LACTATION 



563 



any rate in cow's milk. The percentage of volatile fatty acids 

 is less in human than in cow's milk. 



Lactose or milk-sugar is the carbohydrate of milk, but other 

 carbohydrates (animal gum, dextrin, Sec), have also been stated 

 to occur. Lactose under the influence of certain micro-organisms 

 becomes converted into lactic acid, which causes the souring of 

 milk. 



Milk is rich in calcium and potassium salts (especially in 

 calcium phosphate), but magnesium, sodium, and other salts 

 (chiefly chlorides) are also present in small quantities. The 

 amount of iron in human milk is very slight, while in cow's 

 milk it is practically absent altogether. 



Bunge ^ has pointed out that whereas the inorganic salts in 

 milk are present in different proportions from those found in 

 the blood plasma, these proportions are almost identical with 

 those occurring in the young animal. He supposes this similarity 

 to indicate an adaptation to the needs of the young. This point 

 is iUustrated in the following table, which gives the respective 

 amounts of mineral constituents present in a hundred parts of 

 ash of (1) the young pup ; (2) dog's milk, and (3) dog's serum. 



Small quantities of oxygen, nitrogen, and carbon dioxide 

 gases have been found in solution both in human and in cow's 

 milk. 



' Bunge, Lehrbuch der Physiologiache und Pathologiache Chemie, Leipzig, 

 1887, and various original papers. Gf. Abderhalden, "Die Beziehungen der 

 Wachsthumsgeschwindigkeit der Sauglinge zur Zusammensetzung der Milch," 

 &c., Zeiiachr. f. Phya. Chem., vol. xxvi,, 1899, and other papers by the same 

 author in the same journal (vols. xxvi. and xxvii.). For farther references 

 see Lusk, The Science of Nutrition, Philadelphia, 1906. It is stated also 

 that the rennin of the stomach is specifically adapted for the coagulation of 

 the casein produced by the female of the same race. 



