94 



MANUAL OF PHYSIOLOGY. 



seen that a varying quantity of all chemical classes of food-stuffs 

 are generally present in most kinds of food, whether animal or 

 vegetable. The above diagram shows the proportion of the 

 more important food-stuffs in some examples of the materials 

 commonly used as food. 



Among animal foods are included milk, the flesh of various 

 animals, and the eggs of birds. These may be more fully described 

 as typical examples. 



Milk. For a certain period of their lifetime the secretion of 

 the mammary gland forms the only food of all mammals, and it 

 is the one natural product which when taken alone affords ade- 

 quate nutriment. 



It consists of a slightly alkaline watery fluid, containing: 



1. Proteids, in solution. 



2. Fats, finely divided to form perfect emulsion. 



3. Sugar, in solution. 



4. Salts, in solution. 



Owing to the action of certain organisms which readily propa- 

 gate in milk if exposed to the air at a warm temperature for some 

 time, it loses its alkaline reaction, and becomes sour from the for- 

 mation of lactic acid from the milk sugar, by a kind of fermen- 

 tation, the probable equation for which may be written thus : 



C 6 H,A = 2C 3 H 6 0, 



Milk sugar. Lactic ;iH<l. 



If fresh good milk be allowed to stand, the fatty particles tend 

 to float to the surface, thus forming a layer of cream. 



The milk of different animals is similar in all essential points, 

 but differs slightly in the relative proportion of the ingredients, 

 as may be seen in the following table: 



