TELOCITY OF THE CIRCULATION. 187 



Perhaps a fair approximation to such, an estimate is, that 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. And this is not a slow move- 

 ment ; for if we stop the circulation at the beginning of 

 any superficial vein, and empty the upper part of the vein, 

 immediately upon removing the finger the blood will move 

 along the vein faster than the eye can follow it. The rate 

 at which the. blood moves in the veins gradually increases 

 the nearer it approaches the heart, for the sectional area of 

 the venous trunks, compared with that of the branches 

 opening into them, becomes gradually less as the trunks 

 advance towards the heart 



Velocity of the Circulation. 



Having now considered the share which each of the cir- 

 culatory organs has in the propulsion and direction of the 

 blood, we may speak of their combined effects, especially 

 in regard to the velocity with which the movement of the 

 blood through the whole round of the circulation is accom- 

 plished. 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 vas- 

 cular 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 

 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 



