In cases in which the sinus is directed away from the rectum, the proper course is not to divide the sphincters, but freely to enlarge the external orifice and to maintain free drainage. The treatment of INCOMPLETE INTERNAL RECTAL FISTULÆ invariably demands operative interference at the earliest possible moment after a diagnosis is made; for if left alone its tendency is to burrow. The operation for a blind internal fistula consists in making it a complete fistula and in dividing the intervening structures between the bowel and the sinus. This is best performed by introducing a probe-pointed director, bent at an acute angle, into the bowel, and passing the bent portion through the internal opening. This done, the point of the probe can be felt subcutaneously and cut down upon, and the remainder of the operation completed. In dealing with COMPLEX FISTULÆ the surgeon must be guided by the peculiarities of each case. In operating upon a horseshoe fistula it is essential to recognize the true condition of affairs; for a careless or an inexperienced observer might think that he had two separate fistulæ to deal with, and operate accordingly. Even were he to recognize that he was dealing with a horseshoe fistula, if he followed the usual plan he would slit up first one sinus and then the other, dividing the sphincter in two places obliquely through its fibers, thus endangering the patient's future power of controlling the movements of the bowel. (Fig. 38.) [Illustration: Fig. 38.—Diagram showing wrong method of operating in horseshoe fistula.] [Illustration: Fig. 39.—Diagram showing the method recommended in operating upon horseshoe fistula.] According to Messrs. Cooper and Edwards,[47] "If this fistula can be laid open in such a way as to entail only one division of the sphincter, and that at right angles to its fibers, there will be a minimum amount of risk of subsequent incontinence." The operation can be done in this way (Figs. 39, 41, 43). First pass a probe-pointed director through the internal aperture, and on its point incise the skin in the middle line behind; now push the director through, and slit up. Secondly, slit up the lateral sinuses on directors passed in at the external openings and brought out at the dorsal incision. These lateral sinuses may take a straight, a curved, or even a rectangular direction. Fistulæ taking these different courses are illustrated in Figs. 40 and 42. [Illustration: Fig. 40—A diagram of one variety of horseshoe fistula.] [Illustration: Fig. 41—Diagram of incisions necessary.]