THE CIRCULATION OF THE BLOOD. 161 



the capacity of the veins is about twice or three times as great as that of 

 the arteries, and that the velocity of the blood's motion is, therefore, 

 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 towards 

 the heart. 



(d.) Of the Circulation as a whole. It would appear that a por- 

 tion 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 sub- 

 ject is not slower than in the horse, it may be concluded that half a min- 

 ute 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 masseteric artery, it occupied between fifteen and thirty 

 seconds; to the facial artery, in one experiment, in between ten and fif- 

 teen 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 that 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 

 11 



