THE CEREBRUM. 561 



The Mechanism of the Intra-cranial Circulation. As previously 

 stated, by virtue of the physical relations existing between the blood, the 

 brain, the cerebrospinal fluid and rigid walls of the cranium, the flow of the 

 blood through the brain and cranial cavity, is attended by certain phenomena 

 which are peculiar to this region and present in no other situation. 



Taking as a point of departure the condition of the arteries during the 

 cardiac diastole, the relations of these structures are somewhat as follows: the 

 cerebrospinal fluid occupies all the available lymph space, but under a pres- 

 sure approximately equal to that in the large veins and hence not materially 

 above that of the atmosphere; the pressure in the arteries, capillaries and 

 veins presents the usual values in these different regions of the vascular 

 apparatus; the brain presents a volume which may be termed diastolic. 



With the occurrence of the succeeding cardiac systole, the cerebral 

 vessels, receiving an additional volume of blood, expand and occasion a 

 corresponding increase in the volume of the brain, which is accomplished by 

 a partial displacement of the cerebrospinal fluid into extra-cranial lymph 

 spaces. Because of the fact that the displacement of the cerebrospinal 

 fluid is insufficient to permit of the complete expansion of the brain, there is 

 developed in the intra-cranial lymph spaces a counter-pressure (the so-called 

 intra-cranial pressure) which would keep pace with and finally equalize the 

 rising pressure in the arteries. In consequence of this, the brain tissue, it is 

 believed, would be subjected to a pressure sufficiently great to interfere with 

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

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

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

 approaches that degree of expansion permitted by the displacement of the 

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

 compression by narrowing the lumen of the veins diminishes their capacity 

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

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

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

 in 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 

 increase 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 

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

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

 logic limits. With the diastole of the heart and the recoil of the arteries, 

 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. Thus the rise 

 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, 

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