Anal canal

The anal canal, the terminal portion of the large intestine, begins at the level of the apex of the prostate, and is directed downward and backward, and ends at the anus.

It forms an angle, the anorectal flexure, with the lower part of the rectum at the anorectal junction, and measures from 2.5 to 4 cm. in length.

It has no peritoneal covering, but is invested by the Sphincter ani internus, supported by the Levatores ani, and surrounded at its termination by the Sphincter ani externus.

In the empty condition it presents the appearance of an antero-posterior longitudinal slit.

Behind it is a mass of muscular and fibrous tissue, the anococcygeal body (Symington); in front of it, in the male, but separated by connective tissue from it, are the membranous portion and bulb of the urethra, and the fascia of the urogenital diaphragm; and in the female it is separated from the lower end of the vagina by a mass of muscular and fibrous tissue, named the perineal body.

The anal canal is divided by the pectinate line into two unequal sections, upper and lower.

• The upper 2/3 has longitudinal folds or elevations of tunica mucosa. Its mucosa is lined by simple columnar epithelium. The lumen of the anal canal presents, in its upper half, a number of vertical folds, produced by an infolding of the mucous membrane and some of the muscular tissue. They are known as the anal columns (rectal columns; Morgagni), and are separated from one another by furrows (anal sinuses; rectal sinuses), which end below in small valve-like folds, termed anal valves (Ball's), which join together the lower ends of the rectal columns. The upper 2/3 of the anal canal is supplied by the superior rectal artery which is a branch of the inferior mesenteric artery.

• The lower 1/3 of the anal canal (anal pecten) is lined by stratified squamous epithelium that blends with the skin. It ends at the anocutaneous line, the lower margin of the internal anal sphincter and beginning of the external skin. The anal transition zone is an histological term describing the epithelium of the transition zone between the anal columns and the anocutaneous line. The intersphincteric groove is composed by connective tissue from the muscular coat of the recum blended with connective-tissue from levator ani, and is palpable below the anocutaneous line. The lower third of the anal canal is supplied by the inferior rectal artery which is a branch of the internal pudendal artery.

The whitish line called the pectinate line (Hilton's) indicates the junction between keratinized stratified squamous epithelium and unkeratinized stratified squamous epithelium.