218 



THE CIRCULATORY SYSTEM 



[CIT. XVIII. 



endothelium is made up of cells elongated in the direction of the 

 vessel, but wider than in the arteries. 



Valves. The chief influence which the veins have in the circu- 

 lation is effected with the help of the valves, contained 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 ; but their free margins are turned in the opposite direction, 

 i.e., towards the heart, so as to prevent any movement of blood back- 

 ward. They are commonly placed in pairs, at various distances in 

 different veins, but almost uniformly in each (fig. 229). In the 



smaller veins single valves are 

 often met with; and three or 

 four are sometimes placed to- 

 gether, or near one another, in 

 the largest veins, such as the 

 subclavian, at their junction with 

 the jugular veins. The valves 

 are semilunar; the unattached 

 edge is in some examples con- 

 cave, in others straight. They 

 are composed of an outgrowth of 

 the subendothelial tissue covered 

 with endothelium. Their situa- 

 tion in the superficial veins of 

 the forearm is readily discovered 

 by pressing along their surface, 

 in the direction opposite to the 

 venous current, i.e., from the 

 elbow towards the wrist; when 

 little swellings (fig. 229, c) appear 

 in the position of each pair of 

 valves. These swellings at once disappear when the pressure is 

 removed. 



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 pulmonary 

 veins, the veins in the interior of the cranium and vertebral column, 

 the veins of bone, and the umbilical vein. The valves of the portal 

 tributaries are very inefficient. 



Lymphatics of Arteries and Veins. Lymphatic spaces are present 



FIG. 227. Ramification of nerves and termination 

 in. the muscular coat of a small artery of the 

 frog. (Arnold.) 



