114 HANDBOOK OF PHYSIOLOGY. 



The middle coat is considerably thinner than that of the arteries ; 

 and, although it contains circular unstriped muscular fibres or fibre-cells, 

 these are mingled with a larger proportion of yellow elastic and white 

 fibrous tissue. In the large veins, near the heart, namely the vence cavcB 

 and pulmonary veins, the middle coat is replaced, for some distance from 

 the heart, by circularly arranged striped muscular fibres, continuous with 

 those of the auricles. 



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- 



FIG. 110. A, vein with valves open. B, vein with valves closed: stream of blood passing off by 

 lateral channel. (Dalton.) 



lunar 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 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. 109). 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 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 



