| Anal canal | |
| Latin: | canalis analis |
| Precursor: | Hindgut, proctodeum |
| Artery: | Superior rectal artery (above pectinate line) and inferior rectal artery (below line) |
| Vein: | Superior rectal vein (above pectinate line) and Inferior rectal vein (below line) |
| Nerve: | Autonomic inferior hypogastric plexus (above pectinate line) and somatic inferior rectal nerves (below line) |
| Lymph: | Superficial inguinal lymph node (below pectinate line) and internal iliac lymph nodes (above line) |
The anal canal is the part that connects the rectum to the anus, located below the level of the pelvic diaphragm.[1] It is located within the anal triangle of the perineum, between the right and left ischioanal fossa. As the final functional segment of the bowel, it functions to regulate release of excrement by two muscular sphincter complexes. The anus is the aperture at the terminal portion of the anal canal.
In humans, the anal canal is approximately long, from the anorectal junction to the anus.[2] [3] It is directed downwards and backwards. It is surrounded by inner involuntary and outer voluntary sphincters which keep the lumen closed in the form of an anteroposterior slit.
The canal is differentiated from the rectum by a transition along the internal surface from endodermal to skin-like ectodermal tissue.
The anal canal is traditionally divided into two segments, upper and lower, separated by the pectinate line (also known as the dentate line):
The anal verge refers to the distal end of the anal canal, a transitional zone between the epithelium of the anal canal and the perianal skin. It should not be confused with the pectinate line between the upper and lower zones within the anal canal.
The anal gland secretes lymphal discharge and built-up fecal matter from the colon lining. In some animals this gland expungement can be done routinely every 24–36 months to prevent infection and fistula formation.
The external anal sphincter muscle is the voluntary muscle that surrounds and adheres to the anus at the lower margin of the anal canal. This muscle is in a state of tonic contraction, but during defecation, it relaxes to allow the release of feces.
Movement of the feces is also controlled by the involuntarily controlled internal anal sphincter, which is an extension of the circular muscle surrounding the anal canal. It relaxes to expel feces from the rectum and anal canal.