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 terminal part of the large intestine. It is situated between the rectum and anus, below the level of the pelvic diaphragm. It lies in the anal triangle of perineum in between the right and left ischioanal fossa.
The 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 cm 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.
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.
- The ischioanal fossa lies 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.