252 THE CIRCULATION OF THE BLOOD 



we must conclude that changes in blood supply depend on changes in 

 the velocity of the bloodflow, and that such alterations in velocity are 

 dependent upon changes occurring in the aortic and more especially 

 in the vena-cava pressure. When the aortic pressure rises, more blood 

 will flow into the cerebral arteries and move along them at an increased 

 velocity, the increased pressure probably causing a moderate degree 

 of passive dilatation, to allow extra room for which the numerous 

 small cerebral veins become compressed. This compression of the veins 

 probably does not obstruct the greater flow of blood through them, be- 

 cause, taken as a whole, they are ordinarily much more capacious than 

 need be. On the other hand, if the aortic pressure should remain con- 

 stant, but that in the vena caya increase, then there Avould be obstruc- 

 tion to the passage of blood in the intracranial arteries, and conse- 

 quently a diminished velocity of flow. 



Vasomotor Nerves 



It might be inferred that, since the bloodflow through the cerebral 

 vessels is mainly dependent on vascular conditions elsewhere in the 

 body, there would be no need, as in the vessels of other vascular areas, 

 for vasomotor fibers. Histologists have, however, discovered the pres- 

 ence of such fibers, and it has become necessary for the physiologist to 

 find out if they are really of importance in connection with the regula- 

 tion of the blood supply to the brain. Even if it is admitted that the 

 arterioles could not contract or expand as. a whole without producing 

 local changes in venous pressure or cranial volume, it is yet of course 

 always possible, as has already been pointed out, that one set of arte- 

 rioles might contract at the same moment that another set expanded. 



That the vessels can undergo a process of constriction has been shown 

 by experiments in which the volume of outflow from the vessels of 

 the brain was measured in perfused preparations of brain. When 

 epinephrine was added to the perfusion fluid, curtailment of outflow 

 was observed to occur (Wiggers). Since this drug causes constriction of 

 vessels only when these are supplied with constrictor fibers (see page 

 736), the conclusion may be drawn that the cerebral blood vessels do 

 contain such nerve fibers. Nevertheless, the local vasomotor control of 

 the cerebral blood vessels can not have the significance in connection 

 with changes in blood supply that it has for other vascular areas (Hill 

 and Macleod 20 ). No doubt nerve fibers are present in .the cerebral 

 blood vessels, and presumably under certain conditions they are capable 

 of causing the blood vessels to undergo alterations in caliber, but it is 

 impossible to see of ivhat real value this can be under normal conditions. 



