MAMMARY GLANDS. 381 



structures, and are specially elaborated for the lubrication of 

 the ciliary margin. The glands about the prepuce and clitoris 

 are also analogous to the sebaceous glands; in some animals 

 they secrete a peculiarly odoriferous material (Castor). 



MAMMARY GLANDS. 



The secretion of milk only takes place under certain circum- 

 stances, and continues only for a limited period. As the name 

 of the gland implies, they are present in all mammalian animals. 

 The activity of the gland commences in the latter stages of 

 pregnancy, and then continues, if the secretion be regularly 

 withdrawn from the gland, for some nine to twelve months. 



During pregnancy, the breasts undergo certain preparatory 

 changes prior to the appearance of the milk. They increase in 

 bulk owing to their greater blood supply, and by certain changes 

 in the cell elements of the glands, which are compound saccular 

 glands. Each breast contains a series of some ten to twelve 

 glands with distinct ducts, upon which are dilatations that act 

 as reservoirs, in which, during active lactation, the secretion is 

 stored until it is needed. 



The alveoli are chiefly saccular in form, and are lined with a 

 single layer of glandular epithelium, and during active lactation 

 contain but little fat, though in the later stages of pregnancy, 

 before the secretion is established, the cells contain quantities of 

 large fat globules. 



Milk is a yellowish-white, perfectly opaque, sweetish fluid, 

 with an alkaline reaction, and a specific gravity of about 1030. 

 When exposed to the air, particularly in warm weather, the 

 milk soon loses its alkalinity, first becoming neutral, and then 

 markedly acid; the milk is then said to have "turned sour," 

 but its appearance is not greatly changed. When it has stood 

 a very long time, it may crack or curdle, and separate into two 

 parts one a thick, white curd, and the other a thin, yellowish 

 fluid. This turning sour and ultimate curdling depends upon a 

 change brought about in one of its most important constituents, 

 namely, milk sugar, by means of a process of fermentation. The 

 milk sugar, in the presence of certain forms of bacteria, ferments, 



