sphincter(redirected from pupillary sphincter)
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a ringlike muscle that constricts to close a natural orifice such as the oral or anal opening, or to narrow the passage from one part of a hollow tubular organ to another, for example, from the stomach and bile duct to the duodenum and from the bladder to the urethra. The sphincter is always in a tonic state. The orbicular muscle of the iris is a sphincter. Some sphincters consist of striated muscle and others of smooth muscle innervated by the autonomic nervous system.