188 IRRITABILITY 



products is increased. This is the refractory period. By the 

 metabolic self -regulation in accordance with the principle of 

 chemical equilibrium, the original metabolic equilibrium is 

 restored after every excitation. Irritability, therefore, increases 

 in the same measure as this occurs, that is, in the form of a 

 logarithmic curve, until it again reaches the specific degree of 

 irritability of the particular system. The refractory period 

 diminishes. If the processes of disintegration and self-regulation 

 are delayed, either by want of substance necessary for breaking 

 down or the accumulation of decomposition substances, the 

 refractory period is prolonged and the response to every further 

 stimulation decreased, that is, the system is fatigued. In all 

 aerobic organisms the retardation of the course of excitation and 

 self-regulation under a continuous influence of stimuli is the result 

 of the relative want of oxygen. The processes of oxydative dis- 

 integration are prolonged and restricted by relative deficiency of 

 oxygen and merge more and more into anoxydative decomposi- 

 tion. The products of incomplete oxydative and anoxydative 

 decomposition accumulate. Both factors decrease the strength 

 of the response after every stimulation. Thus the want of oxygen 

 leads to reduced activity. In the anaerobic organisms the refrac- 

 tory period and symptoms of fatigue are, of course, produced by 

 the relative deficiency of other substances. Fatigue in the 

 anaerobic systems has, however, so far not been investigated. We 

 advance very slowly, step by step, in physiology, and, as in every 

 science, an acquirement of a new knowledge means a new prob- 

 lem. In this lies the inexhaustible charm of our scientific 

 research. 



