Is a surgical operation for opening the skull in cases of fracture; a description of which does not form a part of the plan of this work, and we only introduce the subject, in order to describe the instrument by which it is performed, as the principle of its construction may be advantageously applied to other purposes, a represents a thin steel tube, the edge of which is serrated into fine sharp teeth, forming thereby an annular saw; it is fixed in a stout brass collar b, which is adjusted to the end of the axis c, and revolves therewith, when turned by the winch d. There are three screw supports, e e, to the upper and lower plates, which form the frame, and the distance of the plates from each other is adjustable by the screws e e. The end of the axis c is formed into a pointed drill, and extends a little beyond it.

The case which contains this instrument is provided with several sized annular saws, drills, and screws. The surgeons in using this instrument, (after removing a portion of the scalp,) cutout a circular piece of bone, the central pin or drill preventing it from slipping, and the perforation thus made by the drill serves afterwards for the insertion of a screw, by which the removal of the circular piece of bone is ensured. Access is thus gained under the arch of the skull for removing the splinters or raising the depressed parts, occasioned by the fracture. Circular saws of this description have already been applied for cutting out pillars and concentric cylinders from solid blocks of stone; and our mechanical readers will find out many other valuable uses for the application of a similar instrument.

Trepanning 647