Anorectal anatomy

Anatomy

Buttocks

The buttocks are made up of the masses of the gluteal muscles  superimposed by a layer of fat.  The gluteal muscles are the gluteus maximus muscle and the gluteus medius muscle. 

The superior aspect of the buttock ends at the iliac crest, and the lower aspect is outlined by the horizontal gluteal crease. The gluteus maximus has two insertion points:  

• 1/3 superior portion of the linea aspera of the femur
• superior portion of the iliotibial tractus

An intermediate intergluteal cleft separates the masses of the gluteus maximus muscle. The anus is situated in the  intermediate intergluteal cleft.  

Anoderm

The anoderm is the lining of the anal canal immediately inferior to the pectinate line and extending approximately 1.5cm to the anal verge.

It is squamous epithelium which is devoid of hair follicles or glands, and has a pale, smooth, shiny appearance.

Anal verge

The anal verge is the distal end of the anal canal, forming the transitional zone between the skin of anal canal and the perianal skin.

Anal canal

The anal canal is 3-4cm in length, extending from the external anus to the anorectal junction. It is surrounded by powerful sphincter muscles which facilitate defaecation and maintain continence:

• internal anal sphincter - involuntary smooth muscle
• external anal sphincter - voluntary skeletal muscle

The Dentate Line (Pectinate Line) is an important landmark that divides the upper two thirds of the anal canal with the lower third.

The upper two-thirds of the anal canal are lined by columnar epithelium, which is characterised by the Columns of Morgagni, a number of fleshy, vertical folds. Here, pain is absent and lymph drains to the superior rectal and iliac nodes.

The lower third of the anal canal is lined by stratified squamous epithelium and is continuous with the perianal skin. Here, pain is present, and lymph drains into the inguinal nodes. Four to eight anal glands drain into the Crypts of Morgagni at the Dentate Line.

Common anal conditions

There are a range of common anal conditions which may present in an asymptomatic patient.