THE ANIMAL MACHINE 7 



field which would be utter waste, or, at best, serve but 

 for fuel if available only in their raw, unconverted state. 

 4. Intermediate relation of animal to plants and man. — ■ 

 There are about fifteen principal chemical elements of 

 nutrition. They are constituents of the human body, 

 likewise the bodies of animals and plants ; a few of them 

 compose also the three requisites for the maintenance of 

 life, — air, water and soil, the sources of food. We can, 

 therefore, understand the synthetical relationship between 

 man on the one hand, and the original sources of his sub- 

 sistence on the other, with the plants and animals as inter- 

 mediary factors. Some of the hydrogen and oxygen re- 

 quired by man is obtained direct from the water he drinks ; 

 more of his oxygen is taken direct from the air he breathes, 

 and in return he gives carbon dioxide, equally essential 

 for plant respiration. The soil, however, contributes to 

 our support only indirectly through the bodies of the 

 plants grown upon it. These plants also make abun- 

 dant use of the nitrogen of the air, an important function, 

 since nitrogen is a chief constituent of the protoplasm of 

 the human or animal cell, both of which are helpless to 

 draw directly upon the supply in the air. A great wealth 

 of plant products are directly available to man in quantity 

 and variety to meet his nutritive requirements, but the 

 entire body of some plants and the major portion of many 

 others are impossible for human consumption. The corn 

 kernel, after a process of milling, becomes a staple article 

 of food for man, but for every pound of corn there is ap- 

 proximately a pound of stover, which would be absolutely 

 valueless were it not for the ruminant's ability to transform 

 it into digestible, nutritious animal food products, meat 

 and milk. We readily recognize the physical impossibility 

 of a man's consuming sufficient pasture grass or hay to 



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