Mammary glands are the organs that, in the female mammal, produce milk for the sustenance of the young. These exocrine glands are enlarged and modified sweat glands and are the characteristic of mammals. All the milk-secreting tissue leading to a single lactiferous duct is called a "simple mammary gland".In a "complex mammary gland" all the simple mammary glands serve one nipple. Humans normally have two complex mammary glands, one in each breast, and each complex mammary gland consists of 10–20 simple glands.