Structure
The anal canal is the final part of the gastrointestinal tract. It lies at an acute angle to the rectum due to the action of the pubococcygeus and puborectalis parts of levator ani.
The anal canal has three distinct parts. The upper part has mucosa similar to the rectum and is surrounded by the submucosal smooth muscle. The middle part is notable for six to ten vertical folds – the anal columns. The middle part is firmly attached to the perineal body anteriorly. The lower part is separated from the middle by the pectinate line, which unites the anal columns inferiorly. It is lined by smooth mucosa which is non-keratinising and lacks sweat glands and hair. This mucosa is continuous with the skin of the perineum.
The internal sphincter is the inner circumferential band of smooth muscle under autonomic control. It lies deep to the mucosa and superficial to the external sphincter, and extends to the junction of the middle and lower parts. The external sphincter extends inferiorly from the levator ani and also forms a circumferential band around the anus.
Relations
The anus is located inferior to the rectum and levator ani. The ischioanal space is found on either side. In men, the bulb of the penis and the membranous urethra lie anteriorly. The equivalent is the vagina in women. The perineal body lies between the anus and these structures. The coccyx lies about 2 – 3 cm posteriorly.
Microscopic Features
The proximal part and middle parts of the anus are lined with columnar epithelium, similar to the rectum. The inferior part is lined with stratified squamous epithelium, lacking the skin appendages found in skin. The point of changeover is the pectinate line, where the transition is immediate.
See this page at Blue Histology for the changeover point.
Neurovascular Supply
Arterial Supply
The anal canal is supplied predominately by the inferior rectal artery which arises from the interior pudendal artery. It receives some supply from the terminal branches of the superior rectal and median sacral arteries [1].
Venous Drainage
The drainage of the proximal anal canal and internal sphincter is via the superior rectal vein to the inferior mesenteric vein. The drainage of the distal anal canal and external sphincter is via the inferior rectal vein to the internal pudendal vein and thereby the internal iliac artery [1]. Importantly, anastamoses between these two systems exist in the anal canal.
Lymphatics
Lymphatics from the proximal canal and internal sphincter typically drain along superior rectal vessels to the mesorectum. The distal canal and external sphinter drain laterally to inguinal nodes. If the levator ani is involved lymph drains to internal iliac nodes directly[1]. The presacral space may also be involved. Therefore anal cancers have a broad variety of lymph node metastases which must accounted for with radiotherapy.
Innervation
The anal canal receives autonomic and somatic inputs. Sympathetic nerves descend from the lumbar plexus. Parasympathetics and somatic supply is from sacral nerve roots.
Contouring
The anus is rarely contoured as an organ at risk and there are few dose guidelines. More typically it is included as a 'anorectal' avoidance contour when treating genitourinary or gynaecological malignancies. See Gay et al[2].
The anal verge is difficult to identify from CT scan. The most accurate way of identifying the distal end of the anal verge is to place a marking wire at CT simulation. The anorectal junction is also difficult to precisely identify but a good guide is the point at which the mesorectum disappears and the anal sphincter is visible. The anal sphincter is usually included in the contour.
