EXTRACT XLIX.c. 



ON THE CIRCULATION OF THE BLOOD WITHIN 

 THE HEAD. 



THE circulation of the blood, apart from the circulation 

 of the lymph, within the skull, is a subject on which a 

 great deal might be said, but it may suffice merely to 

 draw attention to it by a few references. To begin with 

 the arterial, it may be remarked that, when once inside 

 the skull, the great arterial trunks break up freely, 

 anastomose largely, and altogether distribute themselves 

 so as to secure the most complete and guarded supply of 

 blood to every part and section of the cerebral textures, 

 the two sides uniting and commingling their supplies, 

 so as to take advantage to the full of the principle of 

 anastomosis, and to secure an equal distribution of the 

 vital fluid. The basilar vessels are secured from mechani- 

 cal pressure and impediment to circulation by the presence, 

 throughout every space and inter-space not occupied by 

 proper nervine or meningeal structure, of the ubiquitous 

 cerebro-spinal fluid, and superficially on the walls and 

 crown, by the hollowing of the inner plate of the cranial 

 bones for each artery, so that, when everything is normal, 

 not even arterial pulsation is felt by the sensorium, and 

 the vessels are at liberty to expand and contract without 

 interfering with the freedom of cerebral material move- 

 ment or function. This is explained and secured by the 

 existence, throughout the whole arterial vasculature of the 

 brain, of a peri-vascular spaceage, identical with it in 

 distribution, in which circulates the cerebro-spinal fluid,, 

 accommodating itself to every pulsation and to every 



