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The anal canal is the terminal part of the large intestine. It is situated between the rectum and anus, below the level of the pelvic diaphragm. In humans it is approximately 2.5 to 4 inches long. It lies in the anal triangle of perineum in between the right and left ischioanal fossa.
Image of the anus affected by hemorrhoids
|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 short terminal portion of the rectum through which wastes from the large intestine are excreted from the body. The ring at the terminal portion of the anal canal is called the anus.
The anal canal is between 2.5" and 5" in length and is guarded by two muscles that control the release of waste from the rectum.
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 an extension of the circular muscle surrounding the anal canal. It relaxes to expel feces from the rectum and anal canal.
Anal canal is divided into three parts. The zona columnaris is the upper half of the canal and is lined by simple columnar epithelium. The lower half of the anal canal, below the pectinate line, is divided into two zones separated by Hilton's white line. The two parts are the zona hemorrhagica and zona cutanea, lined by stratified squamous non-keratinized and stratified squamous keratinized epithelium, respectively.
In humans it is approximately 2.5" to 4" long, extending 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.
Behind this lies the anal gland which secretes lymphal discharge and built up fecal matter from the colon lining. In animals, gland expungement can be done routinely every 24 – 36 months to prevent infection and fistula formation.
The anal canal is divided into two unequal sections, upper and lower.
- The upper 2/3 has longitudinal folds or elevations of tunica mucosa. Its mucosa is lined by simple columnar epithelium. Its lower ends are joined together by folds of mucous membrane called anal valves. The upper 2/3 of the anal canal is supplied by the superior rectal artery which is a branch of the inferior mesenteric artery.
- The lower 1/3 of the anal canal is lined by stratified squamous epithelium that blends with the skin. The lower third of the anal canal is supplied by the inferior rectal artery which is a branch of the internal pudendal artery.
A whitish line called Hilton's white line indicates the junction between keratinized stratified squamous epithelium and unkeratinized stratified squamous epithelium.