Erect breast nipple. Artwork of a perpendicular section through a female breast nipple (papilla) in an erect state. The section shows cylindrically arranged muscle cells (red) that cause the erection. The response can be due to cold temperatures, sexual arousal, and breastfeeding. In the latter two cases, the contraction of the smooth muscles in and around the nipple are due to the release of the hormone oxytocin.
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