End portion of the alimentary canal, distinguished from the rectum by the transition from an internal mucous membrane layer to one of skinlike tissue and by its narrower diameter. Waste products move from the rectum to the anal canal. The human anal canal is 1–1.5 in. (2.5–4 cm) long and has three parts: upper, with longitudinal folds (rectal columns); lower, with involuntary and voluntary constrictive muscles (sphincters) to control discharge of feces; and the anal opening itself. Enlargements of the ends of rectal and anal veins are called hemorrhoids.
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The anal canal is the terminal part of the large intestine.
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 halves, upper and lower. The upper half 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 half of the anal canal is supplied by the superior rectal artery which is a branch of the inferior mesenteric artery.
The lower half of the anal canal is lined by stratified squamous epithelium that blends with the skin.
A whitish line called Hilton's white line indicates the junction between simple columnar epithelium and stratified squamous epithelium.