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HANDBOOK OF PHYSIOLOGY. 



method are less satisfactory. For the total quantity of blood, and the 

 capacity of the cavities of the heart, have as yet been only approximately 

 ascertained. 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. 



Local Peculiarities of the Circulation. 



The most remarkable peculiarities attending the circulation of blood 

 through different organs are observed in the cases of the brain, the erec- 

 tile organs, the lungs, the liver, and the kidneys. 



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 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 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 communication with one an- 

 other, enter the brain, and carry into nearly every part of it uniform 

 and equable streams of blood. The arteries are also enveloped in a spe- 

 cial 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 cavity 

 with unyielding walls. These conditions of the brain and skull formerly 

 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 inthe cranium, 



