220 



THE CIRCULATORY SYSTEM 



[CH. XIX. 



tion of these valves is similar to that of the semilunar valves of the 

 aorta and pulmonary artery, already described; but their free margins 

 are turned in the opposite direction, i.e., towards the heart, so as to 

 prevent any movement of blood backward. They are commonly 

 placed in pairs, at various distances in different veins, but almost 

 uniformly in each (fig. 201). In the smaller veins single valves 



FIG. 201. Diagram showing valves of veins. A, part of a vein laid open and spread out, with two pairs 

 of valves. B, longitudinal section of a vein, showing the apposition of the edges of the valves in 

 their closed state. C, portion of a distended vein, exhibiting a swelling in the situation of a pair 

 of valves. 



are often met with; and three or four are sometimes placed 

 together, or near one another, in the largest veins, such as the 



FIG. 202. A, vein with valves open. B, with valves closed ; stream of blood passing off by lateral 



channel. (Dalton.) 



subclavian, at their junction with the jugular veins. They are com- 

 posed of an outgrowth of the subendothelial tissue covered with 



