NATURE AND COMPOSITION OF MILK. 



Milk, by which term is to be understood the nutri- 

 tive secretion of nursing mammalia, consists of water 

 holding fat in suspension, and nitrogenous, saccharine 

 and mineral matters in solution. Cow's milk, being 

 of the greatest importance to the analyst, will receive 

 the largest share, of attention, and will be under- 

 stood to be meant in all cases, unless otherwise 

 stated. 



With rare exceptions, the secretion of milk takes 

 place only as a result of pregnancy and delivery at 

 term, and continues for a variable period. The chem- 

 istry of its formation is not entirely understood. The 

 organic ingredients do not exist in appreciable quanti- 

 ties in the blood, and must, therefore, be elaborated by 

 specific secretory action. The fat has been regarded 

 by many authorities as resulting from the fatty degen- 

 eration of the epithelial cells lining the ducts of the 

 mammary gland. It occurs in the form of minute 

 globules from .0015 mm. to .005 mm. in diameter, 

 under conditions which prevent spontaneous coales- 

 cence. Films of proteid matter are also abundant. 

 A form of sugar, isomeric with cane-sugar, and called 

 lactose, is present. 



B 9 



