238 IRRITABILITY 



away with the assumption of the existence of the most manifold 

 physical types of depression, the analysis of a few processes which 

 have been studied up to now demonstrates the singular fact that 

 a number of these which are brought about by quite different 

 external factors, are based on an absolute uniformity of their 

 mechanism. As we have previously seen, a certain constituent 

 of the metabolic chain can be excitated primarily by very different 

 kinds of stimuli. In like manner there exists in metabolic activity 

 a certain point of predilection for different kinds of stimuli, from 

 which their depressing effects proceed. Here the highly interest- 

 ing fact is shown that this point of predilection, which represents 

 that of the most frequent attack, is the same for excitating as for 

 depressing stimuli. These are the oxydative processes. As our 

 knowledge of the reactions to stimuli in anaerobic organisms is 

 still almost nil it is not possible at present to ascertain which 

 component in the metabolism of these organisms, adapted to life 

 without oxygen, plays an analogous role to that of the oxydative 

 in aerobic systems. Our investigations must, therefore, be re- 

 stricted to the world of aerobic organisms. Here we have seen 

 that the different stimuli which produce an excitating effect in- 

 variably increase the oxydative disintegration of the living system 

 and we now find that these constituent processes of metabolism 

 likewise form a point from which depressing responses to stimuli 

 very readily proceed. 



The prototype of this group of processes of depression in 

 which this is manifested in a most striking manner, is that of a 

 simple asphyxiation by the withdrawal of the oxygen supply 

 from the exterior. If the supply of oxygen is withheld from an 

 aerobic organism, oxydative disintegration is gradually found to 

 be more and more decreased and further breaking down takes 

 place awoxydatively, as oxydative decomposition forms the chief 

 source of energy production, and energy production consequently 

 undergoes a gradual decrease. Excitating stimuli, therefore, meet 

 with less response than when a sufficient supply of oxygen is 

 present, that is, irritability is diminished. As a result of this 

 decrease, a corresponding decrement in the extension of excita- 

 tion takes place, which, in turn, is likewise manifested by the 



