CIRCULATION OF THE BLOOD. 



137 



The internal coat of veins is less brittle than the corresponding coat 

 of an artery, but in other respects resembles it closely. 



Valves. The chief influence which the veins have in the circulation, 

 is effected with the help of the valves, which are placed in all veins sub- 

 ject to local pressure from the muscles between or near which they run. 

 The general construction of these valves is similar to that of the semi- 

 lunar valves of the aorta and pulmonary artery, already described; but 

 their free margins are turned in the opposite direction, i.e., toward the 

 heart, so as to stop any movement of blood backward in the veins. They 

 are commonly placed in pairs, at various distances in different veins, but 

 almost uniformly in each (Fig. 119). In the smaller veins, single valves 

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

 near one another, in the largest veins, such as the subclavian, and at 

 their junction with the jugular veins. The valves are semilunar; the 



FIG. 119. 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, snowing 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. 



unattached edge being in some examples concave, in others straight. 

 They are composed of inextensile fibrous tissue, and are covered with 

 endothelium like that lining the veins. During the period of their in- 

 action, when the venous blood is flowing in its proper direction, they 

 lie by the sides of the veins; but when in action, they close together like 

 the valves of the arteries, and offer a complete barrier to any backward 

 movement of the blood (Figs. 119 and 120). Their situation in the 

 superficial veins of the forearm is readily discovered by pressing along its 

 surface, in a direction opposite to the venous current, i.e., from the 

 elbow toward the wrist; when little swellings (Fig. 119, c) appear in the 

 position of each pair of valves. These swellings at once disappear when 

 the pressure is relaxed. 



Valves are not equally numerous in all veins, and in many they are 

 absent altogether. They are most numerous in the veins of the extremi- 

 ties, and more so in those of the leg than the arm. They are commonly 

 absent in veins of less than a line in diameter, and, as a general rule, 



