The upper 2/3 of the anal canal has longitudinal folds or elevations of tunica mucosa. Its mucosa is lined by simple columnar epithelium. The lumen of the anal canal presents, in its upper half, a number of vertical folds, produced by an infolding of the mucous membrane and some of the muscular tissue. They are known as the anal columns (rectal columns; Morgagni), and are separated from one another by furrows (anal sinuses; rectal sinuses), which end below in small valve-like folds, termed anal valves (Ball's), which join together the lower ends of the rectal columns

This definition incorporates text from a public domain edition of Gray's Anatomy (20th U.S. edition of Gray's Anatomy of the Human Body, published in 1918 – from


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