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The Collaborative International Dictionary
Costotome

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.

Wiktionary
costotome

n. An instrument to cut the ribs and open the thoracic cavity in post mortems.