The anus, or anal canal, is the final segment of the gastrointestinal tract. It acts as the orifice that feces pass through during defecation.
The anus is a short tube at the end of the rectum that ends at the body’s exterior. It is around 1 inch (2-3 cm) long and varies widely in its diameter depending on how distended it is. The superior end of the anus is continuous with the tissue of the rectum and is lined with simple columnar epithelial tissue. This tissue forms folds known as anal columns, with valleys known as anal valves between the folds.
At the inferior end of the anal columns is the pectinate line, where the tissue lining the anus changes from simple columnar epithelium to stratified squamous epithelium. The stratified squamous epithelium is non-keratinized inside the anus, but becomes keratinized epithelium as it approaches the body’s exterior. The change from non-keratinized to keratinized occurs at a visible landmark known as Hilton’s line.
Two sphincter muscles, the internal and external anal sphincters, surround the anus and control its opening and closing. The internal anal sphincter is made of visceral muscle and is continuous with the muscularis layer of the rectum. Closer to the body’s exterior, the external anal sphincter is made of skeletal muscle and is continuous with the surrounding skeletal muscles of the perianal region. A layer of keratinized stratified squamous epithelium covers the external anal sphincter and is continuous with the skin of the perianal region.
The anus plays an important role in controlling the elimination of solid wastes through defecation. When the rectum is empty or slightly filled, both the internal and external anal sphincters remain closed to hold waste material inside the rectum and prevent defecation. Once enough fecal matter fills the rectum to exert pressure on the rectal walls, pressure receptors in the rectum send signals to the brain, which in turn send signals to relax the internal anal sphincter. The external anal sphincter continues to hold feces in the rectum until voluntary signals from the cerebral cortex cause it to relax during the defecation reflex.