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this|anatomical structure|nipple (disambiguation) Image:Nipple.png|right Nipple is, generally, the name given to the mammalian nipple. In its most general form, a nipple is an appurtenance from which a fluid emanates, in this instance breast milk, to nurture a mother's young. In the anatomy of mammals, a nipple or mammary papilla is a small projection of skin containing the outlets for 15-20 lactiferous ducts arranged cylindrically around the tip. The physiological purpose of nipples is to deliver milk produced in the female mammary glands during lactation to the infant. In the male, nipples are often not considered functional with regard to breastfeeding, although male lactation is possible. Mammalian infants have a rooting instinct for seeking the nipple, and a sucking instinct for extracting milk. Image:Female_nipple_profile.jpg|thumb|right|Female human nipple seen in profile Mammals typically have an even number of nipples arranged bilaterally. Rarely, as in the platypus, the mammary glands empty onto the skin without a nipple. In human anatomy, the two nipples are located near the center of the breasts, surrounded by an area of sensitive, pigmented skin known as the areola. The nipple and areola can be erotic receptors, or considered sex organs, of males and females. Stimulation or sexual arousal can cause the nipples to become erect. Breastfeeding or exposure to cold temperatures often have this effect as well. Pregnancy and nursing tend to increase nipple size somewhat, and this increase may remain permanently thereafter. Small non-striated myocytes (muscle cells) arranged cylindrically within the nipple are responsible for this erection of the nipples. Embryology|Embryologically, nipples develop along the 'milk line|milk lines' which in humans extend from the axilla (armpit) down to the pubis (groin) on both sides. Other mammals such as cats and dogs develop more nipples along the milk lines, in number generally approximating the average litter size for that species, rounded up to an even number. Most people develop two nipples (one on each breast) but some have supernumerary nipples. Occasionally, these have lactiferous glands attached. Sometimes, baby|babies (male or female) are born producing milk. This, called "witch's milk", is caused by maternal estrogens acting on the baby and is quite normal. Witches' milk disappears after several days. Male nipples were for a long time considered a biological mystery, since they appeared not to serve any direct biological function, and so their development appeared to present a problem for the theory of evolution. Modern biology now regards the existence of male nipples as being consistent with the theory of evolution. Indeed, the development of male nipples may be regarded as a good example of both the mechanisms by which evolution of the genome expresses itself in the phenotype, and the undirected and contingent nature of evolution. According to this viewpoint, male nipples exist because they are homologous to female nipples, which originate from the same overall genetic and developmental blueprint as male nipples. Female nipples have evolved to support lactation, but there is no biological advantage to changing the male body plan to eliminate them. Human male nipples do have secondary biological function; like female nipples, they are sensitive to touch (but less so), and can trigger sexual arousal.
Category:Integumentary system de:Brustwarze fr:Mamelon ilo:Mungay nl:Tepel sv:Br??stv??rta This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "nipple".
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