CIRCULATION OF THE BLOOD. 167 



approximated to the truth. Still the most careful of the estimates thus 

 made accord very nearly with those already mentioned; and it may be 

 assumed that the blood may all pass through the heart in from twenty- 

 five to fifty seconds. 



Peculiarities of the Circulation in Different Parts. The most 

 remarkable peculiarities attending the circulation of blood through differ- 

 ent organs are observed in the cases of the brain, the erectile organs, the 

 lungs, the liver, and the kidney. 



1. In the Brain. For the due performance of its functions, the brain 

 requires a large supply of blood. This object is effected through the 

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

 vertebrals. It is further necessary that the force with which this blood is 

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

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

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

 AVillis, which thus insures that the supply of blood to the brain is 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 tem- 

 poral 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 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 somo 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 con- 

 dition 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 quan- 

 tity of blood in the brain must be at all times the same. 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, congestion of the cerebral vessels; 

 while in death by prussic acid, the quantity of blood in the cavity of the 



