6i2 TEXT-BOOK OF PHYSIOLOGY 



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, 

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

 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. Hill found it 

 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. It is 

 generally believed that the cerebral vessels are not provided with vaso-motor 

 nerves. Every attempt to prove their existence either by physiologic or 

 histologic methods has thus far failed of convincing proof. In the absence 

 of vaso-motor nerves, the regulation of the circulation in the brain must neces- 

 sarily be dependent on changes affecting the arterial and venous pressures in 

 other regions of the body. 



The most effective factor in increasing or decreasing the blood-supply 

 to the brain resides in the power of the vaso-motor center to cause a contrac- 

 tion or dilatation of the cutaneous and splanchnic vessels. Thus if the 

 vaso-motor center declines in its tonus from any cause whatever, there is a 

 relaxation of the blood-vessels in one or both of these regions, an increase in 

 the volume of the blood flowing into them, and in consequence, a decrease 

 in the volume of the blood flowing through the brain. If on the contrary the 

 vaso-motor center is increased in its tonus, the reverse conditions prevail in 



