458 



FOOD. 



[CH. XXVIII. 



quantities of globulin instead. Microscopically, cells from the 

 acini of the mammary gland are seen, which contain fat 

 globules in their interior ; they are called colostrum corpuscles. 



Eeaction and Specific Gravity. The reaction of fresh 

 cow's milk and of human milk is generally neutral or slightly 

 alkaline. In carnivora the milk is acid. All milk readily turns 

 acid or sour as the result of fermentative change, part of its 

 lactose being transformed into lactic acid. The specific gravity of 

 milk is usually ascertained with the hydrometer. That of normal 

 cow's milk varies from 1028 to 1034. When the milk is skimmed 

 the specific gravity rises, owing to the removal of the light con- 

 stituent, the fat, to 1033 to 1037. In all cases the specific 

 gravity of water, with which other substances are compared, is 

 taken as 1000. 



Composition. Frankland gives the following table, con- 

 trasting the milk of woman, ass, and cow : 



Hence, in feeding infants on cow's milk, it is necessary to dilute 

 it, and add sugar to make it approximately equal to natural 

 human milk. 



The Proteids of Milk. The principal proteid in milk is 

 called caseinogen ; it is precipitable by acids such as acetic acid, 

 and also by saturation with magnesium sulphate, or half satura- 

 tion with ammonium sulphate, so resembling globulins ; it is 

 coagulated by rennet to form casein. Cheese consists of casein 

 with the entangled fat. The other proteid in milk is an albumin. 

 It is present in small quantities only ; it differs in some of 

 its properties (specific rotation, coagulation temperature, and 

 solubilities) from serum-albumin ; it is called lact-albumin. 



The Coagulation of Milk. Rennet is the agent usually em- 

 ployed for this purpose : it is a ferment secreted by the stomach, 

 especially in sucking animals, and* is generally obtained from the 

 calf. 



The curd consists of the casein and entangled fat : the liquid 

 residue called whey contains the sugar, salts, and albumin of the 

 milk. There is also a small quantity of a new proteid called 

 whey-proteid, which differs from caseinogen by not being con- 



