The anus is divided into an upper part which is endodermal in origin which is covered by stratified columnar epithelium and a lower part which is ectodermal in origin and comes from the cloaca embryologically. The lower portion is covered by stratified squamous epithelium. It is surrounded by voluntary muscles  called external anal sphincter and the corrugator cutis ani which is found in the skin around the anus. A submucosa is placed below the mucosa and between the mucosa and the muscles.

The mucosa of the upper 15 mm of the canal has 6-12 longitudinal folds called anal columns. The lower ends are united by short transverse folds called anal valve. The anal valve run transversely to form the pectinate line. The region below this line is called the pectin or transitional zone. Its lower limit forms the Hilton's white line. Below this level is final skin covering the lowest 8-11mm of anal canal.

The internal anal sphincter is a continuation of the longitudinal muscle of the rectum. It extends to the level of Hiltonís white line, which is marked by the attachment of the peri-anal fascia to the wall of the anal canal.









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1 rectum 2 levator ani 3 anal column 4 deep part of external anal sphincter 5 superficial part of external anal sphincter 6 peri-anal fascia 7 subcutaneous part of external anal sphincter 8 internal anal sphincter 9 (Hilton's white line 10 anocutaneous junction 11 annulus hemorrhoidalis 12 pecten 13 anal sinus





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