The anus is the last part of the digestive tract. It is a 2-inch long canal consisting of the pelvic floor muscles and the two anal sphincters (internal and external). The lining of the upper anus is specialized to detect rectal contents. It lets you know whether the contents are liquid, gas, or solid. The anus is surrounded by sphincter muscles that are important in allowing control of stool. The pelvic floor muscle creates an angle between the rectum and the anus that stops stool from coming out when it is not supposed to. The internal sphincter is always tight, except when stool enters the rectum. It keeps us continent when we are asleep or otherwise unaware of the presence of stool. When we get an urge to go to the bathroom, we rely on our external sphincter to hold the stool until reaching a toilet, where it then relaxes to release the contents.
Most of the digestive tract is a tube composed of four layers:
The serosa, or outermost layer.
The muscularis externa, whose two sets of muscles produce the waves of contraction, called peristalsis, that move food along.
The submucosa, which contains blood vessels and nerve tissue that control digestion. The mucosa-epithelial layer, which absorbs digested food.