sphincters

/S F IH1 NG K T ER0 Z/
noun
  1. 1

    A ringlike band of muscle that surrounds a bodily opening, constricting and relaxing as required for normal physiological functioning.

    โ€œthe iris sphincter in the eyeโ€

Source: https://en.wiktionary.org/wiki/sphincter, https://en.wiktionary.org/wiki/sphincters