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 (also known as the dentate line): • upper zone (zona columnaris) • 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 (a branch of the
inferior mesenteric 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 (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. 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==