The anus is part of the digestive 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, and amphibians, have only a single orifice, called the "cloaca" or "vent," used to excrete both liquid and solid waste, and for reproduction and egg-laying.
The section of the digestive tract immediately before the anus in humans and mammals is the rectum. When food has passes through the digestive tract, nutrients and water are both removed, leaving a collection of indigestible wastes called feces. Feces may consist of the indigestible tissues of plants or animals, as well as dead bacteria from the gut. As feces builds up in the rectum, increased pressure causes a series of muscular contractions. These contractions move the feces along the rectum and relax the anus, which opens, expelling it from the body. This process is called defecation.
Many human cultures have important taboos relating to defecation, and some form of cleaning is almost always practiced. This may include washing the anal region with water and mild soap, or wiping it with cloth or paper. Regular cleaning is good for the maintenance of a healthy digestive tract, although cleaning with rough materials may cause painful irritation.
The anus has a high number of nerve endings, which makes it very sensitive. As a result, the anal region is stimulated in some forms of sexual activity. Anal sex is considered taboo or controversial in some cultures. Like all sexual activity, unprotected anal sex can lead to sexually transmitted infections.
Biologists believe that the evolution of the anus has occurred at least twice, based on the fact that there are two different ways in which the anus forms in an embryo. In the earliest stages of embryonic development, an opening called the "blastopore" appears in the small ball of cells that makes up the organism. In deuterostomes, including all vertebrates, the blastopore becomes the anus, while in protostomes, it develops into the mouth. Humans, like all vertebrates, are deuterostomes.