[Anatomy of the anal canal].

The anal canal marks the transition from the end of the alimentary canal to the exterior. Its prime function is to maintain continence by means of the internal sphincter. 1) Level with the mucous membrane, the pectineal line contains the anal valves, which form the limits of a supravalvular area glossy red in colour and a blue-grey subvalvular area in which is sited the linea alba of Hilton. 2) The sphincteral apparatus includes the internal sphincter which contracts automatically and which ensures tight continence, the longitudinal muscular stratum joining the ampulla of the rectum, and the pubo-rectal tract or levator ani, drawing the head of the rectum forwards. 3) The canal is surrounded by cellular perianal tissues, and, phlebologically speaking, is vascularised by a system which joins the portal current; the position of this system explains the topography of hemorrhoids.