in Surgery, the perforating part of the instrument called a trepan. This instrument, which is mentioned by Galen, Fabricius ab Aquapendente, and others, was a conical saw with a circular edge. Modern practitioners, however, prefer the cylindrical form; and various contrivances have been recommended to obviate the danger that may arise from want of dexterity, or from rashness, in performing the operation of trepanning. The instrument most commonly employed in Britain is the trephine, in which the circular saw is moved backward and forward by the hand; but on the Continent the preference is usually given to the trepan, in which the circular saw is attached to a handle like that of a carpenter's brace; and this is preferred by Professor Syme, and some of our other eminent surgeons, to the trephine, as the latter renders the operation more tedious and laborious. See SURGERY.