

62. This is the modification of the Law of Bell to which reference was made in §26. It replaces the idea of two different kinds of nerve, sensory and motor, by that of two different anatomical connections. I need not reproduce here the argument with which I formerly criticised the supposed distinction between sensory and motor nerves; because the old idea is rapidly falling into discredit, and physiologists so eminent as Vulpian and Wundt have explicitly announced their adhesion to the principle of identity,—a principle which, as Vulpian truly remarks, dominates the whole physiology of the nervous system.106

THE HYPOTHESIS OF SPECIFIC ENERGIES.

63. One development of the theory of Bell, respecting the different kinds of nerve, has been the still accredited hypothesis that each nerve has a “specific energy,” or quality, in virtue of which it acts and reacts only in one way. The optic nerve, no matter how stimulated, only responds by a sensation of color, the auditory nerve only by a sensation of sound; and so on. This hypothesis, which (as I learn from a correspondent)107 was originally propounded by Bell himself, was developed and made an European doctrine by Johannes MÜller, first in his remarkable treatise, Über die phantastischen Gesichtserscheinungen (1826), and afterwards in his Physiology. Like all good hypotheses, it has been fruitful; and Helmholtz still holds it to be of extraordinary importance for the theory of perception. Although combated by a few physiologists, it has kept its place firm in the general acceptance; no doubt because it forms a ready explanation of the facts. But, as I often have to remark, explanation is not demonstration.108

64. The first criticism to be made on the hypothesis is that it commits the error of confounding function with property, assigning as a specific quality of the nerve the reaction of the organ innervated. Thus MÜller speaks of the specific energy as “the essential condition of the nerves in virtue of which they see light and hear sound.” But the optic nerve no more sees, than the liver-nerve secretes bile. That the optic nerve is one element in the mechanism on which vision depends, is all that we can say, MÜller declares that it is not sufficient to assume each nerve to be so constituted that it has a susceptibility to certain stimuli rather than to others; but that “with Aristotle we must ascribe to each a peculiar energy as its vital quality. Sensation,” he adds, “consists in the sensorium receiving through the medium of the nerves a knowledge of certain qualities,—a condition, not of the external bodies, but of the nerves themselves,”—and these qualities are different in different nerves. In other words, he assumes a special substance for each special energy. The sensation of color depends on the special Visual substance (Sehsinnsubstanz); the sensation of sound on the Auditory substance (HÖrsinnsubstanz); and so on.

65. We have here an hypothesis analogous to that of Innate Ideas, or a priori Forms of Thought. It is, in fact, only a reproduction of that conception carried into the sphere of Sense. No one thinks of assigning specific energies to the several muscles, yet a movement of prehension is as different from a movement of extension, a peristaltic movement is as different from a movement of occlusion, as a sensation of sound is from a sensation of color. If movement is common to both of the one class, feeling is common to both of the other: the forms and mechanism are different and specific. Muscles have the common property of contracting under stimulation; whatever be the nature of the stimulus, each muscle has its own particular response, or mode of reaction: the flexor always bending, never extending the limb; the sphincter always closing, never opening the orifice. The movements of the heart are not the same as those of the eye; both are unlike the movements of the intestine. There are muscles which respond to some stimuli, and not to others. Those of the eye, or of the vocal chords, respond to impulses which would leave the masseter or biceps unstirred. According to Marey, the hyoglossus of a frog will become tetanic under a stimulus of only ten pulses in a second; whereas the gastrocnemius of that same frog resists a stimulus of less than twenty in a second. We find the retina responding to ethereal pulses which leave the auditorius unaffected; we find the muscles of a gnat’s wing so exquisitely susceptible that the wing beats eight thousand times in a second,—a delicacy in comparison with which even our muscles of the eye are coarse.

66. The facts which the hypothesis of specific energies is called on to explain are more consistently interpreted on the admission of a common property in nerve-tissue, manifesting different degrees of excitability, and entering into different mechanisms, so that the functional results differ. A nerve which may be stimulated from the skin will not respond at all, or not in the same way, if the stimulus be applied under the skin. Are we to suppose that the specific energy resides in one part of the nerve, and not in another?109 That the optic nerve responds to stimuli which will not sensibly excite a motor nerve, depends on the terminal structures through which the stimulation is excited; for the optic nerve itself, apart from the retinal expansion, is as insensible to light as the motor nerve is. And the specific sensation, or movement, which results from stimulation of a nerve depends not on the nerve, but on the mechanism of which the nerve is one element. Sensations of touch, temperature, and pain are assuredly specific; they are as unlike each other as a sensation of taste is unlike a sensation of smell. Yet the same nerves, variously stimulated, produce all three sensations.

67. We conclude, therefore, that the phrase “specific energy” is an elliptical expression for the particular office of a nerve. In this meaning there is no obscurity. The optic nerve is not a vasomotor nerve, the skin nerve is not a muscle nerve; the auditory nerve is a nerve of special sensation, the vagus is a nerve of systemic sensation; and so on. Neither movement nor sensation belongs to the nerves themselves.