in rhizopods and infusoria, in plant and ganglion cells, but finds its most complete demonstration in the nerves. Here these processes can be easily produced with any rapidity desired, accordingly as a relative or absolute want of oxygen is brought about. These same typical results are likewise shown in numerous processes in which the external conditions are quite different in nature. We have previously become acquainted with such a case and studied it in detail. This is the state of _fatigue_. Fatigue is a typical state of depression, that is, a state in which the vital process is retarded and irritability in response to stimuli correspondingly decreased. Fatigue is, however, as we have found, the result of a relative deficiency of oxygen. The amount of oxygen at disposal is not sufficient to allow of disintegration, increased by constant functional activity oxydatively taking place, to develop to its full extent. In consequence the previously cited sequence of processes takes place. A “depression of activity” is produced. Fatigue is true asphyxiation and it is here evident that depression proceeds from the same constituent processes of metabolism as excitation, brought about by a single stimulus. Excitation produced by constant stimuli gradually merges into depression as the amount of oxygen at disposal, even if augmented in the intact organism by the increased blood supply, for instance, is still insufficient to meet the demand made by the increased oxygen consumption as a result of continuous functional activity.