In both sexes it is an erogenous area in that stimulation of the nipple is sexually stimulating, but in men it serves no other purpose. It contains numerous small muscles that contract to make the nipple erect when stimulated by suckling, plucking, cold or anxiety. These muscles are more numerous in women as it is considerably larger than on men.
In women, 15 to 20 ducts from the milk glands in the breast open through the nipple.
The nipple is surrounded by pigmented skin called the areola, which enlarges at puberty, and may darken further (chloasma) after pregnancy or hormonal medication use (eg. contraceptive pill). The areola contains sebaceous glands (Montgomery glands) that give it a bumpy appearance, particularly around its edge.