PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA 261 



and there is never any local change in the latter which would indicate that this potent 

 drug had locally caused these vessels to constrict. 



5. The alterations in systemic pressure induced by the operation of the force of 

 gravity and coming into play when the position of the body is changed, if not per- 

 fectly compensated for by constriction of the splanchnic area, will cause correspond^ 

 ing changes in the intracranial tensions. Under the influence of gravity, for example, 

 the intracranial and the intracranial-venous pressures may fall below zero. 



The comparatively slight amount of extra room which can be provided in the 

 -cranial cavity by compression of the venules and capillaries has suggested to some 

 writers that a self -strangulation of bloodflow might occur when the pressure suddenly 

 rises in the basal and cerebral arteries. The increased pressure would be transmitted 

 un diminished through the incompressible brain substance to the thin-walled vessels and 

 compress them because of the lower pressure within. This is, however, impossible for 

 any curtailment in the bloodflow through the venules and capillaries could only be 

 transitory, since the compression will be overcome by the arrival of the pressure wave 

 through the blood stream itself. For it is obvious that the arterial pressure trans- 

 mitted directly must be greater than that pressure after it has overcome the tension 

 of the arterial wall and is transmitted to the venules through the brain substance. 

 Whenever this readjustment has occurred, the cerebral vessels become expanded to 

 the greatest extent possible and they become virtually rigid tubes comparable with 

 the rubber tube suspended in water in a closed flask, as in the schema referred to 

 above. 



The only variation in intracranial blood supply which can occur is 

 one affecting the velocity of flow or if you prefer the term the mass 

 movement of the blood; the volume can not change. After all, however, 

 that is what is necessary to meet the demands for more blood, and the 

 conceptions which have been formed by studies on expansible vascular 

 areas, such as the kidney and spleen, that increased blood supply runs 

 parallel with increased volume, do not apply. 



That the mass movement of the blood in the cranium increases when 

 the arterial pressure rises has been shown by direct experiment. Hill 

 and Nabarro found it increased from two to six times during the con- 

 vulsions produced by absinthe. 



Local Readjustments of Blood Supply in Different Parts of the Brain. 

 Limited though any change in caliber of the cerebral arteries can be, it 

 is nevertheless sufficient to make it possible that local variations in blood 

 supply might occur as a result of active constriction or dilatation of the 

 vessels. Just as the blood supply of a muscle or gland may be varied 

 independently of any change in general blood pressure, by local changes 

 in the caliber of its blood vessels, so might that of the brain be varied, 

 and this might occur to a limited extent for the supply as a whole, as 

 by constriction of the circle of Willis, or to a greater extent in one or 

 other of the arteries which spring from the circle. By the latter adjust- 

 ment a greater blood supply might be directed into an area which had 

 become especially active, the flow to other relatively quiescent areas 

 being meanwhile somewhat curtailed. 



