Anatomy of the anal canal

The anal canal is about 4cm long. It is surrounded by the anal sphincter mechanism. It begins at the point where the rectal ampulla narrows abruptly, level with the puborectalis muscle.

The upper half of the canal is lined by glandular mucosa - a continuation of rectal mucosa - and the lower half is lined by stratified squamous epithelium - modified skin. The point at which this change from glandular mucosa to squamous epithelium occurs is at the dentate - pectinate - line.

In the upper half of the canal the mucosa is thrown into 6-10 longitudinal folds - the columns of Morgagni - that each contain a terminal branch of the superior rectal artery and vein. These folds are most prominent where the veins form prominent venous plexuses in the left lateral, right posterior and right anterior quadrants. In the upper half of the canal the lymphatics drain to the pelvic and abdominal lymph nodes. The glandular mucosa is relatively insensitive in the upper half of the canal. The pink rectal mucosa becomes a darker reddish-blue when it overlies the submucosal venous plexuses.

In the lower half of the canal there is lymph node drainage to the inguinal nodes. The lower canal skin is highly sensitive.

The anal sphincter is composed of three structures:

• internal sphincter: this represents a downward, thickened extension of rectal wall circular musculature
• external sphincter } encircle the internal sphincter and
• puborectalis } arise from the pelvic floor

The most important contribution to the function of the sphincter is the angle maintained by the action of the puborectalis muscle - one of the levator ani group.