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