DIGESTION OF FOOD. 



275 



soon as if they had been wholly deprived of food. But it has ; 

 also been observed that some animals will all but starve rather / 

 than eat certain kinds of food, though chemically sufficient. ( 

 We must thus recognize something more in an animal than 

 merely the mechanical and chemical processes which suffice to 

 accomplish digestion in the laboratory. A food must be not 

 only sufficient from the chemical and physical point of view, 

 but be capable of being acted on by the digestive juices, and of / 

 such a nature as to suit the particular animal that eats it. 



To illustrate, bones may be masticated and readily digested ! 

 by a hyena, but not by an ox or by man, though they meet the \ 

 conditions of a food in containing all the requisite constituents. J 

 Further, the food that one man digests readily is scarcely di- 

 gestible at all by another ; and it is within the experience of 

 every one that a frequent change of diet is absolutely necessary. ; 



Since all mammals, for a considerable period of their exist-'' 

 ence, feed upon milk exclusively, this must represent a perfect; 

 or typical food. It wLU be worth while to examine the compo- 

 sition of mUk. The various substances composing it, and their 

 relative proportions for different animals, may be seen from the 

 following table, which is based on a total of 1,000 parts : 



The following table, giving the percentage composition of 

 the milk of different animals, may prove instructive. 



