Thus, when a fistula is associated with a stricture of the rectum of a malignant nature, any operative interference on the former lesion will be out of the question. If it is a simple stricture and its existence be not recognized, or if it be left untreated, any operation performed on the fistula will fail to effect a cure. TREATMENT BY INCISION.—In a large majority of cases of fistula in ano, the operation which is sanctioned by experience as the most prompt and certain at the same time that it is the safest in its results is to lay open the sinus into the rectum, dividing with the knife all the tissues intervening between its cavity and that of the bowel. Figs. 28, 29, 30 and 31 represent useful forms of knives for incising a fistula. The preparation of the patient consists in having the bowels moved by means of castor oil or some other mild cathartic on the day preceding the operation, and on the morning of the operation the lower bowel should be evacuated by means of an enema. [Illustration: Fig. 28—Blunt-pointed Knife.] [Illustration: Fig. 29—Curved Knife, useful in certain fistulous cases.] [Illustration: Fig. 30—Gowlland's Bistouries.] [Illustration: Fig. 31—Kelsey's Fistula-Knife.] After etherization the patient should be placed on the side on which the fistula exists, the buttock being brought to the edge of the operating-table. Occasionally the lithotomy posture is preferable, as in cases in which there is a complex fistula. The first step in the operation is to dilate the sphincter muscles, which is to be done in a slow but steady manner by introducing the thumbs into the rectum, back to back, and making gradual pressure around the anal orifice until muscular contraction is overcome. [Illustration: Fig. 32—Probe-pointed Director.] [Illustration: Fig. 33—Kelsey's Fistula-Director.] In dealing with COMPLETE FISTULÆ a flexible probe-pointed director (Figs. 32, 33) is passed through the sinus, and is then brought out of the anus by means of the forefinger of the left hand introduced into the bowel. The tissues lying upon the director are then to be divided with a sharp bistoury. A careful search is now to be made for any diverticula, which if found should be divided. If none exist, the granulations lining the track should be scraped away with a Volkmann's spoon (Fig. 34). The healing process will be facilitated by removing with scissors all overtopping edges of skin and mucous membrane. [Illustration: Fig. 34—Volkmann's Spoon.]