VEINS 189 



The tunica media contains smooth muscle fibers, the most of which 

 are circularly arranged. A somewhat smaller proportion of delicate 

 connective tissue completes this coat. 



The media is best developed in veins of the lower extremities; it 

 forms a thinner layer in veins of the upper extremities, and is rela- 

 tively scant in the large veins of the abdominal cavity. 



The adventitia of the larger veins consists of interlacing bundles of 

 dense white fibers, among which is a network of fine elastic fibers. Occa- 



M^f^^^is^^^l 



FIG. 209. TRANSECTION OF THE WALL OF THE HUMAN VENA CAVA. 



a, tunica intima; 6, tunica media; c, tunica adventitia. The inner portion of the 

 tunica adventitia contains numerous bundles of longitudinal smooth muscle fibers 

 which have been cut across. Hematein and eosin. X 90. 



sional small bundles of longitudinal smooth muscle fibers occur in the 

 adventitia of the largest veins. In these vessels also, a very incomplete 

 external elastic membrane may be demonstrated by the specific stains for 

 elastic tissue. 



Nerve fibers and minute blood-vessels, vasa vasorum, occur in this 

 coat and distribute their terminal branches to the two outer coats of the 

 vessel. The intima of the vein, as in the artery, is non-vascular. The 

 venae vasorum of veins empty directly into the lumen. 



Atypical Veins. In certain tissues the veins present noticeable de- 

 partures from the typical structure. Longitudinal muscle fibers are found 

 in many of the larger veins of the abdominal and thoracic cavities. 



