Nipple  The nipple is a raised region of tissue on the surface of the breast from which, in females, milk leaves the breast through the lactiferous ducts to feed an infant. The milk can flow through the nipple passively or it can be ejected by smooth muscle contractions that occur along the ductal system. The nipple is surrounded by the areola, which is often a darker color than the surrounding skin. A nipple is often called a teat when referring to non-humans. Nipple or teat can also be used to describe the flexible mouthpiece of a baby bottle. In humans, nipples of both males and females can be stimulated as part of sexual arousal. In many cultures, human female nipples are sexualized, or "regarded as sex objects and evaluated in terms of their physical characteristics and sexiness." In mammals, a nipple (also called 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 two 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.