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The rectum is the terminal portion of the large intestine. It begins at the lower end of the sigmoid colon at the third sacral vertebra and opposite the greater sciatic. It ends at the anus a short distance in front of the tip of the coccyx. It is about fifteen centimeters in length. It follows the curve of the sacrum and coccyx, lying behind the bladder, the seminal vesicles and their ducts and the prostate in the male; and behind the uterus and vagina in the female. The upper portion of the rectum is twelve and a half centimeters in length and is the most dilated portion of the rectum. It lies upon the front of the sacrum and extends downward from the third sacral vertebra to about two and a half centimeters in front of the tip of the coccyx. At this point, the rectum bends backward. The lower third rests against the musculofibrous structure situated between the coccyx and anus, called the anococcygeal body, or ligament. Just above the junction of the first with the second portion the bowel is dilated, and forms the rectal ampulla. The peritoneum covers the anterior surface and sides of the upper third, forming the pararectal fossa on each side. It covers the anterior surface of the middle third of. It forms the bottom of the recto-vesical or recto-vaginal pouch. This is about seven and a half centimeters from the anus and about two and a half centimeters above the prostate in the male. The lowest third is covered by the pelvic fascia and is devoid of peritoneum. The lower portion of the rectum, the anal canal, is four to five centimeters in length. It begins at about the apex of the prostate or the tip of the coccyx. Here, it passes downward and backward to the anus. It is encircled throughout almost its entire extent by the internal sphincter ani muscle which permits its walls to separate only during the passage of feces. The external sphincter guards its orifice, the anus. Above the outer sphincter is the levator ani muscle which supports the bowel. The rectum is divided into three parts. The first part, however, is commonly included with the sigmoid colon. The rectum commences opposite the middle piece of the sacrum slightly to the left side of the median line. It follows the concavity of the sacrum and of the coccyx and lies behind the bladder, seminal vesicles and prostate in the male, and the uterus and vagina in the female, ending in the median line of the perineum as the anus.