104 IRRITABILITY 



tal importance of oxygen to the maintenance of irritability of 

 living substance. Oxygen is of greatest importance for a high 

 degree of irritability in all aerobic organisms. All living systems 

 which are characterized by a great capability of activity and 

 evince strong responses under the influence of stimulation, such 

 as the vertebrates and insects, are necessarily aerobic, whereas 

 the living organisms of pronounced anaerobic character, as some 

 bacteria, yeast cells, parasitic organisms, etc., manifest on the 

 average much less capability of activity. 



Finally, to briefly summarize the foregoing, the following 

 picture presents itself of disintegration produced by a momen- 

 tarily acting stimulus. It is immaterial how the stimulus pro- 

 duces 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 mole- 

 cules 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 

 anaerobic organisms this decomposition takes place anoxyda- 

 tively with the cooperation of enzymic processes, and as larger 

 fragments generally result from the disintegration of the com- 

 plex molecule, the production of energy is accordingly smaller. 

 The disintegration of aerobic organisms, on the other hand, occurs 

 in the form of an oxydative splitting up of the complex mole- 

 cules 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 gen- 

 eralize 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 frag- 

 ments, followed by an oxydative splitting up of these into carbon 

 dioxide and water, can in no case as yet be justifiably applied to 



