up to now demonstrates the singular fact that a number of these which are brought about by quite different external factors, are based on an absolute uniformity of their mechanism. As we have previously seen, a certain constituent of the metabolic chain can be _excitated_ primarily by very different kinds of stimuli. In like manner there exists in metabolic activity a certain point of predilection for different kinds of stimuli, from which their _depressing_ effects proceed. Here the highly interesting fact is shown that this point of predilection, which represents that of the most frequent attack, is the same for _excitating_ as for _depressing_ stimuli. These are the _oxydative_ processes. As our knowledge of the reactions to stimuli in anaërobic organisms is still almost nil it is not possible at present to ascertain which component in the metabolism of these organisms, adapted to life without oxygen, plays an analogous rôle to that of the oxydative in aërobic systems. Our investigations must, therefore, be restricted to the world of aërobic organisms. Here we have seen that the different stimuli which produce an excitating effect invariably increase the oxydative disintegration of the living system and we now find that these constituent processes of metabolism likewise form a point from which _depressing_ responses to stimuli very readily proceed. The prototype of this group of processes of depression in which this is manifested in a most striking manner, is that of a simple _asphyxiation_ by the withdrawal of the oxygen supply from the exterior. If the supply of oxygen is withheld from an aërobic organism, oxydative disintegration is gradually found to be more and more decreased and further breaking down takes place _an_oxydatively, as oxydative decomposition forms the chief source of energy production, and energy production consequently undergoes a gradual decrease. Excitating stimuli, therefore, meet with less response than when a sufficient supply of oxygen is present, that is, _irritability is diminished_. As a result of this decrease, a corresponding decrement in the extension of excitation takes place, which, in turn, is likewise manifested by the restriction of the perceptible response to stimulation. In the same degree in which oxydative disintegration becomes less, _an_oxydative breaking down products are accumulated. The accumulation of these products likewise plays a part in the production of depression and increases the decrement in the conduction of excitation. The decrease of energy production by decline of the oxydative decomposition, as well as the accumulation of anoxydative breaking down products, therefore, similarly reduce irritability; that is, their effect is depressing. This whole series of processes, which we have previously considered in detail, takes place on the withdrawal of oxygen and leads to the depression of asphyxiation. It can readily be observed in the most varied kinds of aërobic organisms