Anal canal

Dr Henry Knipe et al.

The anal canal is the terminal part of the gastrointestinal tract. Anatomically, the anal canal is referred to as the terminal alimentary tract between the dentate line and anal verge. However, histologically it extends more proximally and includes the columns of Morgagni and anal sinuses. Surgically, the anal canal is referred to as the portion of bowel between the anorectal sling and the anal verge.

Anal margin is arbitrarily defined as 5 cm skin surrounding the anal verge

The anal canal measures ~4 cm long and is continuous with the rectum at the anorectal junction, which is the right angle the rectum takes at levator ani (i.e. the pelvic floor). 

The anal canal is a muscular tube, just like the rest of the gastrointestinal tract, but the difference being is that the muscular layer is circular forming the external and internal anal sphincters

The dentate (pectinate) line separates the anal canal into an upper and lower parts, not only in structure but also in neurovascular supply (reflecting the differing embryological origin). The dentate line is formed by the anal column, which consists of a series of anal sinuses (which drain anal glands) at approximately the midpoint of the anal canal. 

Note that the dentate line is a "watershed area" and that the exact transition of epithelium and neurovascular supply is varied. 

Above the dentate line the epithelium is a mucous membrane like the rest of the gastrointestinal tract and below the dentate line the epithelium is considered cutaneous (i.e. stratified squamous keratinised with hair and sebaceous glands).

The anal canal is a site of portosystemic anastomosis

  • above dentate line: internal iliac nodes
  • below dentate line: superficial inguinal nodes

The upper part of the anal canal derives from the dorsal compartment of the cloaca (endoderm) and the lower part is derived from proctodeum (ectoderm).

  • imperforate rectum
    • 1 in 1500-5000 newborns
    • failure of the bowel to open to the external world
    • may vary from stenosis to blind anal canal/rectum to absent anal canal

Abdominal and pelvic anatomy
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Article Information

rID: 24535
Section: Anatomy
Synonyms or Alternate Spellings:

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