For the performance of the operation trepans were formerly used, which were replaced by drills which are still used by Buck, Jacobi, Lucae and others, but by most operators they have been set aside, owing to their uncertain and dangerous advance in the deep parts, and on account of their soiling the wound with splinters. The most rational and safe method is by means of the chisel, as recommended by Schwartz, and is performed as follows: The patient being anæsthetized, a perpendicular incision beginning a little above the linea temporalis, extending an inch and a half in length immediately behind the attachment of the auricle. Formerly I employed a straight incision, but recently have followed the suggestion of Politzer, and from the superior end of the perpendicular incision a second one is made backward at right angles, thus forming a flap, which I have found to simplify the operation very much, as it affords a better view of the locality and extent of any pathological changes which may have taken place, and gives more room for operative procedures, and the periosteum can readily be removed to any desired extent. The linea temporalis and the more or less strongly developed protuberance on the posterior superior orifice of the osseous meatus, so strongly urged by authors, are very nice guides theoretically or to point out on an exceptional skull in the class room, but practically are seldom well enough developed to be of any use to the operator. The best guide to go by is to take the superior wall of the meatus as the upper boundary, and the angle formed by the plane of the mastoid with the posterior wall of the external meatus for the anterior boundary when opening the mastoid. This is best determined by pressing the finger into the meatus. Often in children, and when the bone is diseased in adults, the cortical plate of bone can be removed with the hand chisel, and we come at once upon the pus cavity, or diplœ, or cholesteatomatous epidermic masses, or a sequestrum of dead bone, or bleeding granulation tissue, or whatever the case may present. Sometimes the external plate is very thick and we have to chisel our way carefully for almost half an inch before reaching the diplœ, or may find the entire mastoid process sclerossed. No absolute rule can be given as to the depth it is safe to penetrate. Schwartz says "never to go deeper than 25 mm." Buck says "it is better to place the extreme limit at 20 mm," about three-fourths of an inch.