THE CIRCULATION 175 



consists in two points : (1) the cerebral arteries alone of 

 all systemic arteries in the body are not under the direct 

 control of the vasomotor centre ; (2) the circulation 

 takes place in a rigid, enclosed space. Seeing that the 

 cerebral arteries are not endowed with vasomotor nerves, 

 they do not participate in any general constriction of the 

 arteries brought about by the vasomotor centre. On 

 the contrary, as the blood-pressure throughout the body 

 tends to rise as the result of such constriction the blood, 

 choosing the path of least resistance, passes through 

 the intracranial cavity in increased amount. 



General vasoconstriction, therefore, increases the 

 amount of arterial blood which the brain receives, and 

 cerebral hypersemia is a necessary concomitant of high 

 blood-pressure, and may explain some of the symptoms 

 of the latter. 



Seeing, again, that the skull forms a rigid box, 

 and that its contents are incompressible, this increased 

 amount of arterial blood can only be accommodated by 

 the squeezing out of a certain amount of cerebro-spinal 

 fluid or of venous blood. 



If a foreign body such, for example, as a blood-clot 

 be introduced into the intracranial cavity, room can 

 only be found for it at the expense of a certain amount 

 of blood i.e., the brain becomes to a greater or lesser 

 degree anaemic. If the anaemia increases to such a 

 degree as to affect the arterial flow through the medulla, 

 the vasomotor centre experiences its effects, and responds 

 by constricting the arterioles throughout the body, with 

 the result that the general blood-pressure rises and 

 more arterial blood is squeezed through the brain to make 



