1 68 



THE CIRCULATION. 



tion, is effected with" the help of the valves, which are placed 

 in all veins" subject to local pressure from the muscles- 

 between or near which they run. The general construction 

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

 the aorta and pulmonary artery, already described (p. 1 08) ; 

 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. 5 3). 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 semi- 

 lunar; the unattached 

 edge being in som& 

 examples concave, in 

 others straight. They 

 are composed of inex- 

 tensile fibrous tissue, 

 and are covered with 

 epithelium like that 

 lining the veins. 

 During the 'period of 

 their inaction, 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 com- 

 plete barrier to any backward movement of the blood 

 (figs. 54 and 55). 



Valves are not equally numerous in all veins, and in 



* Fig. 53. Diagrams 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. 



