Rene Descartes’ skull.
The French philosopher died in Sweden in 1650 and was interred there for 16 years. When his body was exhumed for return to France, the ambassador appropriated his right index finger (“the instrument of immortal writing”) and, apparently inspired, one of the Swedish guards took the skull, engraving on it “Descartes’ skull, taken and carefully kept by Israel Planstrom when the body was sent to France and hidden since that time.” 
The skull bounced around Europe for 150 years, with various owners carving their names on it; it was discovered missing only when the coffin was opened again in 1819. A Swedish chemist, no doubt rolling his eyes, tracked it down and returned it to the French academy.
via

Rene Descartes’ skull.

The French philosopher died in Sweden in 1650 and was interred there for 16 years. When his body was exhumed for return to France, the ambassador appropriated his right index finger (“the instrument of immortal writing”) and, apparently inspired, one of the Swedish guards took the skull, engraving on it “Descartes’ skull, taken and carefully kept by Israel Planstrom when the body was sent to France and hidden since that time.”

The skull bounced around Europe for 150 years, with various owners carving their names on it; it was discovered missing only when the coffin was opened again in 1819. A Swedish chemist, no doubt rolling his eyes, tracked it down and returned it to the French academy.

via