Nipple (physiology)

The nipple is the part of the breast through which milk exits during breastfeeding. It contains small openings called ducts that carry milk from the milk-producing glands beneath the skin. Nerve endings in the nipple make it sensitive, and stimulation triggers the release of oxytocin, which causes the milk to flow. The skin of the nipple and surrounding areola also contain smooth muscle fibers that can contract, causing the nipple to become erect. The structure of the nipple and its glands play an essential role in nursing, hormonal regulation, and sexual sensation.