Problems of Life and Mind. Second series, George Henry Lewes [e book reading free TXT] 📗

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.

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