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In its most general form, a nipple is a structure from which a fluid emanates. More specifically, it is the projection on the breasts or udder of a mammal by which, in females, breast milk is delivered to a mother's young. In this sense, it is often called a teat, especially when referring to non-humans, and the medical term used to refer to it is papilla. A breast or nipple is sometimes called a "pap". The rubber mouthpiece of a baby bottle or pacifier may also be referred to as a "nipple" or a "teat". In humans, nipples of both males and females are also one of the erogenous zones. In many cultures, human female nipples are sexualized and it is considered a taboo or a public indecency to uncover them in public.
The word "nipple" most likely originates as a diminutive of neb, an Old English word meaning "beak", "nose", or "face", and which is of Germanic origin. The words "teat" and "tit" share a Germanic ancestor. The second of the two, tit, was inherited directly from Proto-Germanic, while the first entered English via Old French.
In the anatomy of mammals, a nipple, mammary papilla or teat 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: these are slowly-adapting and rapidly-adapting cutaneous mechanocepetors. Mechanoreceptors are identified respectively by Type I slowly-adapting with multiple Merkel corpuscle end-organs and Type II slowly-adapting with single Ruffini corpuscle end-organs, as well as Type I rapidly-adapting with multiple Meissner corpuscle end-organs and Type II rapidly-adapting with single Pacinian corpuscle end-organs.
The dominant innervation to the nipple is derived from the lateral cutaneous branches of fourth intercostal nerve. The physiological purpose of nipples is to deliver milk to the infant, produced in the female mammary glands during lactation. Marsupials and eutherian mammals typically have an even number of nipples arranged bilaterally, from as few as two to as many as 19.
Changes in size
The average projection and size of human female nipples is slightly more than 3/8 of an inch (10mm). This is equal to about 5 stacked American quarters. Pregnancy and nursing tend to increase nipple size, sometimes permanently. Pregnancy also deepens the pigmentation.
Nipples of both males and females are one of the erogenous zones, which, when stimulated, leads to sexual arousal. The male and female breast have a similar nerve supply but the nerves lie closer together in men, whereas in a female breast they further apart. After puberty, female breasts turn more sensitive, likely due to hormonal influences. During arousal they generally become stiff, erect and protrude.
Nipple stimulation is sometimes intense enough to elicit an orgasm in some women. Research has suggested the sensations are genital orgasms caused by nipple stimulation, and may also be directly linked to "the genital area of the brain". In women, one study indicated that sensation from the nipples travels to the same part of the brain as sensations from the vagina, clitoris and cervix. Nipple stimulation may trigger uterine contractions, which then produce a sensation in the genital area of the brain.
In male mammals
From conception until sexual differentiation, all mammalian fetuses within the same species look the same, regardless of sex. In humans, this lasts for around 6 weeks, after which genetically-male fetuses begin producing male hormones such as testosterone. Usually, males' nipples do not 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. This may happen whenever the testosterone level drops. Gynecomastia, although not as severe, may occur in pubescent boys undergoing physical changes due to the rapid and uncontrolled release of hormones, including estrogen. The heightened levels of estrogen in pubescent male bodies leads to the swelling of the nipple and surrounding tissue – this can often look similar to a female human nipple – and may cause slight discomfort. Thus, because the "female template" is the default for humans, the question is not why evolution has not selected against male nipples, but why it would be advantageous to select against male nipples to begin with:
The uncoupling of male and female traits occurs if there is selection for it: if the trait is important to the reproductive success of both males and females but the best or "optimal" trait is different for a male and a female. We would not expect such an uncoupling if the attribute is important in both sexes and the "optimal" value is similar in both sexes, nor would we expect uncoupling to evolve if the attribute is important to one sex but unimportant in the other. The latter is the case for nipples. Their advantage in females, in terms of reproductive success, is clear. But because the genetic "default" is for males and females to share characters, the presence of nipples in males is probably best explained as a genetic correlation that persists through lack of selection against them, rather than selection for them. Interestingly, though, it could be argued that the occurrence of problems associated with the male nipple, such as carcinoma, constitutes contemporary selection against them.
In a now-famous paper, Stephen Jay Gould and Richard C. Lewontin emphasize that we should not immediately assume that every trait has an adaptive explanation. Just as the spandrels of St. Mark's domed cathedral in Venice are simply an architectural consequence of the meeting of a vaulted ceiling with its supporting pillars, the presence of nipples in male mammals is a genetic architectural by-product of nipples in females. This idea implies men have nipples because females do.
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