VELOCITY OF THE CIRCULATION. 



effect of inspiration on the venous circulation; and he- 

 mentions the following experiment in proof of it. He 

 introduced one end of a bent glass tube into the jugular 

 vein of an animal, the vein being tied above the point 

 where the tube was inserted ; the inferior end of the tube- 

 was immersed in some coloured fluid. He then observed 

 that at the time of each inspiration the fluid ascended in. 

 the tube, while during expiration it either remained 

 stationary, or even sank. Poiseuille confirmed the truth 

 of this observation, in a more accurate manner, by means of 

 his hsemadynamometer. 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. 



Dr. Burdon Sanderson's experiments 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 



