CEREBRAL CIRCULATION. 151 



tribution of the vessels of the brain, and in the conditions 

 attending the amount of blood usually contained within the 

 cranium. 



The functions of the brain seem to require that it should 

 receive a large supply of blood. This is accomplished through 

 the number and size of its arteries, the two internal carotids, 

 and the two vertebrals. But it appears to be further necessary 

 that the force with which this blood is sent to the brain should 

 be less, or at least, subject to less variation from external cir- 

 cumstances than it is in other parts. This object is effected by 

 several provisions; such as the tortuosity of the large arteries, 

 and their wide anastomoses in the formation of the circle of Wil- 

 lis, which will insure that the supply of blood to the brain maybe 

 uniform, 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 capillaries, which, after frequent communications 

 with one another, enter the brain, and carry into nearly every 

 part of it uniform and equable streams of blood. 



The arrangement of the veins within the cranium is also pe- 

 culiar. The large venous trunks or sinuses are formed so as 

 to be scarcely capable of change of size; and composed, 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 appeared, 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 re- 

 moved by the veins. In accordance with this supposition, the 

 symptoms commonly referred to either excess or deficiency of 

 blood in the brain, were ascribed to a disturbance in the bal- 

 ance between the quantity of arterial and that of venous blood. 



