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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 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 for seeking the nipple, and a sucking instinct for extracting milk.
Mammals typically have an even number of nipples arranged around bilaterally. In the primitive mammals (monotremes such as 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 pigments of the nipple and areola are brown eumelanin (a brown pigment) and to a greater extent pheomelanin (a red pigment). The nipple and areola of males and females can be erotic receptors, or considered sex organs. Stimulation or sexual arousal can cause the nipples to become erect, due to the release of the polypeptide neurotransmitter oxytocin. Breastfeeding or exposure to cold temperatures often has this effect as well.
The average projection and size of human female nipples is slightly more than 3/8 inches (0.4 in./10mm.) . Pregnancy and nursing tend to increase nipple size somewhat, and this increase may remain permanently thereafter. Pregnancy also increases the pigmentation of the nipples. The erection of the nipple is partially due to the cylindrically arranged muscle cells found within it. In many women there are small bulges on the areola, which are called 'Montgomery bodies'.
Embryologically, nipples develop along the 'milk lines' which in humans extend from the axilla (armpit) down to the pubis (groin) on both sides. 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. Most people develop two nipples (one on each breast) but some have supernumerary nipples. Occasionally, these have lactiferous glands attached.
Sometimes, 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. Witch's milk disappears after several days.
Nipples on male mammals
Starting at conception and lasting until about 14 weeks, all mammalian fetuses within the same species look the same, regardless of sex. After 14 weeks, genetically-male fetuses begin producing male hormones such as testosterone. As "female" is the "developmental default" for mammals, by 14 weeks the nipples have already formed. In recent studies, doctors have found female nipples to be erect in the fetal stage of mammals, as well as other forms of arousal.
Most of the time, males' nipples don't change much past this point. However, some males develop a condition known as gynecomastia, in which the fatty tissue around and under the nipple develops into something similar to a female breast. For males who develop gynecomastia during puberty, it is said the effects are temporary unless they are obese. This may happen whenever the testosterone level drops because of medications.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Nipple". A list of authors is available in Wikipedia.|