The causes of increased intracranial pressure in the order of frequency of occurrence are: (1) skull fracture; (2) brain tumor; (3) meningitis; (4) cerebral hemorrhage and thrombosis; (5) subarachnoid hemorrhage; (6) hydrocephalus due to inflammation around ventricular foramina; (7) hypertension associated with headache; (8) acute cerebral edema. It was evident that a clearly formulated plan of treatment had never been devised to cope with the main general result of these varied morbid conditions. It was further evident that the problem could be solved only by the proper application of exact physiologic knowledge regarding intracranial pressure changes brought about by protean factors such as sleep-waking, changes in posture, respiratory variations, and the exact effects of many different drugs. Accordingly, through 1927, extensive experiments were carried out on patients having bone defects in the skull, beneath which defects the dura had been cut and left unsutured. Pressure conditions inside the skull were