Breast, or mammary gland, is the organ concerned in secreting milk. The gland substance proper is surrounded by connective tissue and fat, Which forms a kind of packing and supporting material. The gland itself is made up of a number of lobes, each lobe being further divided into lobules. These lobules are found on microscopical examination to be composed of a number of acini or hollow sacs lined by cubical epithelial cells which all open into a common duct. By the union of such lobular ducts, the main ducts of the gland, the lactiferous or galactophorous ducts, are formed; these are about fifteen in number and, radiating towards the nipple, open by separate orifices upon it. Just before reaching the surface each main duct presents a dilatation, a sort of reservoir for the accumulation of the secretion. The nipple contains in addition to these terminations of the ducts a supporting framework of areolar tissue, unstriped muscle fibres, and numerous blood-vessels. It is surrounded by an areola of pink or brownish skin. In the female at the time of puberty the breasts enlarge; during pregnancy further development occurs, and culminates ultimately in profuse secretion of milk after childbirth.