A circular hole made in the SKULL using a special surgical drill with a rounded tip, called the burr. The operation is done to relieve pressure on the BRAIN. This pressure – raised intracranial tension – is commonly the result of blood collecting between the skull and the brain after a head injury. The presence of PUS or an increase in the amount of CEREBROSPINAL FLUID as a result of infection or tumours in the brain can also cause a potentially fatal rise in intracranial pressure which can be relieved by drilling a burr hole. A neurosurgeon may make several burr holes when doing a CRANIOTOMY, a procedure in which a section of the skull is removed to provide access to the brain and surrounding tissues. Archaeological evidence suggests that modern man’s ancestors used burr holes probably to treat physical ailments and mental illness.