Sphincter. This is a ringlike muscle that opens and closes to let stuff through or to stop it. There are multiple sphincters in the body; the only sphincter we can see from outside the body is the anus. When there’s poop inside the rectum, it keeps it there, and it opens when you need to let it out. During anal sex, it’s through the anal sphincter that someone penetrates the body.

Synonyms:
anus