.378 MANUAL OF PHYSIOLOGY. 



the smooth muscle band which commonly embraces the gland and 

 squeezes it against the hair follicle. 



This secretion, the use of which is to lubricate the surface with 

 a fatty material, is cast off with the desquamated epithelium and 

 the hairs. The Meibomian glands of the eyelids are analogous 

 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 se- 

 crete 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 preg- 

 nancy, and then continues, if the secretion be regularly withdrawn 

 from the gland, for some 9 to 12 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 

 glajids. 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 ap- 

 pearance is not greatly changed. When it has stood a very long 

 time it may crack or curdle, and separate into two parts, one a 



