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 leftischioanal 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.
Structure
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 lumenclosed 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. Anal canal is traditionally divided into two segments, upper and lower, separated by the pectinate 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. 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 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.
