While a method of injecting therapeutic solutions into the internal carotid artery so that these substances might reach the brain directly was being worked on, the technic broadened out into a new method of studying the metabolism of the brain. It seemed theoretically correct that if one could study the blood directly before it reached the brain, and then could study it directly as it came from the brain, without admixture with venous blood from other parts of the body, something might be learned of what takes place within the brain. In order to accomplish such a study, it is desirable to obtain blood from the internal carotid artery (although, theoretically, any arterial blood would be of the same constitution), but it would be necessary to get blood from the internal jugular vein before it receives branches from the face and the neck. We found it possible to do this