In humans , the anus from Latin anus meaning "ring", "circle"   is the external opening of the rectum , located inside the intergluteal cleft and separated from the genitals by the perineum. Two sphincters control the exit of feces from the body during an act of defecation , which is the primary function of the anus. These are the internal anal sphincter and the external anal sphincter , which are circular muscles that normally maintain constriction of the orifice and which relaxes as required by normal physiological functioning. The inner sphincter is involuntary and the outer is voluntary.
S1 nerve is the most efficient nerve rootlet innervating the anal canal and rectum in rats
Causes and Treatment of Rectal Pain in Men
The rectum is the most distal segment of the large intestine , and has an important role as a temporary store of faeces. It is continuous proximally with the sigmoid colon, and terminates into the anal canal. In this article we will discuss the anatomy of the rectum — its structure, anatomical relationships, and clinical relevance. The rectum begins at the level of the S3 as a continuation of the sigmoid colon. It is macroscopically distinct from the colon, with an absence of taenia coli, haustra, and omental appendices.
Sensory nerve-endings and sensation in the anal region of man
The anus is an opening in the lower part of the digestive tract. When stool fills the rectum, the sphincter muscle relaxes, letting stool pass through the anus and out of the body. The external anal sphincter closes off the anus when stool has passed. Lumps that form around the anus — for a variety of reasons — can cause it to feel hard. There may also be swelling, pain , and discharge.