The subject has been placed in a prone position. The lateral ventricles are completely outlined by air. Some air is also contained within the third ventricle (3). The fourth ventricle (2) and the cerebral aqueduct are less distinct and are in part obscured by air in the frontal sinuses (11). Small collections of air (not drawn) can be seen within the subarachnoid spaces in many sulci of the brain. (These films have been reproduced through the courtesy of Dr. Howard L. Jones of the Palo Alto Hospital.)