VELOCITY OF THE CIRCULATION. 177 



except by impelling on the whole mass, and by overcoming 

 the resistance arising from friction in the smaller vessels. 



From the rate at which the blood escapes from opened 

 vessels we can only judge, in general, that its velocity is, as 

 already said, greater in arteries than in veins, and in both 

 these greater than in the capillaries. More satisfactory data 

 for the 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. From eighteen such experiments on 

 horses, Hering deduced that the time required for the 

 passage of a solution of ferrocyanide of potassium, mixed 

 with the blood, from one jugular vein (through the right 

 side of the heart, the pulmonary circulation, 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, 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. 



Poiseuille's observations accord completely with the 

 above, and show, moreover, that when the ferrocyanide 

 is injected into the blood with other substances, such as 

 acetate of ammonia, or nitrate of potash (solutions of 

 which, as other experiments have shown, pass quickly 

 through capillary tubes), the passage from one jugular 

 vein to the other is effected in from eighteen to twenty- 

 four seconds ; while, if instead of these, alcohol is added, 

 the passage is not completed until from forty to forty-five 

 seconds after injection. Still greater rapidity of transit 



