HEART AND BLOOD. 67 



more negligently, this is compensated either by the more fre- 

 quent occurrence or more perfect action of the succeeding valves 

 or in some other way : the veins, in short, as they are the free 

 and open conduits of the blood returning to the heart, so are 

 they effectually prevented from serving as its channels of dis- 

 tribution from the heart. 



But this other circumstance has to be noted : The arm being 

 bound, and the veins made turgid, and the valves prominent, as 

 before, apply the thumb or finger over a vein in the situation 

 of one of the valves in such a way as to compress it, and pre- 

 vent any blood from passing upwards from the hand; then, 

 with a finger of the other hand, streak the blood in the 

 vein upwards till it has passed the next valve above, (N, fig. 4,) 

 the vessel now remains empty; but the finger at L being 

 removed for an instant, the vein is immediately filled from below; 

 apply the finger again, and having in the same manner streaked 

 the blood upwards, again remove the finger below, and again 

 the vessel becomes distended as before ; and this repeat, say a 

 thousand times, in a short space of time. And now com- 

 pute the quantity of blood which you have thus pressed up be- 

 yond the valve, and then multiplying the assumed quantity by 

 one thousand, you will find that so much blood has passed 

 through a certain portion of the vessel ; and I do now believe 

 that you will find yourself convinced of the circulation of the 

 blood, and of its rapid motion. But if in this experiment you 

 say that a violence is done to nature, I do not doubt but that, 

 if you proceed in the same way, only taking as great a length 

 of vein as possible, and merely remark with what rapidity the 

 blood flows upwards, and fills the vessel from below, you will 

 come to the same conclusion. 



