Used for neurosurgical procedures which required drilling through the skull. For example aspiration of subdural blood clots after head injury, or the elevation of depressed skull fractures. The centre piece of the bit allow the drill to be centered and once the outer teeth had engaged it would be withdrawn so as to prevent further intracranial injury. The high quality brass and steel bit is in mint condition and is detachable. It had it own separate guard which is an unusual feature. The wing of the trepanne has age related loss of plating with early pitting. Signed Mathieu.