234 EESPIRATION. 



tion in the animal system ; and that the fatty matters 

 of the food are not directly appropriated by the ani- 

 mals which feed on them. 



602. Bearing in mind, then, that the strength of 

 man and animals depends mainly on muscle, and 

 that the formation of muscle is greatly dependent on 

 the amount of organic subject containing nitrogen in 

 their food, it becomes a matter of the first importance 

 to study the mode of increasing the quantity of these 

 matters in food. 



603. Some animals feed entirely on vegetable 

 food ; others feed entirely, or in part, on flesh : in 

 either case they derive their nitrogen, or the sub- 

 stances containing it, from plants. Animals do not 

 appear to have any power of absorbing nitrogen from 

 the air; all the albumen, fibrin, &c., which they con- 

 tain, is therefore either directly or indirectly obtained 

 from plants. 



604. The most important of the chemical functions 

 of animal life may be classed under the two great 

 heads of nutrition and respiration; and, consequently, 

 food also may be divided into those kinds which con- 

 tribute to the one or other of these two objects. 

 The changes which ordinary food undergoes in pass- 

 ing through the stomach of an animal, are briefly 

 these : mechanical division, eff'ected by chewing, &c.; 

 chemical division or digestion, eff'ected in the stomach ; 

 chemical transformation, conversion of starch, &c., 

 into animal matters ; absorption of azotized matters 

 identical in composition with animal matters, and 



