Anal canal
Encyclopedia
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
, 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.
It is differentiated from the rectum by the transition of the internal surface from endodermal to skinlike ectodermal tissue.
A whitish line called Hilton's white line or pecten of Robert Stroud indicates the junction between keratinized stratified squamous epithelium and unkeratinized stratified squamous epithelium.
Large intestine
The large intestine is the third-to-last part of the digestive system — — in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body...
.
It is situated between the rectum
Rectum
The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...
and anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
, below the level of the pelvic diaphragm. It lies in the anal triangle
Anal triangle
The anal triangle is the posterior part of the perineum. It contains the anal canal.-Structure:The anal triangle can be defined either by its vertices or its sides.* Vertices** one vertex at the coccyx bone...
of perineum
Perineum
In human anatomy, the perineum is a region of the body including the perineal body and surrounding structures...
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
Simple columnar epithelium
A simple columnar epithelium is a columnar epithelium that is uni-layered. In humans, a simple columnar epithelium lines most organs of the digestive tract including the stomach, small intestine, and large intestine...
. The lower half of the anal canal, below the pectinate line
Pectinate line
The pectinate line is a line which divides the upper 2/3s and lower 1/3 of the anal canal. Developmentally, this line represents the hindgut-proctodeum junction....
, 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
Stratified squamous epithelium
A stratified squamous epithelium consists of squamous epithelial cells arranged in layers upon a basement membrane. Only one layer is in contact with the basement membrane; the other layers adhere to one another to maintain structural integrity...
and stratified squamous keratinized
Stratified squamous epithelium
A stratified squamous epithelium consists of squamous epithelial cells arranged in layers upon a basement membrane. Only one layer is in contact with the basement membrane; the other layers adhere to one another to maintain structural integrity...
, respectively.
In humans it is approximately 2.5 to 4 cm long, extending from the anorectal junction to the anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
. It is directed downwards and backwards. It is surrounded by inner involuntary and outer voluntary sphincters which keep the lumen
Lumen (anatomy)
A lumen in biology is the inside space of a tubular structure, such as an artery or intestine...
closed in the form of an anteroposterior slit.
It is differentiated from the rectum by the transition of the internal surface from endodermal to skinlike ectodermal tissue.
Upper and lower divisions
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 epitheliumSimple columnar epitheliumA simple columnar epithelium is a columnar epithelium that is uni-layered. In humans, a simple columnar epithelium lines most organs of the digestive tract including the stomach, small intestine, and large intestine...
. Its lower ends are joined together by folds of mucus membrane called anal valves. The upper 2/3 of the anal canal is supplied by the superior rectal arterySuperior rectal arteryThe superior rectal artery is an artery that descends into the pelvis to supply blood to the rectum.-Structure:The superior rectal artery is the continuation of the inferior mesenteric artery...
which is a branch of the inferior mesenteric arteryInferior mesenteric arteryIn human anatomy, the inferior mesenteric artery, often abbreviated as IMA, supplies the large intestine from the left colic flexure to the upper part of the rectum, which includes the descending colon, the sigmoid colon, and part of the rectum...
.
- The lower 1/3 of the anal canal is lined by stratified squamous epitheliumStratified squamous epitheliumA stratified squamous epithelium consists of squamous epithelial cells arranged in layers upon a basement membrane. Only one layer is in contact with the basement membrane; the other layers adhere to one another to maintain structural integrity...
that blends with the skin. The lower third of the anal canal is supplied by the inferior rectal arteryInferior rectal arteryThe inferior rectal artery is an artery that supplies blood to the lower half of the anal canal.-Structure:The inferior rectal artery arises from the internal pudendal artery as it passes above the ischial tuberosity....
which is a branch of the internal pudendal arteryInternal pudendal arteryThe internal pudendal artery is an artery that branches off the internal iliac artery, providing blood to the external genitalia.The internal pudendal artery is the terminal branch of the anterior trunk of the internal iliac artery...
.
A whitish line called Hilton's white line or pecten of Robert Stroud indicates the junction between keratinized stratified squamous epithelium and unkeratinized stratified squamous epithelium.
Surgical spaces related to the anal canal
- 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.