circular muscle

A sphincter muscle is a ring-like muscle that controls the opening and closing of bodily passages. Important human sphincters include the sphincter pylori in the stomach, which regulates food emptying into the small intestine, and sphincters involved in waste excretion like the sphincter ani externus and the sphincter urethrae, which controls urination. The eye also has a sphincter pupillae muscle that constricts the pupil in bright light. In soft-bodied animals like worms, muscles often work antagonistically, with circular and longitudinal muscles contracting and relaxing to change body shape and facilitate movement.123