222 



THE CIRCULATORY SYSTEM 



[CH. XVIII. 



The principal vessels of the lymphatic system are, in structure, 

 like small thin-walled veins, provided with numerous valves. The 

 beaded appearance of the lymphatic vessels shown in figs. 23G and 



FIG. 232. Capillary blood-vessels from the omentum of rabbit, showing the nucleate d'endothelial 

 membrane of which they are composed. (Klein and Noble Smith.) 



237 is due to the presence of these valves. They commence in fine 

 microscopic lymph-capillaries, in the organs and tissues of the body, 

 and they end in two trunks which open into the large veins near the 



FIG. 233. Network of capillary 

 vessels of the air-cells of the 

 horse's lung magnified, a, a, 

 capillaries proceeding from b, 

 b, terminal branches of the 

 pulmonary artery. (Frey.) 



FIG. 234. Injected capil- 

 lary vessels of muscle 

 seen with a low mag- 

 nifying power. 



(Sharpey.) 



heart (fig. 235). The fluid which they contain, unlike the blood, 

 passes only in one direction, namely, from the fine branches to the 

 trunk, and so to the large veins, on entering which it is mingled with 

 the stream of blood. In fig. 235 the greater part of the contents of 



