Anus and perianal area

Author: Charles Ladoulis, M.D. (see Authors page)

Revised: 26 January 2016, last major update May 2012

Copyright: (c) 2002-2016,, Inc.

Cite this page: Anatomy. website. Accessed October 26th, 2016.
Definition / General
Anal canal:
  • Tubular structure 3-4 cm long
  • Derived from cloaca (distal hindgut) and arises at level of prostatic apex, is directed downward and backward, and ends at anus
  • Boundaries are proximal and distal margins of internal sphincter muscle, and includes part of rectum
  • Embryologically divided by urogenital septum (cloacal membrane) into anterior GU and posterior GI compartments, and separated from perianal ectoderm by anal membrane, which ruptures at week 7 of gestation

Classic anatomic definition of anal canal:
  • Between proximal and distal margins of internal sphincter muscle, which includes part of rectum

Clinical AJCC definition of anal canal:
  • Begins at puborectalis sling at apex of anal sphincter complex (palpable as anorectal ring, but difficult for pathologists to identify)
  • Ends at squamous mucocutaneous junction with perianal skin; includes 1-2 cm of rectal-type glandular mucosa and possibly transitional mucosa at dentate line

Histologic definition of anal canal:
  • Anal transitional zone and squamous epithelium down to the perianal skin; cannot be identified by clinicians
  • Note: columns, valves and sinuses below are macroscopic landmarks, which may not correspond precisely to microscopic structures

  • Anal columns of Morgagni: longitudinal folds just distal to dentate line, analogous to lower rectums rectal columns of Morgagni; less pronounced in adults
  • Anal papillae: raised toothlike projections on anal columns; extend proximally into rectum
  • Anal sinuses of Morgagni: depressions between anal columns
  • Anal crypts of Morgagni: minute pockets with anal valves as boundary; site of discharge of anal glands

Anal valves:
  • Also called semilunar valves or transverse plicae
  • Connect distal ends of anal columns
  • Identifiable in children, often obscured in adults

Anal cushions:
  • Normal structures of anal canal that contribute to anal closure by close apposition to each other
  • Contain blood vessels, connective tissue, smooth muscle; vessels contain abundant smooth muscle
  • Resemble erectile tissue due to numerous arteriovenous communications

Anal verge:
  • Also called Hiltons line or anal margin;
  • Junction between anal canal and anal skin
  • Mucosa contains cutaneous adnexae

  • Corpus cavernosum recti: network formed by peculiar vessels with a complex convoluted appearance
  • Dentate (pectinate) line: midpoint of anal canal, formed by anal valves; circumferential musculature of canal

Musculature of anal canal
  • Muscularis mucosa: continues from rectum through upper anal transitional zone
    • Presence of muscle fibers in lamina propria indicates mucosal prolapse syndrome
  • Musculus submucosae ani: fibers from intersphincteric longitudinal muscle which pass through internal sphincter and from the internal sphincter itself; form a network around the vascular plexus

Internal anal sphincter:
  • Continuation of circular muscle of rectum, but thicker (5-8 mm); ends 5-19 mm below dentate line

Intersphincteric longitudinal muscle:
  • Between internal and external sphincters
  • Contains fibers from longitudinal muscle layer of rectum and levator ani muscles
  • Distally breaks up into septa that diverge fan-wise through subcutaneous layer of external sphincter and ends in corium, which forms characteristic corrugation of perianal skin

External anal sphincter:
  • Consists of superficial, subcutaneous and deep parts; provides voluntary control of defecation

Regional lymph node drainage:
  • Above dentate line - anorectal, perirectal, paravertebral nodes
  • Below dentate line - superficial inguinal nodes

  • Arterial supply: superior, middle and inferior rectal arteries
  • Venous supply: superior rectal vein
Diagrams / Tables
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