The Collaborative International Dictionary of English v.0.48:

Costotome \Cos"to*tome\ (k?s"t?-t?m), n. [Costa + Gr. ???? to
   cut.]
   An instrument (chisel or shears) to cut the ribs and open the
   thoracic cavity, in post-mortem examinations and dissections.
   --Knight.
   [1913 Webster]