The anus is the terminal opening of the gastrointestinal tract (GI tract); the solid residues of digestion are eliminated through the anus. The last 3 to 4 centimeters of the GI tract is the anal canal, which continues from the rectum and opens to the outside at the anus. The mucosa of the rectum is folded to form longitudinal anal columns. The smooth muscle layer is thick and forms the internal anal sphincter at the superior end of the anal canal. This sphincter is under involuntary control. There is an external anal sphincter at the inferior end of the anal canal. This sphincter is composed of skeletal muscle and is under voluntary control.