382 



MANUAL OF PHYSIOLOGY. 



and gives rise to lactic acid. When the quantity of lactic acid 

 is sufficient, it not only makes the milk sour, but also precipi- 

 tates another of its important constituents, namely, casein. This 

 albuminous body in its coagulation entangles the fat of the milk, 

 and we have thus formed the curd of cracked milk, while the 

 whey consists of the acid, salts and remaining milk sugar. 



Although the curdling of milk depends on the coagulation of 

 an albuminous body, it is never produced by boiling fresh milk, 

 because the chief proteid is casein, a form of derived albumin 

 (alkali albumin), which does not coagulate by heat. 



FIG. 167. 



Section of Mammary Gland during active lactation (human). (a) Saccules lined with 

 regular epithelium. (6) Connective tissue between the alveoli. (Cadiat.) 



When milk is preserved from impurities, and kept in a cool 

 place, a thick yellow film soon collects on the topvof the fluid ; 

 the thickness of this layer the cream may be taken as a rough 

 gauge of the richness of the milk, for milk consists of a fine 

 emulsion of fat, the suspended particles of which are kept from 

 running together by a superficial coating of dissolved casein. 

 When left at rest, the light, fatty particles float to the top and 

 form the cream. 



When the mammary glands commence to secrete, the milk 



