148 THE CIRCULATION. 



the velocity with which the movement of the blood through the 

 whole round of the circulation is accomplished. As Miiller 

 says, the rate of the blood's motion in the vessels must not be 

 judged of by the rapidity with which it flows from a vessel 

 when divided. In the latter case, the rate of motion is the 

 result of the entire pressure to which the whole mass of blood 

 is subjected in the vascular system, and which at the point of 

 the incision in the vessel meets with no resistance. In the 

 closed vessels, on the contrary, no portion of blood can be 

 moved forwards 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 ves- 

 sels 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 pulmon- 

 ary 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 trans- 

 mitted 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 occu- 

 pied 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 am- 

 monia, 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 ef- 

 fected 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 has been observed by Mr. J. Blake, who 

 found that nitrate of baryta injected into the jugular vein of 



