456 



ANATOMY FOR NURSES [Chap. XXI 



The secretion of milk. — The secretory portion of the mammary 

 glands are the milk ducts, and these are lined with secreting cells. 

 Some of the constituents of the milk, i.e. water, salts, and sugar, 

 are secreted by these cells from the blood, but it is thought that the 

 cells themselves disintegrate and form the proteins and fat. The 

 sugar contained in the milk is lactose, and the sugar of the blood 

 is glucose, so if the first is derived from the second, some chemical 

 change must take place either during or after secretion. 



Colostrum and milk. — The secretion of the mammary glands 

 during the first few days of lactation is called colostrum. It is a 

 thin, yellowish fluid, composed of proteins, fat, sugar, salts, and 

 water, but not in the same proportion as in milk. It also contains 

 numerous cells containing large masses of fat. These are called 

 colostrum corpuscles, and are secreting cells that are not completely 

 broken down. 



Human milk is specially adapted to the requirements of the 

 infant and so diflFers in some respects from that of all other animals. 

 Cow's milk is most frequently substituted for human milk and the 

 relative composition of the two can be seen in the following 

 table : — 



In substituting cow's milk for human milk the differences that 

 must be taken into consideration are not only the different relative 

 proportions, but also the following : (1) the difference in the pro- 

 teins ; the protein of human milk is one-third caseinogen, and two- 

 thirds lactalbumin, and that of cow's milk is five-sixths caseinogen 

 and one-sixth lactalbumin ; (2) the difference in the curds formed 

 in the stomach ; human milk curdles in small flocculi, and cow's 

 milk curdles in large heavy curds ; and (3) the reaction of both 

 human and cow's milk is amphoteric, but cow's milk is more nearly 

 acid than human milk. 



