Sphincter

Your sphincter is the muscle at the end of your digestive tract.

the sphincter of the bladder

the iris sphincter in the eye

She decided that she would force him to climax first by the sheer strength of her young animal control, the strength of her sphincters […]

The ink sac feeds into the rectum, controlled by a sphincter and in some inking events mucus from another organ, the funnel organ is ejected with water and ink through the anus and the siphon to create a cloud of ink.

Polygraph operators place their subjects on a pad that can sense the clenching of their sphincters, a device known by those who’ve sat on them as the “whoopee cushion.” Maguire took his seat, his sphincter already tight enough to crush walnuts.

Go ahead, use your eye sphincters and watch the roof opening and closing.