NIPPLE (papilla mammae)

Anatomy and Physiology
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, and down to the pubis (groin) on both sides. Those nipples usually disappear before birth, but sometimes remain, resulting in supernumerary nipples which uncommonly have lactiferous glands attached. Sometimes, babies (male or female) are born producing milk. This is called 'witch's milk'; it is caused by maternal estrogens acting on the baby and is quite common. Witch's milk disappears after several days.
In the anatomy of mammals, a nipple or 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. 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.
The pigments of the nipple and areola are brown eumelanin (a brown pigment) and to a greater extent pheomelanin (a red pigment). In many women, there are small bulges on the areola, which are called 'Montgomery bodies'. In humans, the nipple is innervated by the 4th intercostal nerve.
The erection of nipples is not due to erectile tissue, but due to the contraction of smooth muscle under the control of the autonomic nervous system. It is more akin to a hair follicle standing on end than to a sexual erection. Nipple erections are a product of the pilomotor reflex which causes goose bumps. The erection of the nipple is partially due to the cylindrically-arranged muscle cells found within it. Nipple erection can also be caused by a tactile response to cold temperature in both males and females. Nipple erection may also result during sexual arousal in females and males, or during breastfeeding. Both are caused by the release of oxytocin. The nipple and areola of males and females can be erotic receptors, sometimes intense enough to elicit orgasm in some individuals of either sex.
From conception until sexual differentiation, all mammalian fetuses within the same species look the same, regardless of sex. In humans this lasts for around 14 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.