190 



CIECTJLATION 



liP^i 



toughness and thickness of the outer coat gives them 



great strength. 

 Many veins also possess semilunar valves formed by 



pockets located at intervals on the inner surface of the 

 tubes. These are so arranged (see Fig. 73) 

 that the edge toward the heart is free and 

 prevents a backward flow in the tube by pres- 

 sing out and bridging the tube under backward 

 pressure. These valves are found chiefly in the 

 superficial and muscle supply veins. The ar- 

 teries have no such valves except at the 

 entrance into the heart. The location of these 

 valves may be recognized by the knotted ap- 

 pearance of the ^Tills at these points. 



Capillaries. As the arteries decrease in size 

 the outer and middle coats gradually disappear 

 imtil, in the capillaries, only the inner layer 

 is left. This is composed of flattened cells 

 cemented together at the edges. Such a 



structiire is evidently well adapted to the process of 



osmosis, dialysis, and filtration. 



Fig. 73 — 

 Vein, showing 

 semil unar 

 valves. 



