begins to suck, the oxytocin-mediated "let down reflex" ensues, and the mother's milk is secreted--not sucked--from the gland into the infant's mouth.

All the milk-secreting tissue leading to a single lactiferous duct is collectively 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. The opening of each simple gland on the surface of the nipple