MAMMARY GLAND, the organ by means of which the young are suckled, and the possession of which, in some region of the trunk, entitles the animal bearing it to a place in the order of Mammalia. In the male the organ is present but undeveloped.
Anatomy.—In the human female the gland extends vertically from the second to the sixth rib, and transversely from the edge of the sternum to the mid axillary line ; it is embedded in the fat superficial to the pectoralis major muscle, and a process which ex tends toward the armpit is some times called the axillary tail. A little below the centre of the glandular swelling is the nipple, surrounding which is a pigmented circular patch called the areola ; this is studded with slight nod ules, which are the openings of areolar glands secreting an oily fluid to protect the skin during suckling. During the second or third month of pregnancy the areola becomes more or less deeply pigmented, but this to a large extent passes off after lactation ceases. In structure the gland consists of some 15 to 20 lobules, each of which has a lactiferous duct opening at the sum mit of the nipple, and branches in the substance of the gland to form secondary lobules, the walls of which are lined by cubical epithelium in which the milk is secreted. These secondary lobules project into the surrounding fat, so that it is difficult to dissect out the gland cleanly. Before opening at the nipple each lactiferous duct has a fusiform dilatation called the ampulla.
After the child-bearing period of life the breasts atrophy and tend to become pendulous, while in some African races they are pendulous throughout life. Variations in the mammary glands are common; often the left breast is larger than the right, and in those rare cases in which one breast is suppressed it is usually the right, though this does not necessarily include absence of the nipple.
Supernumerary nipples and glands are not uncommon, and are usually situated in the mammary line which extends from the anterior axillary fold to the spine of the pubis; hence, when an extra nipple appears above the normal one, it is external to it, but, when below, it is nearer the middle line. Extra nipples are commoner in males than in females.
Embryology.—The mammary glands are modified and hyper trophied sebaceous glands, and transitional stages are seen in the areolar glands, which sometimes secrete milk. At an early stage of foetal life a raised patch of ectoderm is seen, which later on becomes a saucer-like depression; from the bottom of this 15 or 20 solid processes of cells, each presumably representing a sebace ous gland, grow into the mesoderm which forms the connective tissue stroma of the mamma. Later on these processes branch.
The last stage is that the centre of the mammary pit or saucer like depression once more grows up to form the nipple, and at birth the processes become tubular, thus forming lactiferous ducts. The glands grow little until the age of puberty, but their full development is not reached until the birth of the first child.
Comparative Anatomy.—In the lower Mammals the mam mary line, already mentioned, appears in the embryo as a ridge, and in those which have many young at a birth patches of this develop in the thoracic and abdominal regions to form the mam mae, while the intervening parts of the ridge disappear. The number of mammae is not constant in animals of the same species. When only a few young are produced at a time the mammae are few, and it seems to depend on the convenience of suckling in which part of the mammary line the glands are developed. In the pouched Mammals (Monotremes and Marsupials) inguinal mammae are found, and so they are in most Ungulates as well as in the Cetacea. In the elephants, Sirenia, Chiroptera and most of the Primates, on the other hand, they are confined to the pec toral region, and this is also the case in some Rodents, e.g., the jumping hare (Pedetes caffer). In the monotremes the mammary pit remains throughout life, and the milk is conducted along the hairs to the young, but in other Mammals nipples are formed in one of two ways. One is that already described in Man, which is common to the Marsupials and Primates, while in the other the margin or vallum of the mammary pit grows up, and so forms a nipple with a very deep pit, into the bottom of which the lac tiferous ducts open. The latter is regarded as the primary arrange ment. In the monotremes the mammae are looked upon, not as modified sebaceous glands, as in other Mammals, but as altered sweat glands. In these primitive Mammals the glands are equally developed in both sexes, and it is thought that among the bats the male often assists in suckling the young (see G. Dobson, Brit. Museum Cat. of the Chiroptera, London, 1878). These facts, together with the occasional occurrence of functional activity of the organ in the human male, make it probable that among ances tral Mammals both sexes helped in the process of lactation.
For further details see Comparative Anatomy of Vertebrates, by R. Wiedersheim, adapted by W. N. Parker (1907), and Bronn's Classen and Ordnungen des Thierreichs.