THE CEREBRUM 599 



ts activities, even to the point of unconsciousness. If this is not to occur 

 he maximum expansion of the arteries, and hence the brain, must be checked 

 ind controlled. This is accomplished in the following way: As the brain 

 i/pproaches that degree of expansion permitted by the displacement of the 

 erebrospinal fluid, it begins to exert a compression of the pial veins. This 

 .ompression by narrowing the lumen of the veins diminishes their capacity 

 ind hence increases the pressure of their contained blood until it is equiva- 

 ent to the pressure exerted by the brain against the veins. At this moment 

 he pressures in the arterioles, capillaries and veins approximate each other 

 n value. 



From these factors it will be seen that the circulation through the brain 

 Approximates a circulation through a system of rigid tubes. The result is an 

 ncrease in the velocity of the outflow and a diminution of the blood-pressure. 

 As an additional result the pulse-wave of the arterial system is transmitted 

 to the blood of the large veins and sinuses which therefore exhibit normally 

 pulsations synchronous with those of the arteries. The rise of the pressure 

 n the cerebral veins is regarded therefore as the factor which, by limiting 

 ibrain expansion, checks the rise of the intra-cranial pressure beyond physio- 

 llogic limits. With the diastole of the heart and the recoil of the artenes, 

 the former relation of the blood, brain, cerebrospinal fluid and cranial walls 

 is regained. Because of this change of relation with each heart-beat, 

 the brain pulsates synchronously with the arteries. 



The brain differs from other organs, also, in that normally its volume is 

 more influenced in a positive direction by the expiratory rise of venous pres- 

 sure than by the inspiratory rise of general arterial pressure, 

 of pressure in the thoracic veins which occurs with each expiratory act, 

 causes a damming back of the venous blood in the sinuses and pial veins, 

 resulting in a further increase in the volume of the brain and in the mtra- 

 cranial pressure. The reverse takes place in inspiration. 



It has been ascertained experimentally that the intra-cranial pressure 

 may vary considerably and consciousness still be preserved. H 

 to be 40 to 50 mm. of Hg. in the convulsions of strychnin poisoning and a 

 little less than zero in a patient standing erect. 



The Regulation of the Volume of Blood Entering the Brain. - 

 generally believed that the cerebral vessels are not F^ed wi^^^otor 

 5 T^ sttPmnt to Drove their existence either by physiologic or 



be depende't on changes affecting the arterial and venous pressures 



tt *JS?^ in increasing or decreasing the blood-supply 

 to therresfd'sTthe power of the ^7*""^^^^ 



