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( Nipple)
In its most general form, a nipple is a structure from which a fluid emanates. More specifically, it is the projection on the breasts of a mammal by which breast milk is delivered to 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 skin of the nipple is rich in a supply of special nerves that are sensitive to certain stimuli. The physiological purpose of nipples is to deliver milk to the infant, produced in the female mammary glands during lactation. In the male, nipples are often not considered functional with regard to breastfeeding, although male lactation is possible. Mammalian infants have a rooting instinct (moving their head so as to bring their mouth towards whatever is touching their face) for seeking the nipple, and a sucking instinct for extracting milk. Mammals typically have an even number of nipples arranged around bilaterally. They develop in the embryo, along the 'milk lines'. Most mammals develop multiple nipples along each milk line, with the total number approximating the maximum litter size, and half the total number (i.e. the number on one side) approximating the average litter size for that species. In the primitive mammals (monotremes such as the platypus), the mammary glands empty onto the skin without a nipple. In cetaceans such as whales, the infant cannot form a suction-seal to nurse, due to its mouth structure. Therefore the whale's nipple is unlike that of any other mammal. Rather than requiring a sucking action, the discharge of milk is powered by maternal muscles. The calf takes the extended nipple into its mouth, and the mother ejects or expels her milk into the mouth of the calf. Most humans have two nipples after birth, located near the center of each breast and surrounded by an area of sensitive, pigmented skin known as the areola. Human fetuses develop several more nipples along the milk lines, which extend from the axilla (armpit), along the abdominal muscles, down to the pubis (groin) on both sides. Those nipples usually disappear before birth, but sometimes remain, resulting in supernumerary nipples which occasionally have lactiferous glands attached, but usually do not.
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