Houston's valves (or transverse folds of rectum) are semi-lunar transverse folds of the rectal wall that protrude into the anal canal. Their use seems to be to support the weight of fecal matter, and prevent its urging toward the anus, which would produce a strong urge to defecate. Although the term rectum means straight, these transverse folds overlap each other during the empty state of the intestine to such an extent that, as Houston remarked, they require considerable maneuvering to conduct an instrument along the canal, as often occurs in sigmoidoscopy and colonoscopy. These folds are about 12 mm. in width and are composed of the circular muscle coat of the rectum. 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. [ http://en.wikipedia.org/wiki/Houston_valve ]