Anal canal
The anal canal is the part that connects the rectum to the anus, located below the level of the pelvic diaphragm. It is located within the anal triangle of the perineum, between the right and left ischioanal fossae. As the final functional segment of the bowel, it functions to regulate release of feces by two muscular anal sphincter complexes. The anus is the aperture at the terminal portion of the anal canal.
Structure
In humans, the anal canal is approximately 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.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 :
- upper zone
- * mucosa is lined by simple columnar epithelium
- * features longitudinal folds or elevations of tunica mucosa which are joined inferiorly by folds of mucous membrane known as anal valves
- * supplied by the superior rectal artery
- lower zone
- * divided into two smaller zones, separated by a white line known as the Hilton's line:
- ** zona hemorrhagica - lined by stratified squamous non-keratinized epithelium
- ** zona cutanea - lined stratified squamous keratinized epithelium, which blends with the surrounding perianal skin
- * supplied by the inferior rectal artery
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.
Relations
- 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.
Function
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.