EFFECT OF VENOUS VALVES. 



83 



of water in the two directions. Cut the vein open along one 

 side, and pin it to a piece of shingle, with the inner side out. 

 The thin, pocket-like valves ought now to be seen. They are 

 usually in pairs, but sometimes single or in threes. It is im- 

 portant to note that they all have the mouths of the pockets 

 toward the heart, so that the blood flows freely toward the 

 heart, but is prevented from flowing the other way on account 

 of the filling of the valves by the reflow of the blood stream. 

 When the blood is flowing through the veins toward the 



heart the valves lie against 

 the walls of the veins. 



The valves are most nu- 

 merous in the medium-sized 

 veins, and especially in the 

 veins of the extremities ; 

 more abundant in the leg than 

 in the arm. Valves are absent 

 from the Caval and some 

 other veins, and from the very 

 small veins. While experi- 

 menting with the vein, to see 

 the valves and their actions, note also the smoothness of the 

 lining. Test also the elasticity of the veins. 



Effect of Pressure on the Veins. Since the valves in 

 the veins open toward the heart, any intermittent pressure on 

 the veins helps to push the blood on toward the heart. The 

 valves are most numerous in the superficial veins and those of 

 the muscles. The pressure of the muscles during their action 

 (thickening while shortening) produces pressure on the veins ; 

 and as the muscles act for a short time only, and then relax, 

 this alternate compression and release aids very considerably 

 in moving the blood on toward the heart. It is worthy of 

 remark that this effect is more pronounced at the time the 



Open Shut 



Fig. 29. Venous Values. 



