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 conditions190 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 those191 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,192 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 are193 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.194 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 variations195 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 in196 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 of197 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 feeble198 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,199 and awakens the activity of the muscle or gland with which it is connected, but through the centre affects the whole organism—
“Ein Schlag tausend Verbindungen schl?gt.”
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.
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Fig. 13.
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