in mammals and man, paired cutaneous excretory glands. In sexually mature females, the mammary glands secrete milk during the nursing period (lactation); in males, they are present throughout life in rudimentary form. Evolutionarily, the mammary glands derive from sweat glands.

Mammary glands of the simplest structure are present in cloacal mammals (Prototheria), in which they take the form of a paired aggregate of long tubular pouches lined with smooth musculature and opening into hair follicles on the milk field along the sides of the abdomen. The secretion flows along the hairs and is licked by the young.

In viviparous mammals, the glands are lobular formations with complexly branched ducts and terminal saclike secretory areas called alveoli.

In the embryo, the glands develop in the region of the mammary lines, paired strips of thickened epithelium located along the sides of the ventral surface of the body. Nodules, or compact epithelial rudiments of the glands (mammary tubercles, or points), form first. Growing deeper into the skin, the nodules give rise to the efferent lactiferous ducts and alveoli of the gland. The ducts open onto the milk field, represented by a small area of skin called the nipple.

A distinction is made between true nipples, the tips of which have numerous separate openings for the separate ducts, and false nipples, which have a common lactiferous duct and a single opening at the tip. True nipples are characteristic of the majority of marsupials, lemuroids, apes, and man; false nipples are characteristic of carnivores and ungulates. The number and arrangement of nipples are constant for each species. For example, a species of opossums has 25 nipples arranged in two rows along the length of the body’s ventral surface; ungulates have one or two pairs located in the inguinal region; lemuroids have a pair on the chest and often another on the abdomen; and bats and apes have a single pair on the chest.

In man, the mammary glands are a paired organ, located on the anterior surface of the pectoralis major, between the third and sixth ribs. Functionally, the glands are intimately connected with the female sexual system. In virgins, the weight of the mammary glands is 150–200 g; in nursing mothers, 350–400 g. At the center of each gland is a nipple with a pigmented area of skin called the areola mammae. There are 10–15 lactiferous pores, the openings of the lactiferous ducts (the excretory passages of the mammary lobules). The lobules are formed of terminal sections, or alveoli, whose shape and size vary with the phase of secretion. The alveoli are separated by thin layers of connective tissue, which contain small vessels and capillaries, and by tiny intralobular excretory ducts. In terms of the character of secretion, the mammaries are apocrine glands—glands whose secretion is released along with some of the cytoplasm of the cells of the terminal sections. The glandular lobules form 15–20 mam-mary lobes, which run radially from the nipple, separated from each other by dense connective tissue.

In addition to glandular tissue, the mammary glands consist of fatty tissue, the amount of which increases at the end of the nursing period. The most common pathologies of the mammary glands are mastitis, mastopathy, gynecomastia, and developmental defects.