EXTERNAL SECRETIONS 



485 



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 coagulates the caseinogen in a manner similar to, if not iden- 

 tical with, this process, which appears to be essential to the normal digestion 

 of the milk. 



Fat is present in the condition of a very fine emulsion and 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 34C. 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 to- 

 gether 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 12 H 22 O n . 

 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. 



Inorganic salts are always present and are chiefly those of potassium, 

 sodium, calcium, and magnesium phosphates and chlorids. The following 

 table of Bunge gives the quantitative amounts of these constituents in both 

 human and cow's milk: 



Iron is also present in small amounts possibly from 3 to 5 milligrams per 

 icoo c.c. Citric acid to the extent of 0.05 per cent, is also present. 



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 

 into the lumen of the acinus. During the time these changes are taking 

 place oil-globules make their appearance in the cell protoplasm, some of 



