Nipple

# Overview
In its most general form, a nipple is an appurtenance from which a fluid emanates. More specifically, it is the projection on breast of a mammal by which breast milk is delivered to a mother's young.

# Anatomy
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.  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.
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.
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 (10mm).. Pregnancy and nursing tend to increase nipple size, sometimes permanently. Pregnancy also increases the pigmentation. 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'.
Sometimes, babies (male or female) are born producing milk. This is called 'witch's milk' and 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.