82 Motion of the Heart and Blood 



which is below the valve remaining all the while empty 

 (H, o, fig. 3). 



It would therefore appear that the function of the 

 valves in the veins is the same as that of the three 

 sigmoid valves which we find at the commencement of 

 the aorta and pulmonary artery, viz., to prevent all 

 reflux of the blood that is passing over them. 



Farther, the arm being bound as before, and the 

 veins looking full and distended, if you press at one 

 part in the course of a vein with the point of a finger 

 (L, fig. 4), and then with another finger streak the blood 

 upwards beyond the next valve (N), you will perceive 

 that this portion of the vein continues empty (L N), 

 and that the blood cannot retrogade, precisely as we 

 have already seen the case to be in fig. 2 ; but the 

 finger first applied (H, fig. 2, L, fig. 4), being removed, 

 immediately the vein is filled from below, and the arm 

 becomes as it appears at D c, fig. i. That the blood 

 in the veins therefore proceeds from inferior or more 

 remote to superior parts, and towards the heart, moving 

 in these vessels in this and not in the contrary direc- 

 tion, appears most obviously. And although in some 

 places the valves, by not acting with such perfect 

 accuracy, or where there is but a single valve, do not 

 seem totally to prevent the passage of 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 suc- 

 ceeding 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 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 



