THE CIRCULATION OF THE BLOOD. "229 



is sent to the brain should be less, or at least should be subject to less 

 variation from external circumstances than it is in other parts, and so 

 the large arteries are very tortuous and anastomose freely in the circle 

 of Willis, which thus insures that the supply of blood to the brain is 

 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 tem- 

 poral bone, may prevent any undue distention; and uniformity of sup- 

 ply 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 communication 

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

 uniform and equable streams of blood. The arteries are also enveloped 

 in a special lymphatic sheath. 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 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 distention 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^ cav- 

 ity with unyielding walls. These conditions of the brain and skull for- 

 merly appeared, indeed, enough to justify the opinion that the quantity 

 of blood in the brain must be at all times the same. But it was found 

 that in animals bled to death, without any aperture being made in the 

 cranium, the brain became pale and anaemic like other parts. And in 

 death from strangling or drowning, there was congestion of the cerebral 

 vessels; while in death by prussic acid, the quantity of blood in the 

 cavity of the cranium was determined by the position in which the ani- 

 mal was placed after death, the cerebral vessels being congested when 

 the animal was suspended with its head downward, and comparatively 

 empty when the animal was kept suspended by the ears. Thus, it was 

 concluded, although the total volume of the contents of the cranium is 

 probably nearly always the same, yet the quantity of blood in it is liable 

 to variation, its increase or diminution being accompanied by a simul- 

 taneous diminution or increase in the quantity of the cerebro-spinal 

 fluid, which, by readily admitting of being removed from one part of 

 the brain and spinal cord to another, and of being rapidly absorbed, and 



