SECRETION. 



401 



retards and even prevents the coagulation by rennet, owing to the 

 precipitation of the calcium phosphate. When milk is_ taken into the 

 stomach, it is probable that the rennin coagulates the caseino^en ^in a 

 manner similar to, if not identical with, this process, which appears 

 to be essential to the normal digestion of the milk. 



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

 It is a compound of olein, palmitin, and stearin with small quantities 

 of butyrin and caproin and fatty acids. The melting-point of butter 

 varies between 31 and 34 C. When milk is allowed to stand for 

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

 known as cream. Churning the milk or cream causes the fat-globules 

 to run together and form a coherent mass termed butter. 



Lactose^ is the particular fonn__pf sugar characteristic 



^ 



to th 



12 H 2211 



_ 



saccharose group and has the following composition : 

 + HO. Though incapable of undergoing fermentation 



by the action of the yeast plant, it is readily reduced by the Bacillus 

 acidi lactici to lactic acid and carbon dioxid, the former of which 

 imparts to milk an acid reaction and a sour taste. With the accumu- 

 lation of the lactic acid the caseinogen is precipitated as a more or less 

 consistent mass. 



The inorganic salts of milk are chiefly 'potassium, sodium, 

 calcium, and magnesium phosphates and chlorids. Iron is also 

 present in small amount. The following table of Bunge gives the 

 quantitative amounts of these constituents in both human and cow's 

 milk: 



Mechanism of Milk Secretion. During the time of lactation 

 the mammary gland exhibits periods of secretory activity which 

 alternate with periods of repose. Coincidently with these periods 

 certain histologic changes take place in the secreting epithelium. 

 At the ! close of a period of active secretion and after the discharge of 

 tKeflnillf eacTTacTnus preserftT"the~following features: The epithelial 

 short nihinj, nucleated^ and border a rflaHyffly wi'He 

 a variable q^a r> t?ty o* mifc After the gland 

 has rested^ for some time active metabolism again begins. The 

 cells grow and elongate; the nucleus divides into two or three new 

 nuclei; constriction takes place and the inner portion is detached 

 and discharged into the lumen of the acinus. During the time these 

 changes are taking place oil-globules make their appearance in the cell 

 26 



