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Other Terms: Anal orifice
The anus is the dilatable aperture by which the intestine opens externally at the lower extremity of the alimentary tract. It is maintained in a constricted state by the sphincter muscles that surround it, which gives the surrounding skin a wrinkled appearance. The skin around it is pigmented and thrown into folds, which are very numerous posterior. Numerous muscles are associated with this terminal region of the gut tube – they are the internal anal sphincter (a smooth muscle sphincter within its wall), the levator ani (a pelvic body wall muscle through which the gut tube exits), and the external sphincter (perineal body wall muscles that surround the exit). There are a number of sweat glands called circumanal glands that are located in the integument immediately around the anus. Sebaceous glands are also found in connection with the hair of this region.