98 IRRITABILITY 



upon the capability of movement, above all, in homothermous 

 forms, the metabolism of which takes place on a continuously 

 high level. For this reason, in homothermous animals the res- 

 piration of oxygen is the almost exclusive source of energy 

 _2oductiori._ 



The previously mentioned facts make it clear that in one and 

 the same form of living substance both oxydative and anoxdyative 

 decomposition processes are found, depending upon the condi- 

 tions. This does not apply merely to the individual organic 

 forms, such as the facultative anaerobic organisms, but generally 

 to all aerobic living substance. If oxygen is withdrawn from an 

 aerobic organism the disintegration does not cease in conse- 

 quence. In place of the oxydative we have anoxydative decom- 

 position. The various aerobic organisms are, however, adapted 

 in very different degrees to the possibility of an anaerobic exist- 

 ence. While the facultative anaerobic organisms can continue to 

 exist without oxygen, the homothermous animals become asphyx- 

 iated in a very short time in the absence of oxygen, in that they 

 are poisoned by the products of the anoxydative decomposition, 

 which are eliminated with much more difficulty than carbon 

 dioxide and water. The fact, however, that disintegration also 

 continues in an anoxydative form, if oxygen is withdrawn, has 

 given rise to the thought, which has been accepted especially by 

 plant physiologists with great readiness, that the decomposition 

 of organic respiratory substances of the aerobic organisms inva- 

 riably takes place in two stages ; in that the dextrose molecule to 

 again use this as an example is split up first by an enzyme into 

 larger fragments, which then in the second stage of the process 

 undergo combustion to the formation of carbon dioxide and 

 water. Such a possibility cannot be repudiated. I wish, how- 

 ever, to state that one should be very reluctant in generalization 

 of this assumption for all aerobic organisms. The types of metab- 

 olism in the different organisms are so manifold and of such 

 immense variety that we should be very careful in our general- 

 izations before being in possession of material extending over a 

 great number of groups of organisms. Above all, it does not 

 seem justifiable to also accept this type for life existing at higher 



