In its most general form, a nipple's a structure from which a fluid emanates. More specifically, it's the projection on the breasts of a mammal by which breast milk's delivered to a mother's young. Anatomy In the anatomy of mammals, a nipple or mammary papilla or teat's a small projection of skin containing the outlets for 15-20 lactiferous ducts arranged cylindrically around the tip. The skin of the nipple's rich in a supply of special nerves that're sensitive to certain stimuli. The physiolo… (More on Nipples)