364 EXCRETION. 



starch, fat, or albumen ; but they are replaced by urea and other 

 crystallizable substances, of a different nature. Even the carbonic 

 acid exhaled by the breath, as experience has taught us, is not pro- 

 duced by a direct oxidation of carbon ; but originates by a steady 

 process of decomposition, throughout the tissues of the body, some- 

 what similar to that by which it is generated in the decomposition 

 of sugar by fermentation. These phenomena, therefore, indicate an 

 actual change in the substance of which the body is composed, and 

 show that its entire ingredients are incessantly renewed under the 

 influence of the vital operations. 



