THE VASCULAR MECHANISM. 237 



control of the vaso-motor centre, although the existence of nerve 

 plexuses round them has been observed histologically. The presence 

 of cerebro-spinal fluid in the skull allows for a slight increase in the 

 volume of the blood in the cerebral blood-vessels, since a rise of 

 pressure within these vessels distends them to a certain extent, and 

 forces some of the cerebro-spinal fluid out of the skull into the sheaths 

 of the nerve trunks. Apart from this increase, which amounts only 

 to 2 or 3 c.c., the amount of blood in the cerebral vessels cannot 

 be increased, since the capacity of the skull is constant, and the 

 brain, which, together with the blood-vessels, practically fills it, is 

 incompressible, 



Any increase or decrease in the amount of blood supplying the 

 brain is brought about entirely by variations in the velocity with which 

 the blood flows through the cerebral vessels. It has already been 

 pointed out that if the width of the bed through which the blood is 

 flowing remains constant, the velocity of the flow will vary directly with 

 the pressure which tends to drive the blood along the vessels. In the 

 body this pressure is the general arterial pressure, and since the volume 

 of the cerebral vessels remains constant, the velocity of the flow of 

 blood through them will depend entirely on the arterial pressure, pro- 

 vided there is no obstruction to the escape of blood from the cerebral 

 veins. If the arterial pressure rises, the arteries become rather more 

 distended and occupy more space, and as the total volume of the 

 cerebral vessels remains unaltered, the expansion of the arterioles 

 must be accompanied by narrowing of the capillaries and veins. The 

 narrowing is not sufficient to cause any obstruction to the escape of 

 blood through these vessels, and the amount of blood flowing through 

 the brain is greatly increased. If the general arterial pressure falls, 

 the velocity of the blood flow through the brain diminishes, and the 

 amount of oxygen reaching it in a given time is correspondingly 

 decreased. When the supply of oxygen falls, the vaso-motor centre is 

 stimulated, causing general vaso-motor constriction, and thereby raising 

 the blood pressure to such a level that the flow of blood through the 

 brain is again sufficient to provide an adequate supply of oxygen. 

 Thus the blood supply to the brain is determined almost entirely by 

 conditions outside the brain itself, being increased whenever the 

 abdominal vessels are constricted and diminished when these dilate. 

 The brain is protected from lack of oxygen by the vaso-motor centre in 

 the manner just described. 



If the blood supply to the brain becomes inadequate, the respiratory 

 centre is also stimulated, and the increased respiratory movements 

 bring more blood to the heart, and so enable it to expel more blood at 



