The rectal ampulla becomes the anal canal at the anorectal junction. This is the location of the blood-filled cavernous body, the Corpus cavernosum recti. It is necessary for the involuntary, gas-tight closure of the rectal ampulla. The Musculus sphincter ani internus and Musculus levator ani are responsible are located more caudal. They allow an involuntary water proof closure of the anal canal. The voluntary closure is done by the Musculus sphincter ani externus.