The sphincter muscle is a muscle with a circular muscle fiber course that surrounds an opening in a ring. Circular muscles are developed where something has to be temporarily closed ( anus , mouth , eyelid , urinary bladder , iris ). Such muscles are also known as sphincters . Most circular muscles consist of smooth muscles and are therefore not subject to voluntary motor skills . The external anal sphincter ( muscle sphincter ani externus ) as well as the circular muscle of the mouth ( muscle orbicularis oris ) and the eyelids ( muscle orbicularis oculi ) consist of striated muscles .