TRANSFORMATIONS <>F CARBON AND NITROGEN. 77 



table food. After the nitrogen has assumed this form of pro- 

 teid, or allied bodies, whether in animals or plants, it has been 

 wholly removed from the possibility of serving plants as food. 

 We cannot feed green plants upon eggs or upon flesh. The 

 nitrogen of these compounds is in too complex a condition for 

 plants to use, since they must have their nitrogen in far sim- 

 pler compounds than this. Our next question must be how 

 it is that these highly complex bodies, built by plants, utilized 

 and stored by animals, are reduced to the simple condi- 

 tion in which they can once more serve as plant foods. This 

 passage downward, from the proteid to the nitrate, is controlled 

 in large degree by the action of bacteria. 



The foods taken by animals may be, so far as concerns their 

 future history, divided into two classes : 



1 . Part of the food used by animals is metabolized in the 

 animal body to furnish them with energy and material for 

 growth. As the result of this metabolism the materials are 

 reduced to simpler chemical forms. The sugars and starches, 

 as well as the fats, are quite largely metabolized by the life 

 processes of the animal. Part of these foods which the animal 

 assimilates are, it is true, stored in its body as fat and may 

 remain in the body for post-mortem decomposition after the 

 death of the animal, but the larger part are oxidized in the 

 body almost at once. The carbon present in the foods is com- 

 bined with oxygen taken in by the respiratory organs, the re- 

 sult being that most of the metabolized carbon is eventually 

 exhaled from the body in the form of CO 2 , which joins the 

 store of this gas in the atmosphere and thus completes its 

 cycle. 



2. The proteids consumed by the animal also have a double 

 history, part of them being metabolized immediately, and part 

 of them being stored in the animal and remaining there for 

 post-mortem changes. That part of the proteid which the 

 animal uses for its own purpose and metabolizes is broken to 



