376 



ESSENTIALS OF PHYSIOLOGY. 



being most abundant. Analyses have shown that these salts are 

 present in the milk of any one species in exactly the same proportions 

 in which they occur in the young animal which is nourished on that 

 milk. The large proportion of calcium phosphate is of especial 

 importance in view of the formation of bone in the growing animal. 



When milk is allowed to stand it becomes sour. The acidity is due 

 to the formation of lactic acid from the lactose by the agency of certain 

 organisms, such as bacterium lactis, present in the milk. The growth 

 of these germs is facilitated by warmth. The lactic acid has the same 

 effect on the caseinogen as the addition of acetic acid ; that is, it 

 precipitates the caseinogen, and the latter entangles the fats, forming a 

 curd. The precipitation of caseinogen in this way must not be con- 

 fused with the clotting of milk which is brought about by ferment 

 action in the stomach (p. 304). In the latter process the caseinogen 

 undergoes a chemical change, being converted into casein. 



The suitability of the maternal milk for the needs of the growing 

 animal does not depend only on the fact that the various constituents 

 are in the correct proportions. As has already been pointed out, every 

 protein consists of a characteristic grouping of amino-acids, some of 

 these acids being present to a special extent in one protein, and others 

 in another. Caseinogen is remarkable in that all the amino-acids 

 which enter into the composition of the various proteins are represented 

 in its structure to a greater or less extent, so that it may form a source 

 from which any of the body proteins may be built up. Only glycine 

 is absent from the caseinogen molecule, and glycine can be formed in 

 the body by hydrolysis of the higher acids. 



The caseinogen of human milk does not form a firm clot with the 

 " rennet " ferment as does that of cow's milk, but is thrown down in 

 the form of a flocculent precipitate. For this reason cow's milk, even 

 when diluted, does not form a satisfactory substitute for maternal milk 

 in the case of the human infant. Other drawbacks to the "bottle- 

 feeding" of infants are (1) the difficulty of obtaining sterile cow's milk ; 

 (2) the fact that sterilisation can only be effected at the cost of losing 

 a proportion of the vitamines ; and (3) the loss to the child of anti- 

 bodies, which are present in the maternal milk, and help to protect it 

 from certain infective diseases. 



Food-stuffs derived from Milk. The cream of milk contains 14 to 

 44 per cent, of fats, and is a useful form of administering fat when an 

 extra amount is required in the dietary. 



Butter is obtained by separating the fats from cream, and is almost 

 pure fat with a small percentage of water. 



Cheese is made by the compression of clotted milk so as to express 



