The site of trephining may be fixed by the introduction of a bradawl through the scalp, so indenting the external table that the spot can be verified on turning down the scalp-flap. The pin of the trephine is there applied, and the trephine directed at right angles to the surface of the skull, or at that angle which is suited to the region attacked. The pin is discarded when a sufficient groove has been attained. The trephine must be kept under perfect control, muscular effort only being required during the act of supination. [Illustration: FIG. 11. HORSLEY’S DISK OR BONE ELEVATOR.] [Illustration: FIG. 12. HORSLEY’S DURAL SEPARATOR.] [Illustration: FIG. 13. KEEN-HOFFMANN’S CRANIECTOMY FORCEPS.] The firm nature of the external table will be readily appreciated; so soon as this resistance is overcome the trephine will be felt to be cutting its way through a softer structure, whilst the increased bone-dust and venous oozing will make it evident that the instrument is biting its way through the diploic tissue. Greater resistance is again encountered on meeting the internal table, warning the operator that the time has come when greater care must be exercised. The bone-dust must be wiped away, the teeth of the trephine cleansed with nail-brush or sponge, and the depth of the groove estimated. The groove should be of the same depth throughout its circumference, or of that depth which is suited to the part involved. The readiness of the disk for removal is best demonstrated by digital pressure. When it gives to that pressure, whether in whole or in part, the elevator may be applied and the fragment removed. In the temporal and cerebellar regions special care is required by reason of the absence of diploic tissue. It is impossible to lay too much stress on the necessity of avoiding injury to underlying dura and meningeal vessels. Injury to these structures not infrequently upsets the whole plan of campaign. [Illustration: FIG. 14. LANE’S FULCRUM CRANIECTOMY FORCEPS.] [Illustration: FIG. 15. HORSLEY’S GOUGE FORCEPS.] [Illustration: FIG. 16. DE VILBISS’S CRANIECTOMY FORCEPS.] _The enlargement of the trephine hole to the required size and shape._ Previous to any attempt at enlargement of the trephine hole, the dura mater must be separated from the bone with the aid of Horsley’s dural separator or other suitable instrument. Of the many patterns of craniectomy forceps, the following will be found to suit all requirements:-- _Keen-Hoffmann._ The visceral blade is introduced between the dura and the bone and ‘morcellement’ carried out. It is essential that the surgeon should not be too greedy, resting content with the removal of small fragments at a time. _Lane’s fulcrum forceps._ They possess great mechanical advantage and are mainly suited for the rapid removal of large portions of bone.