244 THE CIRCULATION OF THE BLOOD 



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 



I^IG. 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 has recently (1909) 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 convincing evi- 

 dence 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 cervical sympathetic 

 calls forth vaso-dilatation in the brain even when the medulla is completely 

 destroyed. Weber, therefore, concludes that all these stimuli act reflexly. 

 through a special cerebral vascular center located at some as yet undeter- 

 mined point toward the cerebrum from the general medullary center. An 

 active cerebral vaso-dilatation may be accomplished through this center even 

 in the presence of an accompanying fall in general blood pressure. More 

 often the type of vascular reflex is that of dilatation followed by constriction 

 of the brain vessels. If there is an accompanying sharp rise in general 



