The anal canal

Position

• 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.

Sphincters

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

• 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

Lymphatics

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

Innervation

• 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