The principles which guide the position and size of the preliminary incisions which enable the urethra to be opened are very simple:--(1.) The wound in the perineum should be large enough to give free access to the urethra, and easy egress to the stone; (2.) It should be conical, with its base outwards, so as to favour escape of urine and prevent infiltration; (3.) It should not wound any important organ or vessel; that is, it must avoid the rectum, the corpus spongiosum, especially the bulb, if possible, the artery of the bulb, and in every case should leave the pudic artery intact. So far for broad general principles, which must guide all methods of successful lithotomy. THE LATERAL OPERATION.--_Operation of Cheselden._--(1.) _Instruments required._--A staff with a broad substantial handle, and a longer curve than the ordinary catheter requires, furnished with a very deep and wide groove, which occupies the space midway between its convexity and its left side. The one used should invariably be large enough to dilate fully the urethra. A knife, with its blade three or four inches in length, but sharp only for an inch and a half from its point, its back straight up to within a sixth of an inch of its point, and there deflected at an angle to the point, which again curves to the edge. The angle from the back to the point permits the knife to run more freely along the groove in the staff. A probe-pointed straight knife with a narrow blade may occasionally be useful in enlarging the incision in the prostate, when this is required by the size of the stone. Forceps of various sizes and shapes, some with the blades curved at an angle to reach stones lying behind an enlarged prostate, all with broad blades as thin as is consistent with perfect inflexibility, the blades hollowed and roughened in the inside, but without the projecting teeth sometimes recommended, which are dangerous from being apt to break the stone. A scoop to remove fragments or small stones, sometimes useful with the aid of the forefinger in lifting out a large one. A flexible tube of at least half an inch calibre, and about six inches long, rounded off and fenestrated above, fitted at its outer end with a ring and two eyelet-holes for the tapes, with which it is tied into the bladder. Prior to the operation the patient's health should be attended to, the stomach and bowels regulated, and any disorder of the kidneys or bladder as far as possible alleviated. If his health has been good and habits active, three or four days' confinement to his room on low diet, with a full purge the evening before the operation, is all the preparatory treatment that is necessary.