It is well known that stoppage of the blood supply to the brain or the production of severe anoxemia results in unconsciousness and, if prolonged, in irreparable injury to nerve cells. In view of this fact, it is an easy assumption that all states of unconsciousness are due, wholly or in part, to a defect in the cerebral circulation or in the nutrition of nerve tissues. Although a subject of lively debate, the matter has remained largely in the field of speculation because of lack of direct information concerning physiologic disturbances of the cerebral circulation in the human subject. States of consciousness are difficult of determination in animals, even if anesthesia is not used.
Two methods of direct approach to the study of cerebral circulation in unanesthetized persons are now available, viz., analysis of the oxygen content of the blood leaving the brain and observations of changes in blood flow