128 MICROSCOPIC ANATOMY OF THE ORGANS. 



never as sharply marked as the fenestrated membrane of arte- 

 ries. The mt'tlla of veins is developed weakly in comparison with 

 that of arteries (Fig. 92). It consists of a few circular muscle 

 bands separated by thin elastic fibres. The veins of the lower 

 extremities possess the most strongly developed media. It may 



FIG. 91. 



t 



; X/li-lt'HS of nillitclf 



Media 



Advmtitia 



\ Elastic fibre* 



Part of a cross-section through a medium-sized vein of a dog. X 280. 



be quite wanting in others (e.g., vena cava superior, subclavian, 

 veins of pia and dura mater, veins of bones, retinal veins, etc.). 

 The adventitia-, on the contrary, is usually strongly developed. 

 It consists of white connective-tissue and elastic fibres, with often 

 well-developed longitudinal smooth muscle bundles (Fig. 92). 



The valves of the veins are derived from the intima, and, 

 like it, consist of connective tissue and elastic fibres. Their 

 surface is covered by cells of the endothelial tube, which on 

 the inner side toward the blood stream are long, and on the 

 other side polygonal. On the inner surface of the valves there 

 is under the endothelial cells a network of fine elastic fibres. 



The main points of distinction between arteries and veins 

 are the following : The walls of arteries in relation to the size 

 of their lumina are much thicker than in veins. The elastic 

 tissue and muscle elements in the media are more strongly 

 developed in arteries. After death the muscle of the media 

 contracts, and in arteries throws the intima and elastica interna 

 into folds, giving it a wavy appearance in cross-section. Veins 

 usually contain a small quantity of blood after death ; arteries 

 are often empty. These differences are illustrated by Fig. 88. 



