"Cabanac and his colleague Heiner Brinnel (of the Hopital-Maternite
L'Arbresle, France) wanted to know how the human brain keeps cool when
everything else heats up, such as occurs during intense physical
exercise. Accordingly, they studied the relationship between different
temperature states and the direction of blood flow in cranial veins
(which lack valves) by carrying out intriguing physiological experiments
on six healthy adult men." Dean Falk, Braindance (New York: Henry Holt,
1992), p. 156
"IN the minutes following cold exposure or exercise, the direction of
blood flow was recorded for the parietal and mastoid emissary veins.
This was done by placing an instrument called a Dopler probe directly on
the scalp at the point where the vein passes through the cranium. (Four
of the men were conveniently bald.) The results were startling. In
cold subjects, blood flow was slow and flowed OUT from the cranium,
i.e,, from the brain to the skin. In overheated subjects, however,
blood rapidly drained from the skin INTO the braincase. Further, these
findings were identical to those determined earlier for a third emissary
vein, the opththalmic vein of the face. Thus, when subjects became
overheated, blood flow REVERSED from the normal direction in all three
of these emissary veins. Consequently, blood that had been cooled by
evaporation from the sweating face and scalp was delivered into the
braincase." Dean Falk, Braindance (New York: Henry Holt, 1992), p. 157
(emphasis mine)
The article she cites is M. Cabanac and H. Brinnel, "Blood flow in the
emissary veins of the human head during hyperthermia,' European Journal of
Applied Physicology 54(1985):172-176
A couple of years ago in e-mail correspondence with Falk, she referred me
to an article Wofgang Zenker and Stefan Kubick, Anat Embryol
(1996):193:1-13. They also accept Cabanac and Brinnel's resutls as well as
that of:
Caputa et al, "Ecoulement sanguin reversible dans la veine ophthalmique:
mecanisme de refroidissement selectif du cerveau humain. C. R. Acad. Sci
87D:1011-1013,
Deklunder et al, "Influence of ventilation of the face on thermoregulation
in man during hyper-and hypothermia. Eur. J. Appl. Physiol 62:342-348
and
Hirashita et al, "Blood Flow through the ophthalmic veins during exercise
in humans" Eur. J. Appl. Physiol 64:( 1992):92-97
Zenker and Kubik write:
"Using Doppler flow probes, it could be shown in humans that such movements
of venous blood, obviously serving heat exchange, do indeed take place at
great speed in the veins of emissaria mastoidea and parietalia as well as
in ophthalmic and angularis oculi veins in states of hyperthermia.
Comparable movements of blood may also occur in veins connecting submucosal
veins of oral, nasal and paranasal cavities via plexus maxillaris with the
venous elements of the dura mater at the base of the skull.
"These changes of direction and exxtent of blood flow are associated with
physiological reactions to various influences such as hyperthermia, and
they are probably controlled by the nervous system."Wofgang Zenker and
Stefan Kubick, Anat Embryol (1996):193:1-13, p. 8
They conclude:
"In this study we present a detailed analysis of vascular arrangements
allowing a bulky transfer of venous blood from the skin o fthe head and
from nasal and paranasal mucous membranes to the dura mater, providing an
excellent anatomical basis for convection in cooling caused by evaporation
of sweat or mucus." Wofgang Zenker and Stefan Kubick, Anat Embryol
(1996):193:1-13, p. 11