sphincter (sfink'ter)

Syn.: circidar muscle. An annular muscle which constricts or closes a natural orifice; especially the s. ani. cardiac s. The s. of plain muscle around the esophagus at the opening into the stomach. external s. of anus. The s. of striated muscle around the anus, ileocecal s. The s. of plain muscle around the ileum at its opening into the cecum, internal s. of anus. The s. of plain muscle around the anus. s. of bile duct. A s. of plain muscle around the opening of the bile duct into the duodenum, s. of the bladder. As. of plain muscle around the opening of the bladder into the urethra. See table of muscles, under muscle. [Gr., sphigkter, from sphiggein, to squeeze.]