The word “nipple” derives from the Anglo-Saxon neb meaning a nose, and was described at that time as “neble of a woman’s pappe.”
It is a cylindrical eminence in the anatomic center of the breast, anatomically the mammary gland, and is the outlet for that gland’s secretions: human milk. Normally it is located in the mid-clavicular line at the level of the 4th intercostal space. But since the breast has been likened to Easter as a “moveable feast” it is found at varying levels according to the degree of breast sagging. In fact, there may be more than one nipple each side, placed following the milk line extending from armpit to groin as it does in the sow.
In structure, the outer layer of skin, epidermis, is colored as is the areola. Centrally the 15-20 mammary ducts run both to the very apex of the nipple and to its sides. The tissue between ducts and skin is a connective tissue stroma, comprising a small quantity of fatty tissue, collagen bundles, blood vessels and nerves, and in the female particularly, strands of smooth muscle.
The nipple will swell and become generally more prominent in lactation.
It has, much more in the female than the male, the ability to “erect” or stand out prominently (known vulgarly as “headlights on”) as a result of stimulation; this may be sexual in nature (direct physical contact or merely a thought process), or it may be from the irritation of rubbing on a shirt when running, or a response to a frigid ambience. The erection is brought about by the smooth muscle fibers embedded in the nipple.