出典:Wiktionary
出典:Wikipedia
出典:『Wikipedia』 (2011/03/31 04:51 UTC 版)
The urethral sphincter is a collective name for the muscles used to control the flow of urine (micturition) from the urinary bladder. These muscles envelop the urethra, so that when they contract, the urethra is sealed shut. The muscles originate at the pubic ramus with the insertion point at the median raphe. The function of the sphincter urethrae (external sphincter) is controlled by the pudendal nerve which acts to constrict the urethra. There are actually two urethral sphincters in the human body: