MAMMARY GLANDS which occur only in female mammals and provide milk for feeding the young. These paired organs are usually fully developed only in adult females, but are present in rudimentary form in juveniles and males. In women, the two breasts over-lie the second to sixth ribs on the front of the chest. On the surface of each breast is a central pink disc called the areola, which surrounds the nipple. Inside, the breast consists of fat, supporting tissue and glandular tissue, which is the part that produces milk following childbirth. Each breast consists of 12–20 compartments arranged radially around the nipple: each compartment opens on to the tip of the nipple via its own duct through which the milk flows. The breast enlargement that occurs in pregnancy is due to development of the glandular part in preparation for lactation. In women beyond childbearing age, the glandular part of the breasts reduces (called involution) and the breasts become less firm and contain relatively more fat.