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The anus is part of thedigestive tract. It is an opening at the opposite end of the tract from the mouth, and functions to expel waste from the digestive system. The term comes from the Latin word for circle or ring, and refers to the sphincter muscle which controls the opening and closing of the anus.
In mammals, solid or semisolid wastes are expelled through this opening, while liquid wastes are expelled through the urethra. This physiological characteristic is one that distinguishes mammals from other animals. Birds,reptiles, andamphibians, have only a single orifice, called thecloacaor vent, used to excrete both liquid and solid waste, and for reproduction and egg-laying.