When, therefore, the hypothesis that inhibition is brought about by assimilatory excitation meets with insuperable difficulties, the possibility should be considered if it is not more likely dependent upon dissimilatory depression. These reflections induced me to investigate if conditions could not be produced experimentally wherein dissimilatory depression could bring about inhibitory processes in the nervous system. The most essential requirement was, that dissimilatory depression should quickly develop and pass away with like rapidity, for inhibition of the nervous system sets in momentarily and disappears again momentarily. Another important requisite is, that both interference stimuli are individually capable of producing dissimilatory excitation, for the inhibitory processes of the nervous type may be assumed to be the result of dissimilatory excitation which produce by their interference inhibition, for the nerve fibers, as already stated, are capable of conducting only dissimilatory excitation to the responding organ. As I studied the problem in this manner, it became clear to me that all the conditions necessary for the genesis of inhibition are realized in the existence of the refractory period, and that I had already produced inhibition by prolonging the refractory period, by oxygen withdrawal, in the strychninized frog. If we take a strychninized frog in which the refractory period has been somewhat prolonged by oxygen withdrawal, so that the reaction is simply a short reflex contraction, and rhythmically stimulate the skin, a reaction is only obtained with the first few stimuli, which reactions rapidly decrease until a stage is reached wherein the succeeding stimuli are completely inoperative. (Figure 45.)[174] This inhibition is demonstrated even more clearly by the following experiment. Contractions of the triceps muscle of a strychninized frog are recorded which reflexly follow from stimulation of the central end of the cut sciatic nerve. Oxygen is withdrawn in the manner already referred to. At the proper stage of oxygen deficiency, rhythmic induction shocks applied to the central end of the nerve, the interval between the individual stimuli of which being longer than the duration of the refractory period, elicit reflex contractions of the muscles of the posterior extremity on the opposite side following each individual stimulus. If, however, in the same stage the central end of the nerve is stimulated with induction shocks at intervals briefer than the duration of the refractory period, a contraction is only observed during the very beginning, being brought about by the _first_ stimulus, whereas the subsequent stimuli are ineffective, the muscles remaining at rest during their entire application. (Figure 46.) _Tiedemann_[175] at a later date continued these observations and analyzed them more in detail. In all these experiments, therefore, there is an interference