Nipple, also called nipple, is a cylindrical part protruding from the center of mammalian breast, which is harder than areola and breast. Human female nipple is an important sexual sensitive area, which has the function of breastfeeding. The nipple is composed of tight smooth muscle and connective tissue. Smooth muscles are arranged in a ring or radial pattern. When encountering sexual or mechanical stimulation, smooth muscle contracts, which can make the nipple hard and convex.

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