96 IRRITABILITY 



the material employed for the COj formation and with it the 

 production of energy is carbohydrate, but that, on the other hand, 

 various plant organisms and protistse also use a quantity of other 

 substances, such as fats and protein, indeed even such compara- 

 tively simple organic combinations as alcohol, formic acid and 

 methan. It may be accepted that in all these various instances 

 of excitation of the functional metabolism as a result of stimu- 

 lation, the specific respiratory material of the substance con- 

 cerned is used in greater amount in the decomposition and like- 

 wise invariably yields carbon dioxide. 



The point of most essential interest for the analysis of the 

 excitation processes is, above all, the mechanism of the organic 

 combustion and the associated energy production. Here we may 

 base our observations on the disintegration of carbohydrates, 

 which is most extensive in the animal as well as in the vegetable 

 kingdom. We may now ask how dextrose, for instance, dis- 

 integrates in the living system into carbon dioxide, for it is this, 

 or a sugar of similar chemical nature, which is generally con- 

 cerned. Plant physiology, which here, as in many other respects, 

 is in advance of animal physiology, has indicated two ways by 

 which this can be accomplished in the living substance. One is 

 oxydative, the other, awoxydative disintegration. 



In the oxydative disintegration of dextrose, taking place in 

 aerobic organisms, if sufficient quantities of oxygen are present, 

 there occurs a splitting up of the carbohydrate molecule, as a 

 result of the introduction of oxygen, into simpler substances and 

 finally into carbon dioxide and water, just as the dextrose mole- 

 cule, when subjected to oxdyative processes, is split up into sim- 

 pler molecules. In the living substance the oxydases play the 

 important role of oxygen carriers. It cannot be denied, however, 

 that up to now no carbohydrate splitting oxydases have been 

 obtained from living substance. This, of course, does not prove 

 its nonexistence. But this deserves consideration in connection 

 with an assumption very widely spread among plant physiologists 

 in regard to the aerobic disintegration of the carbohydrate mole- 

 cule, which I shall touch upon presently. If we suppose that 

 oxydases exist, which bring about primarily the oxydative disin- 



