Anal Canal Anatomy 

The anal canal is the most terminal part of the lower GI tract/large intestine, which lies between the anal verge (anal orifice, anus) in the perineum below and the rectum above. Confusion and controversy exist regarding the anatomy of the anorectal region in anatomy and surgical texts. The description in this topic is from below upwards, as that is how this region is usually examined in clinical practice. Images depicting the anal canal can be seen below.^{[1, 2]}

The pigmented, keratinized perianal skin of the buttocks (around the anal verge) has skin appendages (eg, hair, sweat glands, sebaceous glands); compare this with the anal canal skin above the anal verge, which is also pigmented and keratinized but does not have skin appendages.^{[3, 4]}

In anatomy texts, the rectum changes to the anal canal at the dentate line. For surgeons, however, the demarcation between the rectum above and the anal canal below is the anorectal ring. The following is the description of the surgical anal canal.

The anal canal is completely extraperitoneal. The length of the (surgical) anal canal is about 3-5 cm, with two thirds of this being above the dentate line and one third below the dentate line (anatomical anal canal).

The epithelium of the (anatomical) anal canal (between the anal verge below and the dentate line above) is variously described as anal mucosa or anal skin. The author feels that it should be called anal skin (anoderm), as it looks like (pigmented) skin, is sensitive like skin, and is keratinized (but does not have skin appendages).

The dentate line (also called the pectinate line) is the site of fusion of the proctodeum below and the postallantoic gut above. It is a wavy demarcation formed by the anal valves (transverse folds of mucosa) at the inferior-most ends of the anal columns. Anal glands open above the anal valves into the anal crypts. The dentate line is not seen on inspection in clinical practice, but under anesthesia the anal canal descends down, and the dentate line can be seen on slight retraction of the anal canal skin.

From a surgical perspective, the anal canal just above the dentate line for about 1-2 cm is called the transition zone. Beyond this transition zone, the (surgical) anal canal is lined with columnar epithelium. Anal columns (of Morgagni) are 5-10 longitudinal (vertical) mucosal folds in the upper part of the anal canal.

At the bottom of these columns are anal crypts, or sinuses, into which open the anal glands and anal papillae. Three of these columns (left lateral, right posterior, and right anterior, at 3, 7, and 11 o’clock position in supine position) are prominent; they are called anal cushions and contain branches and tributaries of superior rectal (hemorrhoidal) artery and vein. When prominent, veins in these cushions form the internal hemorrhoids.

The anorectal ring is situated about 5 cm from anus. At the anorectal angle, the rectum turns backwards to continue as the anal canal.

Levator ani and coccygeus muscles form the pelvic diaphragm. Lateral to the anal canal are the ischioanal fossae (1 on either side), below the pelvic diaphragm. The anterior relations of the anal canal are, in males, the seminal vesicles, prostate, and urethra, and, in females, the cervix and vagina. In front of (anterior to) the anal canal is the rectovesical fascia (of Denonvilliers), and behind (posterior) is the presacral endopelvic fascia (of Waldeyer), under which lie a rich presacral plexus of veins. Posterior to the anal canal lie the tip of the coccyx and lower sacrum.

Blood supply and lymphatics

The anal canal above the dentate line is supplied by the terminal branches of the superior rectal (hemorrhoidal) artery, which is the terminal branch of the inferior mesenteric artery. The middle rectal artery (a branch of the internal iliac artery) and the inferior rectal artery (a branch of the internal pudendal artery) supply the lower anal canal.

Underneath the anal canal skin (below the dentate line) lies the external hemorrhoidal plexus of veins, which drains into systemic veins. Underneath the anal canal mucosa (above dentate line) lies the internal hemorrhoidal plexus of veins, which drains into the portal system of veins. The anorectum is, therefore, an important area of portosystemic venous connection (the other being the esophagogastric junction). Lymphatics from the anal canal drain into the superficial inguinal group of lymph nodes.

Embryology

The anal canal below the dentate line develops from the proctodeum, while that above the dentate line develops from the endoderm of the hindgut.

The perianal skin is keratinized, stratified squamous epithelium with skin appendages (eg, hair, sweat glands, sebaceous glands, somatic nerve endings that are sensitive to pain). The (anatomical) anal canal skin (anoderm) is also keratinized, stratified squamous epithelium and has somatic nerve endings (sensitive to pain), but without skin appendages. The (surgical) anal canal mucosa is cuboidal in the transition zone and columnar above it; it is insensitive to pain. The rectal mucosa above the anorectal ring is lined by pinkish red, insensitive columnar epithelium.

The anorectal ring is formed by the puborectalis (the innermost fibres of levator ani muscle) and the upper ends of the external and internal anal sphincters. The involuntary internal anal sphincter is the lowermost continuation of the inner, circular muscle layer of the rectum. The external anal sphincter has 3 parts: subcutaneous, superficial, and deep. The external anal sphincter is under voluntary control.

Pathophysiologic anal variants include the following:

• Anal atresia
• Ectopic Anus
• Persistent Cloaca

Anal atresia (imperforate anus) is a low anorectal malformation in which the anus is either atretic (absent) or narrowed and the colon and rectum are normal. If the proctodeum and the postallantoic gut fail to unite, an imperforate anus results.

In ectopic anus, the anus is misplaced, usually anteriorly in the perineum (in males) or in the vagina (in females). Persistent cloaca, is a common passage in which lower GI tract (rectum), lower urinary tract (bladder or urethra), and lower genital tract in female (vagina) are open.

The location of perianal lesions is described in relation to a clock (as seen in the supine position), eg, 2 o'clock, 7 o'clock. Sites of perianal lesions include the following:

• Perianal skin – Abscess, hematoma, external opening of fistula-in-ano
• Anal canal skin (anoderm) – Fissure-in-ano, external hemorrhoids, cancer
• Anal canal mucosa - Internal hemorrhoids, cancer

Infection of an anal gland is considered to be the starting event in the formation of a perianal abscess and then fistula-in-ano. Fissure-in-ano is an ulcer in the sensitive anal canal skin and is a very painful condition.

External hemorrhoids are in sensitive anal canal skin and are painful, while internal hemorrhoids are in insensitive anal canal mucosa and are painless (unless complicated).

The dentate line cannot be felt per rectal examination and is seen on anoproctoscopy. The anorectal ring can be palpated per rectal examination (but not under anesthesia when the muscles relax).