MAMMARY GLANDS. 

 COMPOSITION OF MILK 



141 



Caseinogen is the chief protein 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 appropriate 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 protein. 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 quantity 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: C 12 H 2 2O n . 

 In the presence of the bacilus acidi lactici the lactose is decomposed into 

 lactic acid and carbon dioxide, the former of which will cause a coagulation of 

 the caseinogen. 



Mechanism of Secretion. During the time of lactation the mammary 

 gland exhibits periods of secretory activity which alternate with periods of 



