VALVES OP THE VEINS. 207 



but are turned in the opposite direction, having the edge 

 nearest the heart free and the other fixed. These valves 

 permit blood to flow only towards the heart, for a current 

 in that direction, as in the upper diagram, Fig. 6%, 

 presses the valve close against the side of the vessel, and 

 meets with no obstruction from it. Should any back-flow 

 be attempted, however, 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. Usually _ 



the vein is a little dilated opposite >- 



a valve, and hence in parts where '" 



the valves are numerous gets a _____ 



knotted look. On tying a cord 

 tightly round the fore-arm, so as 



, , , i a L ix FlG - 62. Diagram to illus- 



to Stop the nOW in its SUDCUtaneOUS trate the mode of action of 



., . , ., . . . the valves of the veins. C, the 



Veins and Cause their dilatation, capillary, H, the heart end of 



i-ii the vessel. 



the points at which valves are 



placed can be recognized by their swollen appearance. The 



valves are most frequently found where two veins communi- 



cate 



The Course of the Blood. From what has been said it 

 is cloar that the movement of the blood is a circulation. 

 Starting from any one chamber of the heart it will in time 

 return to it: but to do this it must pass through at least 

 two sets of capillaries; one of these is connected with the 



In what direction do the valves of the veins allow the blood 

 to pass? Make a diagram illustrating the action of the valves. 

 In what veins are the valves most numerous? Why does a vein with 

 many valves appear knotted? How may we see the dilatations of 

 the veins opposite the valves? Where are the valves of the veins 

 most frequently placed? 



If we followed the blood course steadily from one chamber of the 

 heart what would we find in time? Through what must blood pass 

 before returning to the chamber of the heart which it leaves? 



