While a skull may be safely perforated, or an opening in it enlarged with a chisel or gouge and mallet, many surgeons prefer to employ a trephine that was specifically designed for this purpose.  The bone is effectively cut by rotation of the trephine combined with downwards pressure.  Most forms incorporate a center pin, which, like the point of an auger, serves to center the instrument and ensure appropriate tracking. Trephines were useful for cutting away badly-shattered parts surrounding a seat of fracture, enlarging a perforation, for providing an opening in the skull to assist in making a diagnosis, to relieve pressure, or permit some further operation.