VALYES OF VEIXS. 



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 be- 

 ing in some examples 

 concave, in others 

 straight. They are 

 composed of inexten- 

 sile 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 complete barrier to any 

 backward movement of the blood (figs. 54 and 55). 



Valves are not equally numerous in all veins, and in 

 many they are absent altogether. They are most numerous 

 in the veins of the extremities, 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, there are few or none in those which are not subject 

 to muscular pressure. Among those veins which have no 

 valves may be mentioned the superior and inferior vena 

 cava, the trunk and branches of the portal vein, the 

 hepatic and renal veins, and the pulmonary veins ; those 

 in the interior of the cranium and vertebral column, those 



* 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. 



