CEREBRAL CIRCULATION. 181 



as the tortuosity of the large arteries, and their wide anas- 

 tomoses in the formation of the circle of Willis, which will 

 insure that the supply of blood to the brain may be uni- 

 form, though it may by an accident be diminished, or 

 in some way changed, through one or more of the principal 

 arteries. The transit of the large arteries through bone, 

 -especially the carotid canal of the temporal bone, may 

 -prevent any undue distension; and uniformity of supply 

 is further insured by the arrangement of the vessels in 

 the pia mater, in which, previous to their distribution to 

 the substance of the brain, the large arteries break up and 

 divide into innumerable minute branches ending in capil- 

 laries, which, after frequent communications with one 

 another, enter the brain, and carry into nearly every part 

 t)i it uniform and equable streams of blood. 



The arrangement of the veins within the cranium is also 

 peculiar. The large venous trunks or sinuses are formed 

 .so as to be scarcely capable of change of size ; and com- 

 posed, as they are, of the tough tissue of the dura mater, 

 and, in some instances, bounded on one side by the bony 

 -cranium, they are not compressible by any force which the 

 fulness of the arteries might exercise through the substance 

 of the brain ; nor do they admit of distension when the 

 flow of venous blood from the brain is obstructed. 



The general uniformity in the supply of blood to the 

 brain, which is thus secured, is well adapted, not only to 

 its functions, but also to its condition as a mass of nearly 

 incompressible substance placed in a cavity with unyielding 

 walls. These conditions of the brain and skull have ap- 

 peared, indeed, to some, enough to justify the opinion 

 that the quantity of blood in the brain must be at all times 

 the same ; and that the quantity of blood received within 

 any given time through the arteries must be always, and 

 -at the same time, exactly equal to that removed by the 

 veins. In accordance with this supposition, the symptoms 

 commonly referred to either excess or deficiency of blood 



