338 THE HUMAN BODY 



direction (as in the upper diagram, Fig. 108) presses the valve 

 close against the side of the vessel and meets with no ob- 

 struction from it. Should any back-flow be attempted, how- 

 A _, ever, the current closes up the valve and 



bars its own passage as indicated in 

 the lower figure. These valves are most 

 numerous in superficial veins and those 

 of muscular parts. They are absent in 

 the venae cava? and the portal and pul- 

 ac monary veins. Usually the vein is a little 



and H, the heart end of the parts where the valves are numerous 

 gets a knotted look. On compressing 



the forearm so as to stop the flow in its subcutaneous veins and 

 cause their dilatation, the points at which valves are placed can 

 be recognized by their swollen appearance. They are most fre- 

 quently situated where two veins communicate. 



