THE CENTRAL NERVOUS SYSTEM 46 5 



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 altera- 

 tion 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 pres- 

 sure in the carotid and the circulation of the brain remains unaltered. Even 

 when the heart in consequence of disease is scarcely able to carry on the circu- 

 lation, 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. 



