THE VALVES OF THE VEINS, 1 57 



ing in the proper direction, A, from capillaries to heart, 

 is not hindered on its road; but any back-flow in the 

 opposite direction, B, is at once * 



checked by the closing of the valve, c "^^-Si H 



This you may easily observe on 

 the back of the hand of any one 

 who is thin. Select a vein which has c 



no branches for an inch or more. FIG. 41. Diagram to illus- 

 trate the mode of action of the 



PreSS On Its lower end, that IS the valves of the veins. C, the 



capillary, H, the heart end of 



end nearer the fingers, so as to close the vessel, 

 it. Then push the blood out of it by rubbing it with your 

 forefinger in a direction towards the wrist. The vein 

 will remain empty up to the place where the next higher 

 branch joins it. At that place there is a valve, which 

 will noJ; allow blood to flow back into itjji As soon as 

 you remove the pressure from its lower end, however, 

 the vein immediately fills, with blood brought to it from 

 the capillaries of the fingers. We learn from this simple 

 experiment that the valves of the veins allow blood to 

 flow through the veins to the heart, but not from it. If 

 the first vein you try the experiment with, does not be- 

 have as it should, try another, for some of the veins on 

 the back of the hand have branches entering their deeper 

 sides, which you cannot see, and from which they be- 

 come filled. 



When the muscles contract in length and swell in 

 breadth (p. 35) they press on the veins near them. 

 This pressure cannot drive blood back to the capillaries, 

 on account of the valves in the veins. But it drives 

 blood on from the veins towards the heart, and thus pro- 

 capillaries ? How may we observe on the hand the action of the 

 valves of the veins ? How do the muscles promote the circulation of 



