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1. Role of emissary veins
Whole-body cooling takes place when arterial blood is cooled through the effects of evaporation of sweat from the body’s surface, a process that also contributes to regulation of brain temperature via its arterial supply. Michel Cabanac and Heiner Brinnel proposed an additional mechanism for selectively cooling the brain under conditions of intense exercise that results in hyperthermia. Because experimental evidence revealed that blood flows out of the cranium through the mastoid, ophthalmic and parietal emissary veins in hypothermic subjects but into the braincase in hyperthermic subjects, Cabanac and Brinnel reasoned that venous blood that is cooled at the head’s surface through the effects of evaporation on dilated veins is selectively delivered into the braincase under, and only under, conditions of hyperthermia (oral temperature of 37.6oC + 0.18o). The authors noted that innumerable, microscopic emissary veins exist in humans, and demonstrated (by massaging a cadaver’s skullcap) that blood is capable of flowing through this network from the outside of the skull to the diploic veins within the cranial bones and then to the inside of the braincase.
The three emissary veins that were used to record direction of blood flow are located at dispersed points of the network that supplies the entire skull: at the face (ophthalmic), behind the ear (mastoid), and at the top back part of the skull (parietal). (See Figure 1.) Cabanac and Brinnel concluded that when blood flows into the braincase in these three emissary veins, it also does so in the innumerable tiny veins that comprise the entire network. According to this hypothesis, venous blood cooled at the head’s surface under hyperthermic conditions flows into the braincase over a disperse network of tiny veins (the cranial radiator). This is a selective brain cooling mechanism that serves to keep brain temperature in check. Cabanac and Brinnel’s hypothesis became controversial among physiologists who claimed that existence of an anatomical network of cranial veins capable of delivering cooled blood into the braincase was speculative. This point will be returned to in Section III.