128 WILLIAM HARVEY 



... 



single valve, do not seem totally to prevent the passage 

 the blood from the centre, still the greater number of them 

 plainly do so ; and then, where things appear contrived more 

 negligently, this is compensated either by the more frequent 

 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 chan- 

 nels of distribution 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 prevent 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 im- 

 mediately filled from below ; apply the finger again, and hav- 

 ing in the same manner streaked the blood upwards, again 

 remove the finger below, and again the vessel becomes dis- 

 tended as before; and this repeat, say a thousand times, in 

 a short space of time. And now compute the quantity of 

 blood which you have thus pressed up beyond 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. 



