124 NORMAL HISTOLOGY. 



The walls of the veins , like those of the arteries, 

 consist of three layers, and these layers have in 

 general an analogous structure ; but they are 

 neither as distinct, nor are their structural features 

 as constant, as those of the arteries. Moreover, we 

 find that veins of the same calibre present, in dif- 

 ferent parts of the body, marked differences in 

 structure, and, unlike the arteries, the thickness of 

 their walls is not uniformly proportional to the 

 calibre of the vessel. In general we may express 

 the structural difference between veins and arteries 

 by saying that in the walls of the former the elastic 

 and muscular elements are much less, while the 

 connective-tissue elements are more abundant, than 

 in the walls of the latter. Now, since the muscular 

 and elastic elements are the chief constituents of 

 the media, and the fibrillar connective-tissue ele- 

 ments, of the adventitia, we find, in general, that in 

 the veins the media is less, while the adventitia is 

 more developed. The intermediary layer of the 

 intima is entirely absent in small veins ; in many of 

 medium size it appears, and is again absent in the 

 largest vessels. In some veins, such as those of the 

 bone, central nervous system, retina, etc., the mus- 

 cular elements are almost or entirely absent. In 

 certain other veins, on the contrary, such as the v. 

 portarum, v. renalis, the adventitia contains a great 

 abundance of muscular elements arranged parallel 

 with the axis of the vessel. 



