Boring a hole in a patient's skull -- without any anesthesia, keep in mind -- was once considered good medicine. Trepanation is one of the oldest known surgical procedures, dating back as long ago as the Mesolithic era -- 10,000 B.C.E., before humans used metal tools. That also places it long before the medical pioneers Hippocrates in ancient Greece and Galen in ancient Rome got their hands on this technique.
Archaic medicine and cures tended to lean more toward mystical and ritualistic than scientific, and the practice of trepanning began in this way. Archaeologists theorize the technique allowed a practitioner to release the evil spirit (demons were considered the root of mental illness) trapped within the patient. There's evidence it was also used to treat migraines and epileptic seizures, and over the years developed as a neurosurgical intervention for head injuries such as skull fractures and bone contusions.
While it sounds barbaric, there's evidence that many patients survived the procedure. Just be sure not to press too hard when treating skull fractures, warns Galen, or the "patient immediately lose all sensation and becomes motionless" [source: Missios]. While that might sound obvious in the 21st century, that observation was an important discovery in anatomy and the human brain.