ANIMAL NUTRITION. 65 



external to itself. An animal has no such constructive 

 power. The animal frame is built up of substances ex- 

 isting ready formed in the food, or produced by the 

 splitting up or partial combustion of some of the food con- 

 stituents in the body. The animal also derives little or 

 no aid from external force. The temperature of the 

 animal (about 100° Fahr.) is maintained by the heat gene- 

 rated within the body from the combustion of the food 

 consumed ; the force by which all the mechanical work of 

 the animal is performed is also derived from the same 

 combustion of food. The source of force in the animal 

 is thus purely internal. 



It is evident from what has just been said that the food 

 of animals has duties to perform which are not demanded 

 of the food of plants. In plants the food merely provides 

 the matter for building up the vegetable tissues. In the 

 animal, besides constructing tissue, the food has to furnish 

 the means of producing heat and mechanical force. 



1. Food Constituents and their Functions. — The solid 

 ingredients of animal food may be classed generally as — 

 (1) albuminoids ; (2) fat ; (3) carbo-hydrates ; (4) incom- 

 bustible matter, or ash. Besides these general ingredients 

 of food we have in immature vegetable products a fifth 

 class — the amides, which also take part in animal nutri- 

 tion. The albuminoids and amides are nitrogenous 

 substances, the other ingredients of food are non-nitro- 

 genous. 



The various albuminoids occurring in com, roots, and 

 other forms of vegetable food, are quite similar in com- 

 position to those found in milk, blood, and flesh. From 

 the albuminoids of the food are formed not only the al- 



