132 HISTOLOGY. 



not form so compact a layer and their nuclei are oval rather than rod 

 shaped. For some distance from the capillaries muscle fibers are absent 

 although encircling bundles of connective tissue may be present. 



In the larger veins (Fig. 157) the intima consists of an endothelium of 

 polygonal cells resting on connective tissue and bounded by the inner 

 elastic membrane. The latter is structureless in small veins but is repre- 

 sented by elastic nets in the larger ones. In the intima of various veins 

 occasional oblique or longitudinal muscle fibers have been found. (These 

 occur in the iliac, femoral, saphenous and intestinal veins, the intramus- 

 cular part of the uterine veins, and especially in the dorsal vein of the penis 

 near the suspensory ligament.) 



The media is best developed in the veins of the lower extremity 

 (especially in the popliteal), less developed in those of the upper extremity, 

 and still less in the larger veins of the abdominal cavity. It consists of 

 circular muscle fibers, elastic networks, and fibrous connective tissue, the 

 last being more abundant than in the arteries. In many veins the media 



is represented only by connective tis- 

 I S ti ? a - r ~--j^s>5. sue, as in the superior vena cava and 



Media. I - --" 



./ -as**^ ' a. its principal tributaries; the veins of 



Extema with longitu- ) .., the retina and of the bones; and 



dinal smooth muscle i -vsa? * -..'. '--* <3; , i < ,1 j j 



fibers cut across. | ',.'-^' those of the pia and dura mater. 



Thin walled veins of large diameter 



FIG. 158. PART OP CROSS SECTION OF THE 



HUMAN RENAL VEIN, x so. J n the dura and elsewhere are called 



sinuses. 



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

 sists of crossed bundles of connective tissue, elastic fibers, and longitudinal 

 smooth muscle which, as in the trunk of the portal vein and in the renal 

 vein (Fig. 158), form an almost complete muscle layer. The blood and 

 nerve supply of veins is similar to that of arteries. The vasa vasorum are 

 said to be more numerous in veins, into which they empty. 



The valves of veins are paired folds of the intima, each shaped like 

 half of a cup attached to the wall of the vein so that its convex surface 

 is toward the lumen. In longitudinal section they appear like the valves 

 of the lymphatic vessel shown in Fig. 164. The valves are generally 

 found distal to the point where a branch empties into the vein, and they 

 prevent its blood from flowing away from the heart. The valves do not oc- 

 cur in small veins. They are most numerous in the veins of the extremities, 

 but appear also in the intercostal, azygos and spermatic veins. Elsewhere 

 they are absent. The endothelial cells on the surface of the valve toward 

 the lumen of the vein are elongated parallel with the current, but on the 

 side toward the wall of the vein they are transversely placed. Under the 



