Blood flow in the emissary veins of the human head during hyperthermia.

The direction of the blood flowing in the emissary veins (vena emissaria mastoidea and v. e. partietalis) was recorded in human subjects during moderate hyperthermia and hypothermia. During hyperthermia the blood flowed rapidly from skin to brain. During hypothermia either no flow could be detected or the blood flowed slowly from brain to skin. On two fresh cadavers the calvaria was removed with the scalp adhering. Gentle massaging of the scalp produced abundant drops of blood on the inner surface of the bone each time the scalp was massaged, thus showing that cutaneous blood can flow inward through the bone. These results support the hypothesis of selective brain cooling in hyperthermic humans by offering a possible mechanism.