FOOD CONSTITUENTS 111 



temperature of the animal (about 100° Fahr.) is main- 

 tained by heat generated within the body by the com- 

 bustion of the materials consumed as food. The energy 

 by which all the mechanical work of the animal is 

 performed is also derived from the same source. The 

 source of heat and 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 vege- 

 table tissues. In the animal, besides constructing 

 tissue, the food has to furnish the means of producing 

 heat, and executing mechanical work; to accomplish 

 this result it is burnt in the body. 



(1) Food Constituents and their Functions. — The soHd 

 ingredients of vegetable food may be classed generally 

 as — (1) albuminoids (proteids) ; (2) fats; (3) carbo- 

 hydrates ; (4) salts. Besides these general ingredients 

 of food, we have in immature vegetable products a fifth 

 class — the amides — which also take part in animal 

 nutrition. The albuminoids and amides are nitro- 

 genous substances, the other ingredients of food are 

 non-nitrogenous. 



The albuminoids occurring in grain, roots, and other 

 forms of vegetable food, are similar in general com- 

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

 From the albuminoids of the food are formed not 

 only the albuminoids of the animal frame, but also 

 the gelatinoids, the hair, wool, horn, &c., and, under 

 some circumstances, the fat. By the combustion of 

 albuminoids in the body heat and mechanical force will 

 also be developed. Albuminoids thus supply in them- 



