Anorectal anatomy

«Note the anal crypts and glands; 90 percent of anorectal fistulas originate in a cryptoglandular abscess. Also note the relationship of the crypts and glands to the internal and external sphincters.»

Source: UpToDate [Internet]. Uptodate.com. 2019 [cited 2019 May 26]. Available from: https://www.uptodate.com/contents/image?imageKey=SURG%2F72874&topicKey=SURG%2F17127&search=rectum&source=outline_link&selectedTitle=1~150.

«The anal canal is 2.5 to 3.5 cm long and begins superiorly where the rectal ampulla is narrowed by the puborectalis sling (the levator ani muscle, which is palpable as the anorectal ring). It ends at the intersphincteric groove. Externally, the anal canal is surrounded by the internal and external anal sphincter muscles.

The superior half of the anal canal contains a series of longitudinal ridges called the anal columns (of Morgagni), which extend from the anorectal junction superiorly to the anal valves inferiorly. The anal valves form an irregular line called the dentate (or pectinate) line (colored purple in the diagram), which is an important anatomical landmark. The portions of the anal canal superior and inferior to it have different origins of arterial supply, nerve innervation, venous/lymphatic drainage, and epithelial lining.

The anal canal is internally lined with mucous membrane above the dentate line and anoderm below it. The upper portion of the anoderm (anal verge) consists of smooth, hairless skin; the lower portion of the anoderm (perianal skin) contains pigmented skin containing hair follicles and glands.»

Source: UpToDate [Internet]. Uptodate.com. 2019 [cited 2019 May 26]. Available from: https://www.uptodate.com/contents/image?imageKey=ONC%2F62539&topicKey=ONC%2F106743&search=rectal%20cancer&source=outline_link&selectedTitle=1~150.