THE FOOD AND DIGESTION 319 



The chief proteid of milk is caseinogen, a nucleo-proteid 

 with a very small amount of phosphorus, which exists as a 

 soluble calcic compound. It is held in solution in milk, but 

 under the influence of various agents it clots or curds. It is 

 split by the action of acids, and the casein is precipitated. 

 Under the influence of rennet it is also split into whey 

 albumin which remains in solution, and calcic paracasein 

 which is insoluble. In cow's milk a small amount of an 

 albumin is also present. 



The fats of milk occur as small globules of varying size 

 floating in the fluid, each surrounded by a proteid envelope 

 which must be removed by means of an alkali or an acid 

 before the fat can be extracted with ether. The fats are 

 chiefly olein with smaller quantities of palmatin and stearin, 

 and still smaller amounts of such lower fats as butyrin, 

 capronin, and caprylin. 



The carbohydrate of milk is lactose, a disaccharid, which 

 splits into dextrose and galactose. 



The ash of milk is rich in phosphoric acid, calcium, and 

 potassium poor in sodium and iron. 



Butter and Cream are simply the fats of the milk more or 

 less completely separated from the other constituents. 



Cheese is produced by causing the coagulation of the 

 casein, which carries with it a large amount of the fats. If 

 cheese is made before the removal of the cream it is rich in 

 fats, if after the removal of the cream it is poor in fats. 

 Cheese contains between 25 and 30 per cent, of proteid, and 

 between 10 and 30 per cent, of fat. It is as a source of 

 proteid that it is of chief value. 



Cheese, when allowed to stand, affords a suitable nidus 

 for the growth of micro-organisms by the action of which 

 the proteids are digested into peptones and simpler bodies, 

 and the fats split up into glycerine and the lower fatty 

 acids. These free fatty acids give the peculiar flavour to 

 ripe cheese. The lactose is in part converted into lactic 

 acid. 



2. Flesh. Under this head may be included not only the 

 muscles of various animals, but also such cellular organs as 

 the liver and kidneys. 



When free of fat they contain about 20 per cent, of pro- 



