[TA] smooth muscle sphincter of the common bile duct immediately proximal to the hepatopancreatic ampulla and organized into a superior and inferior sphincter; it is this sphincter that controls the flow of bile into the duodenum. SYN: musculus sphincter ductus choledochi [TA], musculus sphincter ductus biliaris*, Boyden sphincter, choledochal sphincter, sphincter muscle of common bile duct.