sphincter

noun

1. A ringlike band of muscle that surrounds a bodily opening (such as the anus or the openings of the stomach), constricting and relaxing as required for normal physiological functioning.

   “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.”