FOOD AND DIGESTION 333 



about 1030, and in man and herbivorous animals its reaction 

 is alkaline. 



The chief protein of milk is caseinogen, a nucleo-protein 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. Its calcic 

 compound is split by the action of acids, and casein is pre- 

 cipitated. Under the influence of rennet caseinogen splits 

 into whey albumin which remains in solution, and calcic para- 

 casein which is insoluble and separates as a curd containing 

 the fat. 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 protein 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. Under the action of various 

 micro-organisms it is split to form lactic acid, thus causing the 

 souring of milk. 



Phosphorus Compounds. In addition to caseinogen, milk 

 contains other organic phosphorous compounds. Among these 

 is lecithin and a compound which has been called phosphocarnic 

 acid, the constitution of which is not fully understood. In 

 human milk the greater part of the phosphorus is in organic 

 combinations, while in cow's milk the amount in inorganic 

 compounds is much greater. 



Milk is specially rich in calcium and potassium, but the 

 amount of iron in milk is very small, and therefore, when the 

 child has used up the store of iron which it has in its body 

 at birth, it is necessary to replace the milk-diet by foods 

 containing more iron. 



The average composition of the milk of the cow, mare, and 

 bitch is shown in the table on page 334. 



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, 



