MAMMARY GLANDS. 

 COMPOSITION OF MILK. 



157 



Caseinogen is the chief proteid constituent of milk, and is held 

 in solution by the presence of calcium phosphate. On the addition of 

 acetic acid or of sodium chlorid up to the point of saturation, the 

 caseinogen is precipitated as such, and may be collected by appro- 

 priate chemic methods. When taken into the stomach caseinogen is 

 coagulated that is, it is separated into casein or tyrein and a 

 small quantity of a new soluble proteid. The ferment which induces 

 this change is known as rennin. The presence of calcium phosphate 

 is necessary for this coagulation. 



The fat of milk is more or less solid at ordinary temperatures. It 

 is a composition of olein, palmitin, and stearin, with a small quan- 

 tity of butyrin and caproin. When milk is allowed to stand for 

 some time the fat-globules rise to the surface and form a thick layer, 

 known as cream. When subjected to the churning process, the fat 

 globules run together and form a cohesive mass the butter. 



Lactose is the particular form of sugar characteristic of milk. It 

 belongs to the saccharose group and has the following composition : 

 CiaHasOn. In the presence of the bacillus acidi lactici the lactose 

 is decomposed into lactic acid and carbon dioxid, the former of 

 which will cause a coagulation of the caseinogen. 



