The Animal Body. 215 



such power. The animal tissues are built up from the complex 

 substances existing ready-formed in the food. The animal de- 

 rives no aid from external energy. The temperature of the 

 animal body (about 100 F.) is maintained by heat generated 

 within the body and by the combustion of the material consumed 

 as food. The energy by which all the mechanical work of the 

 animal is performed, comes from the same source. The source 

 of heat and force in the animal is thus purely internal. 



It is apparent from what has been said that the food of animals 

 has duties to perform which are not demanded of the food of 

 plants. In plants the food chiefly provides material for build- 

 ing up the vegetable tissues. In the animal, besides constructing 

 tissue, the food must furnish the means of producing heat and 

 performing mechanical work; to accomplish this result, it must 

 be burned in the animal body. 



Functions of food constituents. The solid ingredients of 

 vegetable food may be classed, as (1) proteins; (2) fats; (3) car- 

 bohydrates; (4) salts. Besides these general classes of food 

 constituents, we have in immature vegetable products, as hays, 

 roots, etc., a fifth class the ammo-acids and amides which also 

 take part in animal nutrition. They are the simple intermed- 

 iary nitrogenous substances, formed from the nitrates absorbed 

 by the plant, and eventually take part in the construction of 

 the complex proteins of seeds and plant tissue. 



The proteins occurring in seeds, roots and other forms of 

 vegetable food, have a general similarity in composition to those 

 found in milk, blood, and flesh, but are by no means identical. 

 From the proteins of the food are formed not only the proteins 

 of the soft tissues of the animal, but also such a class of proteins 

 as the albuminoids, which differ so materially in properties from 

 the proteins of blood and muscle. It is also very probable that 

 fat, a non-nitrogenous body, may be formed from protein. This 

 is still a much disputed question and it remains for future in- 

 vestigations to definitely decide this point. 



Proteins can also serve as a source of energy. In the case of 



