The secretion of milk only takes place under certain circumstances and continues for a limited period. As the name of the glands implies, they are present in all mammalian animals. The activity of the gland commences in the later stages of pregnancy, and then continues, if the secretion be regularly withdrawn from the gland, for some 9 to 12 months.

Section of Mammary Gland during active lactation (human), (a) Saccules lined with regular epithelium.

Fig. 167. Section of Mammary Gland during active lactation (human), (a) Saccules lined with regular epithelium. (6) Connective tissue between the alveoli. (Cadiat ).

During pregnancy the breasts undergo certain preparatory changes prior to the appearance of the milk. They increase in bulk, owing to the greater blood supply, and to 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 these there are dilatations that act as reservoirs, in which, during active lactation, the secretion is stored until 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.