(3) descending. The sigmoid flexure, a device of nature that prevents excessive pressure by the contents of the bowel upon the muscles of the rectum and the anus, lies between the descending colon and the rectum, whose lower opening, the anus, is guarded by a strong circular muscle. The sigmoid flexure thus interrupts the straight fall from the transverse portion of the large intestine to the rectum and acts as a retaining pouch.
From this description it will be seen that there are three positions in which the colon may receive a supply of water sufficient to soften its contents and to wash them away from its walls. These are the right-side, the knee-chest, and the flat-on-the-back. The latter, except in bed-ridden cases and in children, is inconvenient to assume, but the two former postures are found to be comfortable and easily taken.
When the patient in taking the injection lies on the left side, gravity can assist the flow only as far as the transverse colon, which in this position is a perpendicular tube forbidding further passage of the fluid of the enema. Hence only one-third of the bowel is possible of flushing. The right-side posture permits the water to flow along the
