THE CENTRAL NERVOUS SYSTEM 465 



the cranial vault, the brain bulges into the opening. By screwing a tube 

 covered with a membrane into the trephine opening, we can find the pressure 

 necessary to force the brain back to its previous position. This is known 

 as the brain pressure, and is approximately equal, as might be expected, to the 

 cerebro-spinal pressure and to the pressure in the venous sinuses. It is 

 closely dependent on the latter. Forced expiratory efforts, such as may 

 occur in the convulsions of strychnine poisoning, may raise the pressure from 

 30 to 50 mm. Hg. In the vertical position in man, the pressure may be 

 slightly negative in consequence of the tendency of the venous blood to run 

 downwards towards the heart. 



REGULATION OF THE BLOOD SUPPLY TO THE BRAIN 



No satisfactory evidence has been brought forward of the existence of 

 vaso-motor nerves controlling the calibre of the cerebral blood vessels. Nor 

 indeed are such nerves necessary. The brain, as the master tissue of the 

 body, controls through the medullary centres the circulation through all 

 other parts of the body. It is therefore able to regulate the blood supply 

 through its arteries by allowing less or more blood to pass through other parts 

 of the body. For the exercise of its normal functions it requires a certain 

 blood supply, which again will depend simply on the pressure in the carotid 

 arteries and circle of Willis. If this pressure fails, the functions of the 

 brain are affected and loss of consciousness rapidly ensues. This is what 

 occurs when a person who is weak from long illness faints on suddenly getting 

 up from bed. In the normal individual the change in the circulation with 

 alteration of bodily position, which would be produced by the action of 

 gravity, is at once counteracted through the vaso-motor system. The 

 splanchnic area is contracted or dilated according to the necessities of the case, 

 but the pressure in the carotid and the circulation of the brain remains unal- 

 tered. Even when the heart in consequence of disease is scarcely able to carry 

 on the circulation, the arterial pressure undergoes little or no alteration. 

 Any other tissue of the body, even the heart itself, may suffer, but the brain 

 at all costs must receive its proper supply of blood. 



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