Finally, a third kind of secondary effect of stimulation claims our interest. This is the _secondary extension of the result of stimulation_ from the part of a living organism directly and primarily affected by the stimulus, to the surrounding structures. All living substance has the capability of conducting an excitation, which is produced locally through a catalytic stimulus, to a neighboring part, not directly affected by the stimulus. It finds its highest development in the nerve, but in no living structure is it completely absent. This capability has been frequently termed “_conductivity of stimulation_.” It is more precise, however, to speak of conductivity of excitation, for it is not the primary influencing external stimulus which is conducted in the living substance, but the excitation which it has produced. I have intentionally considered only the excitating effects of stimulation, and not those of the depressing reactions, as only excitations, not depressions, are conducted by the living substance. These questions, however, demand a closer analysis. Here we were concerned only with a survey of the general effects of stimulation. If I, therefore, once more summarize the results which have been gained, this is most clearly demonstrated by the following scheme: PRIMARY EFFECTS OF STIMULATION Excitation Depression Functional Cytoplastic Functional SECONDARY EFFECTS OF STIMULATION Secondary excitation Secondary depression Conduction of excitation, Metamorphic processes, Self-regulation of metabolism This, however, is simply a scheme, like all other schemes, having for its purpose a superficial survey of the subject. It brings to some extent order into the overwhelming mass of manifold effects of stimulation but tells us nothing of the mechanism and genesis. Our further task must, therefore, be a more thorough analysis of this field. CHAPTER V THE ANALYSIS OF THE PROCESS OF EXCITATION _Contents_: Indicators for the investigation of the process of excitation. Latent period. The question of the existence of assimilatory excitations. Dissimilatory excitations. Excitations of the partial components of functional metabolism. Production of energy in the chemical splitting up processes. Oxydative and anoxydative disintegration. Theory of oxydative disintegration. Dependence of irritability on oxygen. Experiments on unicellular organisms, nerve centers and nerve fibers. Restitution after disintegration by metabolic self-regulation. Organic reserve supplies of the cell. The question of a reserve supply of oxygen of the cell. Metabolic self-regulation as a form of the law of mass effect, and metabolic equilibrium as a condition of chemical equilibrium. Functional hypertrophy.