166 HAND-BOOK OF PHYSIOLOGY. 



about twice or three times as great in the arteries as in the veins, 8 inches 

 (about 200 mm.) a second. The rate at which the blood moves in the 

 veins gradually increases the nearer it approaches the heart, for the sec- 

 tional area of the venous trunks, compared with that of the branches 

 opening into them, becomes gradually less as the trunks advance toward 

 the heart. 



Velocity of the Circulation as a whole. It would appear that a 

 portion of blood can traverse the entire course of the circulation, in the 

 horse, in half a minute. Of course it would require longer to traverse 

 the vessels of the most distant part of the extremities than to go through 

 those of the neck: but taking an average length of vessels to be traversed, 

 and assuming, as we may, that the movement of blood in the human 

 subject is not slower than in the horse, it may be concluded that half a 

 minute represents the average rate. 



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 

 another. 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 seconds; 

 from the jugular vein to the m'asseteric artery, in between fifteen and 

 thirty seconds; to the facial artery, in one experiment, in between ten 

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

 five seconds; in its transit from the jugular vein to the metatarsal 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 to 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 be con- 

 tained 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 estimates both of the total quantity 

 of blood, and of the capacity of the cavities of the heart, have as yet only 



