Finally, to briefly summarize the foregoing, the following picture presents itself of disintegration produced by a momentarily acting stimulus. It is immaterial how the stimulus produces an excitating effect in the given case, whether through changes in the ion concentration of the living system, by increase of intramolecular atomic movement or in any other manner, it invariably accelerates the disintegration of the complex molecules concerned in functional metabolism, the nature of which varies in the special cases. In the great majority of instances nitrogen-free organic combinations serve as material for the functional constituent members of metabolic processes. In the anaërobic organisms this decomposition takes place anoxydatively with the coöperation of enzymic processes, and as larger fragments generally result from the disintegration of the complex molecule, the production of energy is accordingly smaller. The disintegration of aërobic organisms, on the other hand, occurs in the form of an oxydative splitting up of the complex molecules into carbon dioxide and water so that the production of energy attains a high value. The details concerning the manner in which the individual stages of this decomposition take place and the interactions by which its end products are reached is at present beyond our knowledge. It would be a mistake to generalize in this connection from the behavior of certain groups of organisms. The assumption that under certain conditions the disintegration occurs in two phases, the splitting up resulting from enzymic action of the complex molecule into larger fragments, followed by an oxydative splitting up of these into carbon dioxide and water, can in no case as yet be justifiably applied to all conditions and all aërobic organisms. This is more or less the impression which we derive of the functional excitation process as seen today. Under normal conditions the functional excitation is at once followed by a succession of secondary processes, the “_self-regulation of metabolism_.” Self-regulation after a functional excitation is a fact demonstrated by experience. But in what manner does it take place in detail?