PHYSIOLOGICAL PARADOXES. 



obtained by cutting down across the middle 

 of the pockets instead of between them, and 

 here the vein itself is shown distended by the 

 pressure of the blood which is filling the pockets 

 and pressing together the loose edges of the 

 flaps so as to close the opening against its own 

 return. This position of affairs is also shown 

 in cross-section at Z, which gives the view 

 obtained by cutting across the vein just above 



FlQ. 62 THE VALVES IN THE VEINS. 



the valve, and looking down into the pocket 

 at each side between the vein wall and the 

 flap-edge, which is pressed hard against the 

 flap-edge of the other pocket. 



A study of these three views will readily 

 show what an excellent as well as simple 

 arrangement we have here for securing the 

 blood against being driven in the wrong direc- 

 tion. When any cause tends to make it move 

 upwards, it easily separates the loose flaps, which 



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