218 THE HUMAN BODY. 



Structure of the Veins. In these the same three pri- 

 mary coats as in the arteries may be found: the inner and 

 middle coats are less developed while the outer one remains 

 thick, and is made up almost entirely of white fibrous tissue. 

 Hence venous walls are much thinner than those of the 

 corresponding arteries, and the veins collapse when empty 

 while the stouter arteries remain open. But the tenacity 

 and toughness of their outer coats give the veins great 

 strength. 



Except the pulmonary artery and the aorta, which pos- 

 sess the semilunar valves at their cardiac orifices, the 

 arteries possess no valves. Many veins on the contrary have 

 such, formed by semilunar pouches of the inner coat, at- 

 tached by one margin and having that turned towards the 

 heart free. These valves, sometimes single, oftener in 

 pairs, and sometimes three at one level, permit -blood to 

 flow only towards the heart, for a current in that direction 

 (as in the upper diagram, Fig. 85) presses the valve close 



against the side of the vessel and 

 ====== meets with no obstruction from it. 



c ^==^ ff 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 



ln superficial veins and those of 



muscular parts. They are absent 

 end of the vessel. in the venae cavae and the portal 



and pulmonary veins. Usually the 



vein is a little dilated opposite a valve and hence in parts 

 where the valves are numerous gets a knotted look. On 

 compressing the forearm so as to stop the flow in its sub- 

 cutaneous veins and cause their dilatation, the points at 

 which valves are placed can be recognized by their swollen 

 appearance. They are most frequently found where two 

 veins communicate. 



