THE PELVIS
Computed tomography
The rectum
Axial CT scanning may also demonstrate the muscles of the pelvic floor (see Figs 6.4 and 6.6).
This is about 12 cm long. It commences anterior to S3 and ends in the anal canal 2-3 cm in front of the tip of the coccyx. It forms an anteroposterior curve in the hollow of the sacrum. The rectum has no sacculations or mesentery. Its upper third is covered by peritoneum on its front and sides; its middle third has peritoneum on its anterior surface only, and its lower third has no peritoneum. The lower part of the rectum is dilated into the rectal ampulla. In the resting state the mucosa of the rectum has three to four longitudinal mucosal folds, known as the columns of Morgagni. The taenia coli fuse in the rectum. Some longitudinal shortening of the rectum occurs, giving the rectum a slight S shape. This creates three lateral mucosal folds - left, right and left from above downward. These are known as the valves of Houston.
THE SIGMOID COLON, RECTUM AND ANAL CANAL (Figs 6.8 and 6.9; see Fig. 5.20) The sigmoid colon The sigmoid colon is variable in length (12-75 cm, average 40 cm). It is covered by a double layer of peritoneum, supported on its own mesentery which is attached to the posterior and left lateral pelvic wall. It has the same sacculated pattern as the rest of the colon in young people. In older subjects it may appear featureless. The redundant sigmoid lies on the other pelvic structures. Blood supply (see Fig. 5.23) The arterial supply is from the inferior mesenteric artery via the sigmoid arteries, which join in the arterial cascade of the large bowel. Venous drainage is to the portal system via the inferior mesenteric vein. Lymph drainage This is via the blood supply to preaortic nodes around the origin of the inferior mesenteric artery.
Posteriorly are the sacrum and coccyx. Anteriorly, loops of small bowel and sigmoid colon lie in the peritoneal cul-de-sac between the upper two-thirds of the rectum and the bladder or uterus. The lower third is related to the vagina in the female and to the seminal vesicles, prostate and bladder base in the male. The rectum is surrounded by perirectal fat. Fascia known as the perirectal fascia surrounds the perirectal fat, and lateral to the perirectal fascia is the pararectal fat. The perirectal fascia separates the seminal vesicles and prostate from the rectum anteriorly.
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