A trephine is a crown saw used for perforating the skull and removing a circle of tissue or bone. Trepanning is an ancient operation in which a hole is drilled, chiselled or cut into the skull. In the eighteenth century it was carried out, for example, to treat serious skull fractures. Trephination as a therapeutic or magico-religious procedure dates back to Neolithic times, with trephines (or 'trepans') often used as a method of releasing evil spirits from the patient. Over the centuries, this surgical procedure continued to be used to treat a variety of symptoms, from migraine to mental illness.