Trepanning
the operation of cutting a circular opening into the skull by means of a surgical instrument called a trepan or trephine. This consists of a handle, to which is fixed a small hollow steel cylinder, of about 1/2 to 1 inch in diameter, having teeth cut on its lower TREPANNING edge so as to form a circular saw. The operation of trepanning is resorted to for the purpose of relieving the brain from pressure; such pressure may be caused by the depression of a portion of the cranium, or it may be produced by an extravasation of blood, or by the lodgment of matter betwixt the skull and the dura mater, occasioned by a blow upon the head, or the inflammation of the membranes of the brain.