anal sphincter

The anal sphincter is a ring-like muscle that controls the opening and closing of the anus, the terminal part of the digestive tract. It is crucial for the voluntary control of defecation. The anal canal, where the sphincter is located, transitions from a mucous membrane to skin-like tissue and is about 1 to 1.5 inches long. It contains an internal sphincter made of smooth muscle, which is not under voluntary control, and an external sphincter made of striated muscle, which allows for voluntary control of fecal passage. The external sphincter is composed of subcutaneous, superficial, and deep parts. When the rectum fills, the internal sphincter relaxes, and shortly after, the external sphincter also relaxes, allowing for defecation.123