246 DIGESTION. 



Animals cannot subsist on any but organic substances, 

 and these must contain the several elements and com- 

 pounds which are naturally combined with them : in other 

 words, not even organic compounds are nutritive unless 

 they are supplied in their natural state. Pure fibrin, pure 

 gelatin, and other principles purified from the substances 

 naturally mingled with them, are incapable of supporting 

 life for more than a brief time. 



Moreover, health cannot be maintained by any number 

 of substances derived exclusively from one only of the two 

 chief groups of alimentary principles mentioned above. A 

 mixture of nitrogenous and non-nitrogenous organic sub- 

 stances, together with the inorganic principles which are 

 severally contained in them, is essential to the well-being 

 and, generally, even to the existence of an animal. The 

 truth of this is demonstrated by experiments performed for 

 the purpose, and is illustrated by the composition of the 

 food prepared by nature as the exclusive source of nou- 

 rishment to the young of Mammalia, namely, milk. 



COMPOSITION OF MILK. 



Human. Cows. 



Water . . . . 890 . . . .858 

 Solids . . . . no . . . . 142 



1,000 1,000 



Casein . . . 35 . . . .68 



Butter . . . . 25 . . . . 38 



Sugar (with extractives) 48 . . . .30 



Salts . . . . 2 . . . 6 



no 142 



In milk, as will be seen from the preceding table, the 

 albuminous group of aliments is represented by the casein, 

 the oleaginous by the butter, the aqueous by the water, 

 the saccharine by the sugar of milk. Among the salts of 

 milk are likewise phosphate of lime, alkaline, and other 

 salts, and a trace of iron ; so that it may be briefly said 



