The milk-producing organs (possibly modified sweat glands) of female mammals, which provide food for the young (see milk; colostrum). Their number (2 to 20) and position (on the chest or abdomen) vary according to the species. In most mammals the gland openings project as a nipple or teat. Nipples have a number of milk-duct openings; teats have one duct leading from a storage cavity.