The mammary glands are the milk-producing organs. The number varies among species from two to about twenty, with a rough correlation between the number of young born and the number of glands present. The glands are located on the ventral surface of the body, either in the thoracic (in humans, for example) or abdominal region (in horses and cows) or in two lines extending almost the length of the body (in dogs and rodents). Both male and female mammals have mammary glands, because in early mammalian development the basic body plan of male and female embryos is identical. The mammary glands of males are nonfunctional, however, since they lack the hormonal stimulation necessary for lactation.
Internally, the mammary glands of all mammals follow the same basic plan, consisting of alveoli that produce milk and ducts that carry the milk to openings on the surface of the skin. The alveoli are surrounded by myoepithelial cells that contract to squeeze the milk into the ducts during suckling by the young.
Externally, considerable variation exists among the mammals in the appearance of the mammary glands and their associated openings. In the spiny anteater and platypus, the many lobes of the mammary glands each open directly to the surface of the abdominal skin through individual ducts, and the young suck the hair-covered skin to obtain the milk. In other mammals, the mammary glands are more obvious as swellings beneath the skin, with a raised area, the nipple or teat, that contains the duct openings. In some four-legged animals (cows, horses, and goats) the mammary glands are located in a baglike structure called the udder, from which are suspended the elongated teats. In humans, the nipple, which contains the openings of fifteen to twenty-five ducts, is surrounded by pigmented skin, the areola. The areola contains glands (tubercles of Montgomery) that secrete a lubricating fluid.
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