188 



THE BLOOD VASCULAR SYSTEM 



Venules. In the venule the tunica intima consists of little more 

 than the endothelial lining. Its media and adventitia are not as yet 

 distinctly differentiated, the former being distinguished only by the in- 

 complete layer of circularly disposed smooth muscular fibers. The ex- 

 tremely thin adventitia is composed almost wholly of white fibers, the 

 greater part of which are circularly disposed. Very few elastic fibers 

 occur even in vessels of this size. 



Small Veins. In the small veins the three coats are fairly distinct, 

 the vascular wall being, however, much thinner than in the artery of 

 corresponding size. 



The endothelium of the tunica intima is supported by a very delicate 

 connective tissue membrane which as yet contains but few elastic fibers. 



- - - a 



i 



_., j 



FIG. 208. TRANSECTION OF AN ARTERIOI AND VENTJLE. 

 X 250. (After Schafer.) 



The tunica media consists of a thin layer of circularly arranged 

 smooth muscle fibers intermingled with a delicate fibrous tissue; elastic 

 fibers are relatively scarce. The adventitia, though considerably the 

 thickest of the three coats, is as yet a thin membrane. It consists of 

 fibrous connective tissue, elastic fibers being scarcely demonstrable except 

 by means of the specific stains for this tissue. 



Larger Veins. The wall of the larger veins closely resembles that of 

 the corresponding artery, except that the venous wall is much thinner 

 and contains far less elastic tissue. The tunica intima of the medium 

 and lar^-e veins presents a lining endothelium, a thin layer of delicate 

 connective tissue fibers, and an incomplete internal elastic membrane. 

 The last named is never so prominent as in the artery. 



