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Clean the internal aspect of the rectum and anal canal, and attempt to identify three transverse rectal folds (of Houston) which project into its lumen. On the internal wall of the anal canal, identify the vertical anal columns and anal sinuses . At the distal end of the anal columns identify anal valves. They are semilunar-shaped folds of mucous membrane which span between adjacent columns.

Links and References:
Grant's: 3.42
Netter (1ed.): 369 (2ed.): 365
Rohen/Yokochi: 335