KESPIRATION 127 



respiration we must therefore turn our attention to the 

 metabolic changes which are taking place normally in the 

 living substance. From the instability which we have 

 noticed in the protoplasmic material, we can infer that its 

 own molecules are in a constant state of decomposition 

 and reconstruction, new material being incorporated and 

 certain other substances cast off. Besides these, we are 

 probably not wrong in concluding that other changes also 

 take place in the various substances which are contained 

 in it, into which its own molecules do not enter. Pro- 

 cesses of slow oxidation and gradual reduction are taking 

 place there continually, excited, however, in all probability 

 by the changes in the protoplasm itself. Even these, how- 

 ever, are by no means simple, and the direct oxidation of 

 either carbon or hydrogen has probably no place among 

 them. An instance of them may be seen in the oxidation 

 of alcohol in the cells of Mycodermi aceti, a fungus which 

 converts alcohol into acetic acid. This process, into which 

 the molecule of protoplasm apparently does not enter, can 

 only go on in the living cell. Other similar instances 

 could be quoted. 



The probable course of events is that the oxygen in 

 some way unites with the protoplasm, rendering it un- 

 stable, and initiating a series of decompositions which 

 result in the successive formation of many bodies of less 

 complex composition, each successive decomposition pro- 

 ducing simpler ones, till finally carbon dioxide and water are 

 formed. Simultaneously, reconstruction of the protoplasm 

 goes on, many of these residues being in whole or in great 

 part built up again into its substance, together with new 

 material supplied to it in the shape of food. If the 

 temperature is low, the breaking down of the protoplasm 

 proceeds but slowly, and reconstruction is rapid, so that 

 under these conditions the quantity of oxygen absorbed 

 or fixed as intramolecular oxygen by the protoplasm is 

 greater than the quantity of carbon dioxide formed by its 

 decomposition. At a higher temperature decomposition is 



