The nature of the operation varies according to the nature of the case for which it is performed. Thus (1.) it may be performed through the uninjured cranium in the hope of evacuating an abscess of the diploe or dura mater, or of relieving pressure caused by suppuration in the brain itself, or by extravasation into the brain or membranes; or (2.) it may be required in cases of punctured and depressed fracture for the purpose of removing projecting corners of bone and allowing elevation of the depressed portions; or (3.) it is sometimes used to remove a circular portion of bone in cases of epilepsy in which pain or tenderness is felt at some limited portion of the cranium. 1. _In cases where the cranium and its coverings are entire._--There are certain positions where, if it is possible, the trephine should _not_ be applied. These are the longitudinal sinus, the anterior inferior angle of the parietal bone, where the middle meningeal artery is in the way, the occipital protuberance, and the various sutures. These being avoided, a crucial incision is to be made through the skin, and its flaps reflected. The pericranium should then be raised from the centre, for a space large enough to hold the crown of the trephine. The pericranium should never be removed, but carefully raised and preserved, as its presence will greatly aid in the restoration of bone.[80] The centre pin should then be projected for about the eighth of an inch and bored into the bone. On it as a centre the saw is then worked by semicircular sweeps in both directions alternately, till it forms a groove for itself. Whenever this groove is deep enough the pin should be retracted, lest from its projection it pierce the dura mater before the tables of the skull are cut through. Were the cranium always of the same thickness, and even of similar consistence, the operation would always be exceedingly easy; but in both these particulars different skulls vary much from each other, and thus by a rash use of the instrument the dura mater may possibly be injured. The tough outer table is more difficult to cut than the softer and more vascular diploe, and the inner table is denser than either, but more brittle. In many old skulls, however, the diploe is wanting altogether, and the two tables are amalgamated, and often very thin.