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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
length from the center toward the ends of the loop, where it disappears, so that the loop is fixed at its junctions with the iliac colon and rectum, but enjoys a considerable range of movement in its central portion. Behind the sigmoid colon are the external iliac vessels, the left Piriformis, and left sacral plexus of nerves; in front, it is separated from the bladder in the male, and the uterus in the female, by some coils of the small intestine.
  The Rectum (intestinum rectum) (Fig. 1077) is continuous above with the sigmoid colon, while below it ends in the anal canal. From its origin at the level of the third sacral vertebra it passes downward, lying in the sacrococcygeal curve, and extends for about 2.5 cm. in front of, and a little below, the tip of the coccyx, as far as the apex of the prostate. It then bends sharply backward into the anal canal. It therefore presents two antero-posterior curves: an upper, with its convexity backward, and a lower, with its convexity forward. Two lateral curves are also described, one to the right opposite the junction of the third and fourth sacral vertebræ, and the other to the left, opposite the left sacrococcygeal articulation; they are, however, of little importance. The rectum is about 12 cm. long, and at its commencement its caliber is similar to that of the sigmoid colon, but near its termination it is dilated to form the rectal ampulla. The rectum has no sacculations comparable to those of the colon, but when the lower part of the rectum is contracted, its mucous membrane is thrown into a number of folds, which are longitudinal in direction and are effaced by the distension of the gut. Besides these there are certain permanent transverse folds, of a semilunar shape, known as Houston’s valves (Fig. 1078). They are usually three in number; sometimes a fourth is found, and occasionally only two are present. One is situated near the commencement of the rectum, on the right side; a second extends inward from the left side of the tube, opposite the middle of the sacrum; a third, the largest and most constant, projects backward from the forepart of the rectum, opposite the fundus of the urinary bladder. When a fourth is present, it is situated nearly 2.5 cm. above the anus on the left and posterior wall of the tube. These folds are about 12 mm. in width, and contain some of the circular fibers of the gut. In the empty state of the intestine they overlap each other, as Houston remarks, so effectually as to require considerable maneuvering to conduct a bougie or the finger along the canal. Their use seems to be, “to support the weight of fecal matter, and prevent its urging toward the anus, where its presence always excites a sensation demanding its discharge. 1
  The peritoneum is related to the upper two-thirds of the rectum, covering at first its front and sides, but lower down its front only; from the latter it is reflected on to the seminal vesicles in the male and the posterior vaginal wall in the female.


FIG. 1078– Coronal section of rectum and anal canal. (See enlarged image)

  The level at which the peritoneum leaves the anterior wall of the rectum to be reflected on to the viscus in front of it is of considerable importance from a surgical
Note 1.  Paterson (“The Form of the Rectum,” Journal of Anatomy and Physiology, vol. xliii) utilizes the third fold for the purpose of dividing the rectum into an upper and a lower portion; he considers the latter “to be just as much a duct as the narrower anal canal below,” and maintains that, under normal conditions, it does not contain feces except during the act of defecation. [back]

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