C1I. XVIII.] 



THE VALVES OF VEINS. 



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 move- 

 ment of blood backward. They are commonly placed in pairs, at 

 various distances in different 

 veins, but almost uniformly in 

 each (fig. 220). 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 sub- 

 clavian, at their junction with 

 the jugular veins. The valves 

 are semilunar; the unattached 

 edge u in some examples con- 

 cave, in others straight. They 

 are composed of an outgrowth 

 of the subendothelial tissue 

 covered with endothelium 

 Their situation in the super- 

 ficial veins of ths forearm is 

 readily discovered by pressing 

 along their surface, iu the 



direction Opposite to the Ven- Fig. 219. Transverse section through a small 



ous current, i.e. , from the elbow 

 towards the wrist ; when little 

 swellings (fig. 220, c) appear 

 in tha 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 alto- 

 gether. They are most numer- 

 ous in the veins of the extremi- 

 ties, 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 



artery and vein of the mucous membrane 

 of a child's epiglottis ; the artery is thick- 

 walled and the vein thin-walled. A. Artery, 

 the letter is placed in the lumen of the 

 vessel, . Endothelial cells with nuclei clearly 

 visible ; these cells appear very thick from the 

 contracted state of the vessel. Outside it 

 a double wavy line marks the elastic layer 

 of the tunica intima. TO. Tunica media, 

 consisting of unstriped muscular fibres cir- 

 cularly arranged ; their nuclei are well seen. 

 ". Part of the tunics adventitia showing 

 bundles of connective-tissue fibre in section, 

 with the circular nuclei of the connective- 

 tissue corpuscles. This coat gradually 

 merges into the surrounding connective- 

 tissue. v. In the lumen of the vein. The 

 other letters indicate the same as in the 

 artery. The muscular coat of the vein (m) 

 is seen to be much thinner than that of the 

 artery, x 350. (Klein and Noble Smith.) 



