VASCULAR REGULATION IN THE BRAIN 255 



another, and capable of being rapidly absorbed and as readily effused, would 

 serve as a kind of supplemental fluid to the other contents of the cranium 

 to keep it uniformly filled. Although the arrangement of the blood vessels 

 insures to the brain an amount of blood which is tolerably uniform, yet with 

 every beat of the heart, and every act of respiration, and under many other 

 circumstances, the quantity of blood in the cavity of the cranium is con- 

 stantly varying. Roy and Sherrington are responsible for the view now 

 generally current that the brain, therefore, is largely if not entirely dependent 

 upon the general blood-pressure for variations in the quantity of blood which 

 it receives. During a high blood-pressure the amount of blood that flows 

 in a given unit of time is greater, and during low blood-pressure less. Howell 

 has shown that in the decapitated dog's brain the flow of blood is directly 

 proportional to the difference in pressure. 



Numerous attempts have been made to show vaso-motor mechanisms 

 for the cerebral arteries, but with generally unconvincing success. Huber 

 and others have shown nerve endings in such arteries by histological 

 methods. Bayless, Hill, and Gulland make the statement that "no- 

 evidence has been found of the existence of cerebral vaso-motor nerves, 

 either by means of stimulation of the vaso-motor center or central end of 

 the spinal cord, after division of the cord in the upper dorsal region, or 



FIG. 209. Vaso-dilatation in the Brain from Stimulation of the Cerebral Cortex 

 in the Presence of Complete Destruction of the Medulla in the Cat. The upper trace 

 is of the carotid pressure; the lower trace is of the brain oncometer. (Weber.) 



by stimulation of the stellate ganglion, and that is to say the whole 

 sympathetic supply to the carotid and vertebral arteries." However, 

 Ernest Weber (1908) reinvestigated the control of the blood flow in the 

 brain. He admits that the blood flow in the brain is sharply dependent 

 on the general blood-pressure, but he presents plausible evidence that 

 both vaso-constrictors and vaso-dilators exist for the brain vessels. The 

 most striking facts are obtained upon stimulating general sensory nerves, 

 the central end of the sectioned cord, the cerebral cortex, and the cervical 

 sympathetic. The stimulation of the cerebral cortex calls forth vaso- 

 dilatation in the brain even when the medulla is completely destroyed. 

 Weber, therefore, concludes that these stimuli act reflexly through a 

 cerebral vascular center located at some as yet undetermined point in 

 the brain stem above the general medullary center. An active cerebral 

 vaso-dilatation may be accomplished through this center even in the 



