Anal canal

The anal canal is the terminal part of the gastrointestinal tract, whilst the anus (plural: anuses or ani) specifically refers to the opening separating the anal canal from the outside, at the distal most aspect of the anal verge. 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. 

The anal margin is arbitrarily defined as the 5 cm of skin (radius) surrounding the anal verge. 

Gross anatomy

The anal canal measures ~4 cm long and is continuous with the rectum at the anorectal junction, which is the right angle (the anorectal angle) the rectum makes at the 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 columns, 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 keratinized with hair and sebaceous glands).

Arterial supply

• above dentate line: superior rectal artery (from inferior mesenteric artery); small contributions from middle rectal artery (directly from internal iliac artery) and median sacral arteries
• below dentate line: inferior rectal artery (from internal pudendal artery)

Venous drainage

Venous drainage richly anastomoses with the rectal venous plexus

• above dentate line: superior rectal vein to inferior mesenteric vein (portal venous system)
• below dentate line: inferior and middle rectal veins to internal iliac veins

The anal canal is a site of portosystemic anastomosis. 

Lymphatic drainage

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

Innervation

• above dentate line and internal anal sphincter
  • sympathetic: pelvic plexus
  • parasympathetic and afferent sensory: pelvic splanchnic nerves
• below dentate line and external anal sphincter
  • inferior rectal branches of the pudendal nerve

Development

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).

Variant anatomy

• imperforate anus
  • 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