858 THE PHYSIOLOGY OF REPRODUCTION. 



creting alveoli are not fully formed until the first pregnancy. Dur- 

 ing the period of gestation the epithelial cells multiply, the alveoli 

 are formed, and after parturition secretion begins. As the liquid 

 is formed it accumulates in the enlarged galactophorous ducts, and 

 after the tension has reached a certain point further secretion 

 is apparently inhibited. If the ducts are emptied, by the infant 

 or otherwise, a new secretion begins. The emptying of the ducts, 

 in fact, seems to constitute the normal physiological stimulus to 

 the gland-cells, but how this act affects the secreting cells, whether 

 reflexly or directly, is not known. 



Composition of the Milk. The composition of milk is com- 

 plex and variable. The important constituents are the fats, held 

 in emulsion as minute oil droplets and consisting chiefly of olein 

 and palmitin; casein, a nucleo-albumin which clots under the in- 

 fluence of rennin; milk-albumin or lactalbumin, a proteid resem- 

 bling serum-albumin ; lactose or milk-sugar ; lecithin, cholesterin, 

 phosphocarnic acid, urea, creatin, citric acid, enzymes, and min- 

 eral salts. It is well known also that many foreign substances 

 drugs, flavors, etc. introduced with the food are secreted in the 

 milk. An average composition is: proteids, 1 to 2 per cent.; 

 fats, 3 to 4 per cent.; sugar, 6 to 7 per cent.; salts, 0.1 to 0.2 per 

 cent. The fact that casein and milk-sugar do not exist preformed 

 in the blood is an argument in favor of the view that they are formed 

 by the secretory metabolism of the gland cells. The special com- 

 position of the milk-fat and the histological appearance of the 

 gland cells during secretion lead to the view that the fat is also 

 constructed within the gland itself. Bunge has called attention 

 to the fact that the inorganic salts of milk differ quantitatively 

 from those in the blood-plasma and resemble closely the propor- 

 tions found in the body of the young animal, thus indicating an 

 adaptive secretion. This fact is illustrated in the following table 

 giving the mineral constituents in 100 parts of ash: 



YOUNG PUP. DOGS' MILK. DOGS' SERUM. 



K 2 8.5 10.7 2.4 



Na 2 8.2 6.1 52.1 



CaO 35.8 34.4 2.1 



MgO 1.6 1.5 0.5 



Fe 2 O 3 0.34 0.14 0.12 



P,O 5 39.8 37.5 5.9 



Cl 7.3 12.4 47.6 



On account of the use of cows' milk in place of human milk in 

 the nourishment of infants much attention has been given to 

 the relative composition and properties of the two secretions. 

 The chief difference between the two lies apparently in the casein. 

 The casein of human milk is smaller in amount, curdles in looser 



