MILK. 205 



both breasts, in twenty-four hours, averages about 2 pounds and 14 

 ounces, or 22 grains for every 1,000 grains of her weight. A cow 

 secretes only 10.4 grains to 1,000, or 13 pounds and 4 ounces in 

 all. 1 (Lehmann.) 



Origin. Milk is secreted by the epithelial cells of the ultimate 

 follicles or cceca of the lacteal glands. It is not to be, hence, in- 

 ferred, however, that its constituents exist preformed in the blood ; 

 for true secretion (the urine alone, perhaps, excepted) always implies 

 that the fluid secreted is formed in the cells from the elements in 

 the plasma of the blood, and therefore contains elements not to be 

 found in the latter. It has already been stated that the sugar is 

 formed in the mammary gland (p. 72); and the assertion of Mialhe, 

 that caseine exists in the blood, though probable, has not been 

 fully confirmed (p. 88). The caseine of the milk is pretty certainly 

 derived from the albumen in the blood. 



The ducts and terminal follicles of the lacteal gland are shown 

 by Figs. 114 and 115. 



Fig. 114. Fig. 115. 



Milk-ducts terminating in clusters of follicles. Terminal coeca (follicles) of lacteal gland, with 



their secreting cells (a, a) ; nuclei (b, b). 



The epithelial cells lining the follicles (Fig. 115) may be seen to 

 contain the milk-globules, and are therefore proved to be the real 

 agents in this secretion. On bursting, they set the globules free in 

 the follicles, which, communicating with the ducts (Fig. 114), pour 

 the milk into the latter. 



Uses. Milk is the normal food of all the mammalia during the 

 first period after birth. It therefore combines all the elements ne- 

 cessary for perfect nutrition and rapid growth. A discussion of all 

 its important physiological relations would, however, be out of 

 place here. The caseine is its nutritive element (p. 89). 



If, however, milk is the proper nourishment for the infant, while 



1 More than double the quantity here mentioned by Lehinann is very generally 

 secreted by the cow, in this country. 



