As a result of all these investigations, linked together in a systematic series, the proof has now been obtained that the nerve like all other living substances is fatigable. Its fatigue is solely the manifestation of a prolonged refractory period and the extension of the latter by continuous stimulation is, as in all aërobic substances, a result of relative deficiency of oxygen. [Illustration: Fig. 38. Curve of irritability as demonstrated by action current of two nerves in nitrogen, which are alternatively stimulated (plain line) and at rest (dotted line). Recovery in nitrogen is always merely partial and relative. It only increases on introduction of oxygen. (After _Thörner_.) ] To briefly summarize in conclusion, I will repeat that just as all living systems show a refractory period after an excitation, in which irritability is reduced, all living systems are likewise capable of fatigue. Both are most intimately connected and are based fundamentally on the facts of metabolism. An excitating stimulus disturbs the metabolic equilibrium of rest by suddenly bringing about increased decomposition of certain substances. During and directly after the breaking down, irritability is reduced in the same degree as the amount of substances required for disintegration in response to a succeeding stimulus is decreased and the quantity of the decomposition products is increased. This is the refractory period. By the metabolic self-regulation in accordance with the principle of chemical equilibrium, the original metabolic equilibrium is restored after every excitation. Irritability, therefore, increases in the same measure as this occurs, that is, in the form of a logarithmic curve, until it again reaches the specific degree of irritability of the particular system. The refractory period diminishes. If the processes of disintegration and self-regulation are delayed, either by want of substance necessary for breaking down or the accumulation of decomposition substances, the refractory period is prolonged and the response to every further stimulation decreased, that is, the system is fatigued. In all aërobic organisms the retardation of the course of excitation and self-regulation under a continuous influence of stimuli is the result of the relative want of oxygen. The processes of oxydative disintegration are prolonged and restricted by relative deficiency of oxygen and merge more and more into anoxydative decomposition. The products of incomplete oxydative and anoxydative decomposition accumulate. Both factors decrease the strength of the response after every stimulation. Thus the want of oxygen leads to reduced activity. In the anaërobic organisms the refractory period and symptoms of fatigue are, of course, produced by the relative deficiency of other substances. Fatigue in the anaërobic systems has, however, so far not been investigated. We advance very slowly, step by step, in physiology, and, as in every science, an acquirement of a new knowledge