"Mr Scoresby, you know the Tartars make holes in people's heads?"

This is a process called trepanning, and it exists in our world as in Lyra's.

It involves drilling or scraping a small hole in the skull. Evidence of the process has been found from Neolithic times onwards. Cave paintings suggest it was believed that trepanation would cure epileptic fits, migraines and mental illnesses, although Lee tells Lyra that the Tartars believe it will allow them better communication with the gods, and that it is a great privilege to have it performed.