But it may be necessary, in order to elevate portions that are wedged under the sound part of the cranium, to take away a considerable portion of the latter. One or more circular pieces must be removed by the trephine, and it may, perhaps, be necessary to cut out the parts between these apertures by means of the straight-edged saw. The size of the crown of the trephine must be varied according to the object which is in view. The trepan is now disused, and the trephines best suited to the purpose are those fluted on the side of the crown, with the perforator made to slide and fix by means of a proper screw. The centre pin, or perforator, is fixed on a sound and firm part of the bone, and the edge of the crown made to project slightly over the fractured margin. A few turns will suffice to fix the instrument. The saw is then made to turn steadily and lightly, pressure being made when the instrument is moving from left to right, until a pretty deep sulcus is made. The centre pin is then withdrawn, the saw being sufficiently retained by its own groove. The centre pin can scarcely be used at all in children, the cranium being at that age soft and thin. I once had occasion to operate with an old-fashioned trepan, at a distance from town, on a child with abscess under the bone, occasioned by a punctured wound from the point of a spinning top. The centre pin was long, very sharp, and screwed in; and, if it had been used, would have perforated skull, dura mater, and nearly half an inch of the brain, before the saw could come in contact with the bone. I was obliged to use the crown of the trepan, without a centre pin.