1 86 THE CIRCULATION. 



his heemadynamometer. And a like confirmation has been 

 since furnished by Valentin, and in minute details by 

 Ludwig. 



The effect of inspiration on the veins is observable only 

 in the large ones near the thorax. Poiseuille could not 

 detect it by means of his instrument in veins more distant 

 from the heart, for example, in the veins of the extremi- 

 ties. And its beneficial effect would be neutralized were 

 it not for the valves ; for he found that, when he repeated 

 Sir D. Barry's experiments, 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, byDr.Burdon Sanderson, have 

 proved more directly that inspiration is favourable to the 

 circulation, 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 th'e 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. 



