CH. XIX. J THE VEINS 221 



endothelium. Their situation in the superficial veins of the fore- 

 arm 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. 201, 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 canal, 

 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 

 in the coats of both arteries and veins. In the external coat of large 



I- n.. 'J03.- -Surface view of an artery from the mesentery of a frog, ensheathed in a perivaseular lym- 

 phatic 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 simple 

 endothelial membrane. (Klein and Noble Sn ith.) 



vessels they form a plexus of more or less tubular vessels. In smaller 

 vessels they appear as spaces lined by endothelium. Sometimes, as 



