The nipple is a raised region of tissue on the surface of the breast from which, in lactating females, milk from the mammary gland leaves the body through the lactiferous ducts to nurse an infant. The milk can flow through the nipple passively, or it can be ejected by smooth muscle contractions that occur along with the ductal system. The nipple is surrounded by the areola, which is often a darker colour than the surrounding skin. When referring to non-humans, the female nipple is often called a teat. "Nipple" or "teat" can also be used to describe the flexible mouthpiece of a baby bottle. Male mammals also have nipples, but without the same level of function or prominence. In humans, the nipples of both females and males can be sexually stimulated as part of sexual arousal. In many cultures, female nipples are sexualized, or regarded as sex objects and evaluated in terms of their physical characteristics and sex appeal. As such, there are often mores for their coverage.
Etymology The word nipple most likely originates as a diminutive of neb, an Old English word of Germanic origin meaning "beak", "nose", or "face". The words teat and tit share a Germanic ancestor. The latter, tit, was inherited directly from Proto-Germanic, while the first entered English via Old French.
Structure
In mammals, a nipple (also called a mammary papilla or teat) is a small projection of skin containing the outlets for 15–20 lactiferous ducts arranged cylindrically around the tip. Marsupials and eutherian mammals typically have an even number of nipples arranged bilaterally, from as few as 2 to as many as 19. 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 mechanoreceptors. 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 nerve supply to the nipple comes from the lateral cutaneous branches of fourth intercostal nerve. The nipple is also used as an anatomical landmark. It marks the T4 (fourth thoracic vertebra) dermatome and rests over the approximate level of the diaphragm. The arterial supply to the nipple and breast originates from the anterior intercostal branches of the internal thoracic (mammary) arteries; lateral thoracic artery; and thoracodorsal arteries. The venous vessels parallel the arteries. The lymphatic ducts that drain the nipple are the same for the breast. The axillary nodes are the apical axillary nodes, the lateral group and the anterior group. 75% of the lymph is drained through the axillary lymph nodes located near the armpit. The rest of the drainage leaves the nipple and breast through infroclavicular, pectoral, or parasternal nodes. Since nipples change throughout the life span in men and women, the anatomy of the nipple can change and this change may be expected and considered normal.
In male mammals
Almost all mammals have nipples. Why males have nipples has been the subject of scientific research. Differences among the sexes (called sexual dimorphism) within a given species are considered by evolutionary biologists to be mostly the result of sexual selection, directly or indirectly. There is a consensus that the male nipple exists because there is no particular advantage to males losing the trait. In consequence, some biologists would call the male nipple a spandrel. In humans, the nipples are often surrounded by body hair.
Function
The physiological purpose of nipples is to deliver milk, produced in the female mammary glands during lactation, to an infant. During breastfeeding, nipple stimulation by an infant will stimulate the release of oxytocin from the hypothalamus. Oxytocin is a hormone that increases during pregnancy and acts on the breast to help produce the milk-ejection reflex. Oxytocin release from the nipple stimulation of the infant causes the uterus to contract even after childbirth. The strong uterine contractions that are caused by the stimulation of the mother's nipples help the uterus contract to clamp down the uterine arteries. These contractions are necessary to prevent post-partum haemorrhage. When the infant suckles or stimulates the nipple, oxytocin levels rise and small muscles in the breast contract, moving the milk through the milk ducts. The result of nipple stimulation by the infant helps to move breast milk out through the ducts and to the nipple. This contraction of milk is called the "let-down reflex". Latching on refers to the infant fastening onto the nipple to breastfeed. A good attachment is when the bottom of the areola (the area around the nipple) is in the infant's mouth and the nipple is drawn back inside his or her mouth. A poor latch results in insufficient nipple stimulation to create the let down reflex. The nipple is poorly stimulated when the baby latches on too close to the tip of the nipple. This poor attachment can cause sore and cracked nipples and a reluctance of the mother to continue to breastfeed. After birth, the milk supply increases based upon the continuous and increasing stimulation of the nipple by the infant. If the baby increases nursing time at the nipple, the mammary glands respond to this stimulation by increasing milk production.
Clinical significance
Pain Nipple pain can be a disincentive for breastfeeding. Sore nipples that progress to cracked nipples is of concern since many women cease breastfeeding due to the pain. In some instances, an ulcer will form on the nipple. One reason for the development of cracked and sore nipples is the incorrect latching-on of the infant to the nipple. If a nipple appears to be wedge-shaped, white and flattened, this may indicate that the attachment of the infant is not good and there is a potential of developing cracked nipples. Herpes infection of the nipple is painful. Nipple pain can also be caused by excessive friction of clothing against the nipple that causes a fissure.
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