Among other investigations, which since this time have been made to explain the mechanism of inhibition, those of _Gaskell_,[170] _Hering_[171] and _Meltzer_[172] have received widest consideration. These theories are built upon the existence of the two phases of metabolism, and assume that inhibition, in contradistinction to dissimilatory excitation processes, depends upon an increase of the assimilative processes. The principal evidence which _Gaskell_ advances is that when the vagus nerve of the tortoise heart, a typical inhibitory nerve, is stimulated, a positive variation of the demarcation current of the heart muscle occurs, whereas when a motor nerve of a skeleton muscle is stimulated the attached muscle shows a negative variation of the demarcation current. I must confess that this explanation of inhibitory processes, from the standpoint of an interpretation of processes in the living substance, seems very plausible, and I have accepted this even in my address on excitation and depression before the Frankfurter Naturforscher Versammlung.[173] I have since then endeavored to obtain experimental evidence to substantiate this theory, in that I attempted to prove that increase of the assimilatory processes brought about by stimulation would be associated with a reduction of the specific irritability. For this purpose I have sought for such cases in which a stimulus primarily and momentarily increases assimilative processes in a system in a state of metabolic equilibrium. I was disappointed, when, after years of investigation, I could not find such cases. There is only one kind of stimulus of which we can say with positiveness that it primarily increases the assimilative processes, that is, increased supply of food. But here the increase in the processes of assimilation never occurs momentarily, and indeed this increase is so extremely slight that it can only be demonstrated over a long course of time. These totally negative results of my investigation had awakened strong doubts concerning the assimilation hypothesis of inhibition. Above all, this explanation seemed to me to be impossible for the nervous system. I searched, therefore, for another explanation for the processes of inhibition in the nervous system. If the increase of energy production resulting from the application of a stimulus is dependent upon an excitation of a dissimilative nature, then one is justified to look upon the reduction of functional energy production as an expression of an antagonistic process to that of dissimilatory excitation. In this respect the _Gaskell-Hering_ hypothesis of inhibition rests upon a firm foundation. When, however, this hypothesis assumes an antagonism between dissimilatory and assimilatory excitation, then it must not be overlooked that a second antagonism is possible between dissimilatory excitation and dissimilatory depression. The antagonism need not involve the two types of metabolism, it may depend upon variations of _one_ type.