1002 THE PHYSIOLOGY OF REPRODUCTION. 



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 protein resem- 

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

 cholesterin, phosphocarnic acid, urea, creatin, citric acid, enzymes, 

 and mineral salts. It is well known also that many foreign sub- 

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

 in the milk. An average composition is: proteins, 2 to 3 per cent.; 

 fats, 3 to 4 per cent.; sugar, 6 to 7 per cent.; salts, 0.2 to 0.3 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 suggest 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 O.. 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, 0.34 0.14 0.12 



P,O fi .. 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 loosei 

 flocks than that of cows' milk, and seems to dissolve more easily 

 and completely in gastric juice. The former also contains rela- 

 tively more lecithin and less ash, particularly the lime salts. On 

 the other hand, cows' milk contains less sugar and fat. In using 

 it, therefore, for the nutrition of infants it is customary to add 

 water and sugar. The composition of cows' milk is so well known 

 that it is easy to modify it for special cases according to the in- 

 dications. The rules for this procedure will be found in works 

 upon pediatrics. 



* For data as to composition and hygienic relations, see Bulletin 41, 

 "Hygienic Laboratory," Public Health and Marine Hospital Service, U. S., 

 Washington, 1908. 



