The Physical Basis of Mind / Being the Second Series of Problems of Life and Mind.

by George Henry Lewes

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—

“Ein Schlag tausend Verbindungen schlägt.”

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.

Fig. 13.

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.