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Anatomy of the anus and rectum
|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 terminal segment of the large intestine between the rectum and anus, located below the level of the pelvic diaphragm. It is located within the anal triangle of 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 aperture at the terminal portion of the anal canal is known as the anus.
In humans, the anal canal is approximately 2.5 to 4 cm long, from the anorectal junction to the anus. 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.
Anal canal is traditionally divided into two segments, upper and lower, separated by the pectinate line (also known as the dentate line):
- upper zone (zona columnaris)
- lower zone
- divided into two smaller zones, separated by a white line known Hilton's line:
- supplied by the inferior rectal artery (a branch of the internal pudendal artery)
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.
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 ischioanal fossa are on each side of the anal canal.
- The perianal space surrounds the anal canal below the white line.
- The submucous space of the canal lies above the white line between the mucous membrane and internal anal sphincter muscle.
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.