This theory, founded on facts gained entirely from investigation, would involve those forms of energy which play the rôle of activator in the extension of the breaking down of the molecule from cross section to cross section, namely, the osmotic and the electrical energy. Based on the general properties of physical chemistry and those of morphology of the living substances, they would be applicable to all vital systems. It would be premature to attempt to extend this assumption and further develop its specific details, above all to make it responsible for the specific differences in the process of the conduction of excitation in various forms of living substance. For this our knowledge of the properties of living substance is still far too incomplete. Nevertheless, it furnishes us even now with various points of view for the further analysis of a series of vital manifestations, as, for instance, the facts concerning the production of electricity, of galvanotaxis, chemotaxis and so on. This, however, exceeds the limits of the task we have here mapped out. We are concerned here solely with the general principle on which the conductivity of excitation in the living substance is founded. CHAPTER VII THE REFRACTORY PERIOD AND FATIGUE _Contents_: Conception of specific irritability. Alteration of specific irritability during and after excitation. Refractory period in various forms of living substance. Absolute and relative refractory period. Curve of irritability during refractory period. Dependence of the duration of the refractory period on the rapidity of the course of the metabolic processes in the living substance. Dependence on temperature. Dependence on supply of oxygen. Theory of refractory period. Refractory period as basis of fatigue. Fatigue as a form of asphyxiation. Alterations of irritability and the course of excitation in fatigue. Recovery from fatigue. The rôle played by oxygen in recovery. Fatigue as an expression of the prolongation of the refractory period conditioned by the relative want of oxygen. Fatigue of the nerve. Every living system possesses, as we know, a peculiar and characteristic manner of reacting to stimulation. The muscle responds with a contraction, the salivary cell with production of saliva, the luminous cell with the emission of light. This is the _specific energy_ in the sense of _Johannes Müller_. Every living system is likewise characterized by a certain degree of irritability, which can be expressed by the threshold value of the stimulus at which the specific reaction is just perceptible. This degree of irritability, by which the system concerned is distinguished, may be termed its _specific irritability_.