nipple [ME, neb, beak] , a small cylindric, pigmented structure that projects just below the center of each breast. The tip of the nipple has about 20 tiny openings to the lactiferous ducts. The skin of the nipple is surrounded by the lighter pigmented skin of the areola. The depth of pigmentation of the nipple and areola in nulliparas varies from rosy pink to brown, depending on the complexion of the individual. In pregnancy the skin of the nipple darkens but loses some of its pigmentation when lactation is completed. Stimulation of the nipple in men and women causes the structure to become erect through the contraction of radiating smooth muscle bundles in the surrounding areola. In women the nipple enlarges somewhat and becomes more sensitive after puberty. Also called mammary papilla, papilla mammae, papilla mammaria.
