Since then the inhibitory processes in nerves have been frequently investigated by _Schiff_ (1823–1896), _Goltz_ (1834–1901) and others, who gave us a theory concerning the same. Only a small number of inhibitory processes were known at that time, as for instance the inhibition of the croak reflex of the frog, or the inhibition of the grasp reflex during copulation of these animals through skin stimuli, and a few other cases. They regarded the inhibitory nervous processes as a special state, of which the inhibition of the heart through the vagus was the best illustration. Further, the Russian physiologist _Setschenow_ succeeded by directly stimulating certain parts of the central nervous system, especially the optic lobes of the frog, in producing inhibition. It was, therefore, frequently assumed, as _Setschenow_ did, that in the brain there exist special inhibitory centers, just as there are motor centers. This view was later shown to be untenable. It is only quite recently, and especially since _Sherrington_ has shown that inhibition plays a part in all antagonistic muscle movements, that we have obtained a broad and more thorough understanding of the inhibitory processes in the life of the organism, and a physiological explanation of this important group of activities of the central nervous system. This inhibitory effect of stimulation, brought about by the involvement of the central nervous system in the normal organism, was studied side by side with the depressing effects of stimulation. _Claude Bernard_ (1813–1878)[9] first discovered that the excitation of all living substance could be depressed or totally suspended through the influence of certain anæsthetics, such as ether or chloroform. By a series of experiments, as simple as they were convincing, the French scientist showed that irritability could be depressed in mimosa leaves, the growth of germinating plant seeds and the ferment action of yeast cells stopped, likewise the disintegration of the carbon dioxide in the cells of the green leaf, as well as the development of the egg cells, and also the movements of the animal organism and the sensations of man. By this means he recognized that not only does all living protoplasm possess irritability, but that it can also by means of certain substances be put into the condition of “anæsthesia,” a state dependent upon a change of the protoplasm, which he termed “semi-coagulation.” Finally, besides the more apparent processes of excitation and those less so, belonging to the group of inhibition and depression, in the last century the knowledge of the subject was greatly increased by the addition of another group, which recently in consequence of various reasons has met with particular interest. These being effects of stimuli on the direction of movements of motile organisms, it became more and more recognized that these curious manifestations of irritability, which appeared to have such a surprising likeness to