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.
The anal canal is completely extraperitoneal. 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.
Anal columns (of Morgagni) are 5-10 longitudinal (vertical) mucosal folds in the upper part of the anal canal. At the bottom of these columns are anal crypts, or sinuses, into which open the anal glands and anal papillae.
About 5 cm from anus is the anorectal ring . At the anorectal angle, the rectum turns backwards to continue as the anal canal.
Lateral to the anal canal are the pyramidal ischioanal (ischiorectal) fossae (1 on either side), which communicate with each other behind the anal canal. The anterior relations of the anal canal differentiate between genders. In males, they are the seminal vesicles, prostate, and urethra while in females, the cervix and vagina with perineal body in between. Anterior to the anal canal is the rectovesical fascia (of Denonvilliers), and posterior is the presacral endopelvic fascia (of Waldeyer), under which lie a rich presacral plexus of veins. Posterior to the anal canal lie the tip of the coccyx and lower sacrum.
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 epithelium. 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 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 or pecten of Jon 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.