VEINS 



173 



This portion is enlarged below 



Endothelium. 

 \ 



round (Fig. 165). Small veins full of blood may be round, however, 

 and the arteries are sometimes irregularly contracted. 



The walls of the veins, like those of arteries, are composed of three 

 layers, the intima, media, and externa. The intima includes the primary 

 endothelium, which is composed 

 of polygonal cells, generally shorter 

 and broader than those of arteries. 

 The endothelium rests on a thin 

 layer of subendothelial fibrous 

 tissue. The inner elastic mem- 

 brane of arteries is represented in 

 the smaller veins by a thin homo- 

 geneous membrane, but in larger 

 veins it is replaced by a network 

 of elastic fibers (Fig. 165). In 

 addition to these structures the 

 intima of certain veins contains 

 scattered oblique and longitudinal 

 muscle fibers; they are said to 

 occur in the iliac, femoral, saphen- 

 ous and intestinal veins, the intra- 

 muscular part of the uterine veins, 

 and especially in the dorsal vein of 

 the penis near the suspensory liga- 

 ment. 



The media shows great varia- 

 tions. It is generally a thin layer 

 consisting of circular muscle fibers, 

 elastic networks and relatively 

 abundant connective tissue, and is 

 best developed in the veins of the 

 lower extremity (especially the 

 popliteal). In those of the upper 

 extremity it is not so well marked, 

 and it is still thinner in the larger 

 veins of the abdominal cavity; it FJG . I68 ._ r 

 is reduced to fibrous tissue and is 

 essentially absent from the vena 

 cava superior, the veins of the 

 retina, of the pia and dura mater, and of the bones. 



The externa is the most highly developed layer of the veins. It con- 

 sists of interwoven bundles of connective tissue, elastic fibers, and longi- 

 tudinal bundles of smooth muscles which are more abundant than in the 



A CROSS SECTION OF A HUMAN SUPRA- 

 RENAL VEIN, STAINED WITH H^KMATOXYLIN. X 240. 

 , Circular muscle fibers of the media; b, connective tis- 

 sue; c, d, longitudinal muscle fibers of the externa; 

 e, connective tissue; f, small vein; g, fat cell. 



