sphincter

Ring of muscle, such as is found at various points in the alimentary canal, that contracts and relaxes to open and close the canal and control the movement of food. The pyloric sphincter, at the base of the stomach, controls the release of the gastric contents into the duodenum. After release the sphincter contracts, closing off the stomach. The external anal sphincter closes the anus; the internal anal sphincter constricts the rectum; the sphincter vesicae controls the urethral orifice of the bladder. In the eye the sphincter pupillae contracts the pupil in response to bright light.