Trepanning
2 of the 6 encyclopedias on this shelf carry an entry for Trepanning. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
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 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. TREPOMONADIDÆ, in zoology, a family of pantostomatous Flagellata, with a single genus, Trepomonas. Animalcules naked, free-swimming, asymmetrical; two flagella separately inserted; no distinct oral aperture.
Aiton's Encyclopedia (1910)
a well known surgical operation. It consists in the removal of a portion of the skull to relieve the brain from pressure. The operation is performed usually with an instrument known as the trepan, crown saw, or trephine. The crown saw consists essentially of a cylinder fastened on a central shaft or pin. The end of the cylinder which is to perform the work is provided with sharp, saw-like teeth. The central pin holds the instrument in position. When twirled between the fingers or driven by power it cuts out a circular piece of bone neatly, making an orifice varying according to the instrument from the size of a dime to that of a silver dollar. If performed with care the wound heals again and no evil results follow. The term is to be distinguished from trapan, sometimes, but less properly, spelled trepan, to take in a trap or snare.
What Has Changed in the Last Century
The purpose Collier's gave in 1921 — relieving pressure on the brain from a depressed fracture or a blood clot between skull and dura — is still exactly why surgeons open a skull today, and emergency burr holes for an extradural haemorrhage remain a life-saving procedure. What has changed is everything around it. Since CT scanning arrived in the early 1970s, surgeons no longer guess at the site of a clot from the patient's symptoms; the bleed is located on an image first. The hand-twirled crown saw described by Aiton's in 1910 has given way to a power drill and a craniotome that cuts between burr holes, so a whole bone flap can be lifted and then wired or plated back into place rather than a coin-sized disc removed for good.
Written for this edition, 2026.