VELOCITY OF THE '"CIBCUL ATION. 147 



heart for example, in the veins of the extremities. And its 

 beneficial effect would be neutralized were it not for the valves ; 

 for he found that, when he repeated Sir D. Barry's exper- 

 iments, and passed the tube so far along the veins that it went 

 beyond the valves nearest to the heart, as much fluid was forced 

 back into the tube in every expiration as was drawn in through 

 it in every inspiration. 



Some recent experiments, by Dr. Burdon Sanderson, have 

 proved more directly that inspiration is favorable to the cir- 

 culation, inasmuch as, during it, the tension of the arterial 

 system is increased. And it is only when the respiratory orifice 

 is closed, as by plugging the trachea, that inspiratory efforts 

 are sufficient to produce an opposite effect to diminish the 

 tension in the arteries. 



On the whole, therefore, the respiratory movements of the 

 chest are advantageous to the circulation. 



Velocity of Blood in the Veins. 



The velocity of the blood is greater in the veins than in the 

 capillaries, but less than in the arteries ; and with this 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 proportionally 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. A usual 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 to- 

 wards the heart. 



Velocity of the Circulation. 



Having now considered the share which each of the circu- 

 latory organs has in the propulsion and direction of the blood, 

 we may speak of their combined effects, especially in regard to 



