66 PRINCIPLES OF PHYSIOLOGY 



(1) Oxidation is the most common chemical 

 reaction. By continuous processes of oxida- 

 tion complex bodies are split up into simpler 

 ones. Thus by oxidation, protein, such as 

 exists in white of egg, may be split up into 

 leucin, tyrosin, glycine, and the fatty acids ; 

 uric acid into urea, allantoin, oxalic acid and 

 carbonic acid. Oxidation has been carried 

 out by the chemist, with the production of 

 chemical substances the same as those found 

 in the fluids and tissues ; and the inference 

 is that in the living body they are also pro- 

 duced by oxidation. But in living matter 

 the processes are obscure, and there are 

 probably intermediate steps still unknown. 

 There can be little doubt that oxidations 

 occur almost wholly in the tissues ; but 

 we must avoid taking too mechanical a 

 view of the nature of oxidation in living 

 matter. There is no such phenomenon as 

 the direct union of oxygen with carbon, or 

 with hydrogen, as in burning a candle. The 

 combustion of a candle will always yield, for a 

 given weight, the same amount of carbonic 

 acid and of water, and the same amount of 



