Anus

Synonyms

Anal canal

Anatomy

Classically, the generic term “anus” encompasses two parts: the anal canal and the anal margin or perianal skin. This terminology applies to the classification of anal tumors, which can be located either in the anal canal or in the anal margin. However, only the anal canal is precisely defined, and this term is often employed synonymously to the term anus.

The anus constitutes the distal 30–40 mm of the gastrointestinal tract. It extends from the rectum superiorly to the perianal skin inferiorly. The anal canal is defined surgically by the borders of the internal anal sphincter (Parks 1958). Anatomists use the levels of the anal valves and the anal orifice, respectively, to mark the upper and lower limits of the anus (Fenger 2007).

The mucosa comprises successive zones, with in the upper part vertical folds known as the anal columns, joined at their lower end by the anal valves marking the position of the dentate or pectinate line (see section “Microscopy”).