JACOPIN / SCIENCE PHOTO LIBRARY JACOPIN / SCIENCE PHOTO LIBRARY
The rectum in frontal section. The rectum is composed of the rectal ampulla (upper part) and of the anal canal (lower part), delimited by the pectinated line ; anal canal leads to the anus. The rectal ampulla forms curves, at the origin of folds of the internal mucosa of the rectum : the Houston's valves. It also presents vertical folds, the anal columns. The wall of the rectum includes three layers of tissue : the muscularis, external, composed of muscle fibers insuring the defecation, the submucosa, intermediary, and the mucosa, internal. The anal sphincter includes an external sphincter of striated muscles (voluntary control) and an internal sphincter of smooth muscles (involuntary control). The levator ani pulls the anal canal towards the top during defecation. The rectum is largement vascularized by the rectal venous plexus (hemorrhoidal veins).
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