VEINS AND HEART. 



cation of the surrounding tissues, just as, for 

 the same reason, the forcing has to be done 

 by compression of the whole apparatus, in- 

 stead of by movement of a separate part, such 

 as a piston. 



We will take, first, the pumping machinery 

 of the veins as the simplest modification of 

 the pipes or tubes which convey the fluid. 



The veins contain blood, which has to be 

 urged towards the heart. Their walls are very 

 soft and flexible, so that they can easily be 

 flattened by squeezing. Squeezing them flat 

 drives out the blood to some other part of the 

 veins ; but it must be driven towards the 

 heart, not away from it. And that which has 

 been driven on must not be allowed to return 

 and take up the room of the further quantities 

 which ought to follow. These two objects are 

 secured by an arrangement of valves at inter- 

 vals along the course of the veins. The valves 

 are so arranged as to allow a current to flow 

 easily towards the heart, but not away from 

 it, so that blood which has once moved past 

 a certain point cannot return. 



The valves consist of pockets, arranged in 

 pairs, one on each side of the vein. Fig. 62, X, 

 shows the view obtained by slicing a vein 

 down into two parts, so as to leave on each 

 half one complete pocket, ABDFCF. ABD 

 is the loose upper edge. The flap is attached 

 round A F C F D. At Y is seen the view 



T 289 



