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HANDBOOK OF PHYSIOLOGY. 



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 prevent any movement of blood backward. They are 

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

 almost uniformly in each (fig. 161). 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 junc- 

 tion 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 

 endothelium 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 come together 

 like the valves of the arteries (figs. 161 and 

 162). Their situation in the superficial 

 veins of the forearm is readily discovered 

 by pressing along its surface, in a direc- 

 tion opposite to the venous current, i.e., 

 from the elbow toward the wrist; when 

 little swellings (fig. 161, c) appear in the 

 position of each pair of valves. These 

 swellings at once disappear when the pres^ 

 sure is removed. 



Valves are not equally numerous in all 

 veins, and in many they are absent al- 

 together. 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 pres- 

 sure. 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 of the bones, and 

 the trunk and branches of the umbilical vein are also destitute of valves. 



Lymphatics of Arteries and Veins. Lymphatic spaces are present 

 in the coats of both arteries and veins; but in the tunica adventitia or 

 external coat of large vessels they form a distinct plexus of more or less 



Fig. 163. Surface view of an artery 

 from the mesentery of a frog, en- 

 sheathed in a peri-vascular lymphatic 

 vessel, a. The artery, with its circular 

 muscular coat (media) indicated by 

 broad, transverse markings, with an 

 indication of the adventitia outside. 

 I. Lymphatic vessel, its wall is a sim- 

 ple endothelial membrane. (Klein and 

 Noble Smith. 



