Mammary Glands, the organs which secrete the nutritive fluid, milk, by which the young of man and the mammalia are nourished during the early periods of life. They vary from two in the human female to 10 or 12 in the low^er mammals, and may be pectoral as in the former, or pectoral and abdominal, or only abdominal, as in the latter. Each gland is made up of a number of separate lobules, more or less closely connected by fibrous tissue and fat, and bound down by the same to the pectoral or abdominal muscles. The lactiferous tubes arising from the minute ultimate follicles of the lobules terminate in the mam-millary tubes of the nipple, 10 or 12 in the human female, straight but of variable size; at the base of the nipple, and extending into the gland, are reservoirs for containing a constant supply during lactation; these are often much larger in the lower animals than in woman. The skin covering them is very delicate and smooth; the colored circle around the nipple is called the areola, which becomes darker during and after gestation; the irregular surface of the nipple is covered with a very sensitive skin, and much erectile tissue enters into its substance. The tubes are lined with a very vascular mucous membrane, which has its own secretion sometimes in considerable quantity.
These glands," especially during lactation, are well supplied with blood from branches of the subclavian and axillary arteries; their nerves come from the brachial plexus and the mter-costals, and the sympathetic plexus accompanying the mammary arteries. The inner surface of the follicles is covered with a layer of epithelium cells, the real agents in the secreting process. They present no great difference in size in the sexes until near the age of puberty when a considerable enlargement takes place in the female; from the increased supply of blood during gestation, there is a sense of tenderness and distention which is one of the earliest and most valuable signs of pregnancy. These glands in the male are miniatures of those of the female, but the essential structure is the same, as is shown by the authentic «. in which they have become sufficiently developed in men to produce a secretion of true milk. Though the functional activity of these glands is naturally limited to the period succeeding parturition, their secretion is sometimes seen in virgins and in aged women, in whom a strong desire to furnish milk and a-continual irritation of the nipple by the infant's mouth have stimulated the organs to unnatural activity.
The prolonged secretion of milk in domestic cows, which usually lasts for about ten months after calving, is simply a continue d action of these glands due to artificial treatment. The presence of these organs lias given the name to the mammalia, the highest class of vertebrated animals, implying a mode of intra-uterine and extra-uterine development not found in birds, reptiles, or fishes. Physiologically these glands belong to the generative system, and are gradually removed from the caudal to the pectoral region, as we ascend from cetaceans to the human female; the forward, outward, and upward direction of the nipples is exactly adapted to the position of the child lying in its mother's arms, and the greater abundance of the lactiferous tubes at the lower portion of the breast forms a soft cushion for its head to rest upon. In the African and sometimes in other races, after I tation, the skin covering the breasts becon so lax, and the organs so elongated, that they can be thrown over the shoulders like bages The mammary glands are subject to many painful and dangerous diseases, among which may be mentioned acute and chronic inflammations, abscesses, and encysted, fibrous, and cancerous tumors; they are sometimes enormously overloaded with fat.