. A manual of operative surgery . a slit-like passage. The upper bor-ders of the muscles may at times befelt during life as distinct ridges. This portion of the canal passesdownwards and backwards to end atthe anus, and averages in length oneand a half inches. As the rectum passes through thelevatores ani the outer part of itslongitudinal muscle fibres unite withthese muscles and then pass on out-side the internal sphincter to join theskin around the anus. The circular muscle fibres are muchthickened in this portion of the canalto form the internal sphincter, whichis united above with the leva
. A manual of operative surgery . a slit-like passage. The upper bor-ders of the muscles may at times befelt during life as distinct ridges. This portion of the canal passesdownwards and backwards to end atthe anus, and averages in length oneand a half inches. As the rectum passes through thelevatores ani the outer part of itslongitudinal muscle fibres unite withthese muscles and then pass on out-side the internal sphincter to join theskin around the anus. The circular muscle fibres are muchthickened in this portion of the canalto form the internal sphincter, whichis united above with the levatoresani and passes downwards for aboutone inch. The external sphincterforms a muscular cylinder surround-ing the lower two-thirds of the analcanal, and has an attachment an-teriorly to the central point of theperineum, and posteriorly by meansof the recto-coccygeus muscle to thetip of the coccyx; when contracted itis tightly stretched between these twoattachments, leaving only a narrowantero-posterior slit between its from FrG. 190.—VASCULAR SUPPLY rectum. (After Testut.), Peritoneal coat, reflected at 2,anterior surface of rectum ; 3, Levatorani; 4, Sphincter ani; 5, Anal orifice ;6, Superior hemorrhoidal artery (thetermination of the inferior mesenteric);7 and 7, Division of the superiorhemorrhoidal artery into two mainbranches about five inches above theanus : these are seen further dividing soas to form four descending trunks infront of and behind the lower part ofthe rectum ; 8 and 8, Anastomosingbranches from the middle hemor-rhoidal; 9, Inferior hemorrhoidalbranches from internal pudic ; 10, Ter-minal anastomotic plexus, region ofinternal piles ; 1, Wall of rectum. chap, xxxi] ANATOMY OF THE RECTUM 705 There is a loose layer of areolar tissue forming a submucous coat inwhich the haemorrhoidal plexus of veins lies. Mucous membrane.—In its upper half this shows a number of per-manent vertical folds, the columns of Morgagni ; they are about halfan inch in l
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