SECRETION. 



443 



conversion into an insoluble protein, casein or tyrein. To this process the 

 term coagulation has been given. The presence of calcium phosphate 

 appears to be essential to this process, inasmuch as it does not take place if 

 the milk be completely decalcified by the addition of potassium oxalate. 

 After coagulation, the more or less solid mass of milk separates into a liquid 

 portion, the serum, and a solid portion, the coagulum. The former, gener- 

 ally termed whey, consists of water, salts, lactalbumin, sugar; the latter, the 

 curd, consists of the casein and entangled fat. Boiling the milk 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 coagules the caseinogen 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. 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 form of sugar characteristic of milk. It belongs 

 to the saccharose group and has the following composition: C^H^O^. 

 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 accumulation of the lactic acid the caseinogen is precipi- 

 tated 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 the milk each acinus presents the follow- 

 ing features: The epithelial cells are short, cubical, nucleated, and border a 

 relatively wide lumen, in which is found a variable quantity of milk. 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 



