The processes of _excitation_ of all the effects of stimulation are those which have invariably claimed place in the interest of physiologists. The study of the processes of _depression_, on the other hand, has remained more or less in the background. This is readily understood when it is considered how much more apparent the processes of excitation are than those of depression. Nevertheless, these latter possess no less importance for the course of vital phenomena than those of excitation. Without depression no excitation can take place in the vital activity of the organism, for, as we have seen, every excitation is secondarily followed by a refractory period. To this must be added the great number of _primary_ depressions, directly brought about by the most varied stimuli, such as cold, want of oxygen, poisons, etc., without the presence of a preceding excitation. Thus it is essential that the processes of depression should be studied with no less interest than those of excitation, and it is much to be desired that the former should receive a more detailed analysis than has up to now been the case. Even as it is, extensive material has been obtained for the analysis of this group of reactions. With the closer study of the process of excitation the facts in connection with the refractory period and fatigue make it necessary that the processes of depression be taken into consideration. Toxicology and pharmacology likewise furnish innumerable effects of depression produced by poisons and drugs. Unfortunately the investigation of these reactions has been in the main purely superficial. This arises from the recency of the development of these sciences. Even later than physiology they are only now beginning to extend their investigations, directed up to the present to the grosser organic reactions, to the cellular analysis of the effects of poisons. How rarely we find instances in which the effect of some drug is studied at the point of attack and systematically followed to the specific cell form, and its primary excitating or depressing effect on this or that constituent process of the metabolic activities ascertained. And how great, on the other hand, is the number of “medicines” making their appearance each year in pharmacology of which nothing further is known than a few secondary effects on the action of the heart, the blood pressure, the secretion and excretion and on some other outwardly perceptible organic actions! This deplorable condition of present-day pharmacology must be ascribed to the regrettable circumstances that pharmacological research is only in a very small degree the result of careful investigations, carried out by biologically and chemically trained pharmacologists, but is for the most part undertaken at the instigation of chemical manufacturers. This eager haste to obtain superficially practical results has lessened in great degree the interest in the close and painstaking theoretical analysis of reaction to poisons.