22$ HANDBOOK OF PHYSIOLOGY. 



Satisfactory data for these estimates are afforded by the results of 

 experiments to ascertain the rapidity with which poisons introduced into 

 the blood are transmitted from one part of the vascular system to an- 

 other. The time required for the passage of a solution of potassium 

 ferrocyanide, mixed with the blood, from one jugular vein (through the 

 right side of the heart, the pulmonary vessels, the left cavities of the 

 heart, and the general circulation) to the jugular vein of the opposite 

 side, varies from twenty to thirty seconds. The same substance was 

 transmitted from the jugular vein to the great saphena in twenty sec- 

 onds; from the jugular vein to the masseteric artery in between fifteen 

 and thirty seconds; to the facial artery, in one experiment, in Letw r een 

 ten and fifteen seconds; in another experiment in between twenty and 

 twenty-five seconds; in its transit from the jugular vein to the metatar- 

 sal artery, it occupied between twenty and thirty seconds, and in one 

 instance more than forty seconds. The result was nearly the same 

 whatever was the rate of the heart's action. 



In all these experiments, it is assumed that the substance injected 

 moves with the blood, and at the same rate, and does not move from 

 one part of the organs of circulation t;> another by diffusing itself 

 through the blood or tissues more quickly than the blood moves. The 

 assumption is sufficiently probable to be considered nearly certain, that 

 the times above mentioned, as occupied in the passage of the injected 

 substances, are those in which the portion of blood, into which each 

 was injected, was carried from one part to another of the vascular system. 



Another mode of estimating the general velocity of the circulating 

 blood, is by calculating it from the quantity of blood supposed to bo 

 contained in the body, and from the quantity which can pass through 

 the heart in each of its actions. But the conclusions arrived at by this 

 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 about twenty-five 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, i\\Q3rec- 

 tile organs, the hings, the liver, and the 'kidneys. 



In the Brain. For the due performance of its functions the hrain 

 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 



