176 THE CIRCULATION. 



fact may be remembered the relative capacities of the 

 arterial and venous systems ; for since the veins return to 

 the heart all the blood that they receive from it in a given 

 time through the arteries, their larger size and propor- 

 tionally greater number must compensate for the slower 

 movement of the blood through them. If an accurate 

 estimate of the proportionate areas of arteries and the veins 

 corresponding to them could be made, we might, from the 

 velocity of the arterial current, calculate that of the venous. 

 An iisual 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. 

 Some doubt has, however, been lately expressed regarding 

 the accuracy of this calculation, and the matter, therefore, 

 must be considered not yet settled. 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 



