n. - A crown-saw or cylindrical saw for perforating the skull,
turned, when used, like a bit or gimlet. See Trephine.
n. - A kind of broad chisel for sinking shafts.
v. t. & i. - To perforate (the skull) with a trepan, so as to
remove a portion of the bone, and thus relieve the brain from pressure
or irritation; to perform an operation with the trepan.