The anal canal


  • Terminal part of the large intestines

  • Extends from the anorectal angle

  • Descends postero inferiorly between the anococcygeal ligament and the central perineal tendon

  • Sorrounded the levator ani

Interior of the anal canal


Anal colums:

  • A series of longitudinal ridges / folds

  • in the superior half of the mucous membrane of the anal canal.

  • Contain terminal branches of superior rectal artery and vein

  • The superior ends of the anal columns indicate the anorectal line where the anus joins the rectum

Anal pectinate line

  • The inferior comb shaped limit of the anal valve.

  • It indicates the site of junction of the superior part of the anal canal and inferior parts.



The internal sphincter

External sphincter

  • Involuntary
  • Stimulated by parasympathetic nerves
  • Sorrounds the superior 2/3rds of the anal canal
  • Involuntary
  • Supplied by S4 via the inferior rectal nerve

Anorectal ring


Comprises of the following structures:

  • Puborectalis muscle
  • External anal sphincter
  • A few involuntary fibres from the internal anal sphincter

It is responsible for maintaining anal continence

External anal sphincter

  • Voluntary and surrounds the inferior end of the anal canal

  • Lies in the perineum

  • Has three parts
    • Subcutaneous part (1)

      (+) Consists of slender fibres

      (+) Sorrounds the anus (crossing anterior and posterior to it)

      (+) Has no bony attachments.

    • Superficial part (2)

      (+) Fibres extend anteriorly from:
      the tip of the coccyx and the Anococcygeal ligament around the anus
      to the central perineal tendon.

    • Deep part (3)

      (+) Arises from the central perineal tendon

      (+) Fuses with the puborectalis part of the lavetor ani superiorly

      (+) Blends with lavetor ani, and is not sharply distinguished from it.

    • All the components are innervated by perineal branch of S4; and inferior rectal branch of the pudendal nerve.

Functions of the external anal sphincter;

  • Closes the anus
  • Draws anal canal anteriorly, thereby increasing the anorectal angle.
  • Deep fibres are assisted by puborectalis

Blood supply


  • Superior rectal – above pectinate line
  • Inferior rectal – below pectinate line
  • Middle rectal anastomoses the two
  • The veins correspond



  • Above the pectinate line – internal iliac- common iliac and aortic lymph nodes
  • Below pectinate line – superficial inguinal – deep inguinal and then to iliac nodes



  • The part above the pectinate receives autonomic innervation and is sensitive only to stretch
  • The part below the pectinate line is innervated by inferior rectal branches of the pudendal nerve. This is somatic innervation and is sensitive to pain, touch and temperature

Summary of Pelvic Viscera