162 THE ORGANS 



tissue separating the endothelium from the media. In the subclavian, the media 

 contains longitudinal muscle cells. Longitudinally running muscle cells occur 

 in the adventitia of the umbilical arteries, in the iliac, splenic, renal, superior 

 mesenteric and dorsalis penis. The radial, femoral and coeliac arteries have 

 comparatively little clastic tissue, while in the common iliac, carotid, and axillary 

 the elastic tissue is in excess of the muscular. 



Veins 



The walls of veins resemble those of arteries. There are the 

 same three coats, intima, media, and adventitia, and the same elements 

 enter into the structure of each coat (Fig. 95). Venous walls are not, 

 however, so thick as those of arteries of the same calibre, and the 

 coats are not so distinctly differentiated from one another. The 

 transition from capillary through the precapillary vein to the small 

 vein is similar to that described under arteries (page 157). Unlike 

 the artery, the thickness of the wall of a vein and its structure a,re 

 not directly proportionate to the size of the vessel, but depend also 

 upon other factors such as the position of the vein and the support 

 given to its walls by surrounding structures. 



Of the INTIMA the endothelial layer and the intermediary layer 

 are similar to those of the artery. The elastic layer is not always 

 present, is never so distinct, and is not wavy as in the artery (Fig. 

 95). The result is a lack of demarcation between intima and media, 

 the connective tissue of the intermediary layer of the intima merging 

 with the mixed muscle and connective tissue of the media. Project- 

 ing at intervals from the inner surfaces of the walls of some veins are 

 valves. These are derived entirely from intima and consist of loose 

 connective tissue covered by a single layer of endothelium. Beneath 

 the endothelium of the convex surface of the valve (the surface 

 directed peripherally or against the blood current) is a rich network 

 of elastic fibres continuous with the elastic tissue of the intima of the 

 adjacent vein wall. The connective tissue along the opposite margin 

 of the valve is entirely free from or contains very few elastic fibres. 

 Valves are especially large and strong in the larger veins of the lower 

 limbs. They are absent in the veins of the brain and cord and their 

 membranes, in the veins of bones, in the umbilical vein, and in most 

 of the visceral veins with the exception of some branches of the 

 portal. 



The MEDIA- of veins is thin as compared with that of arteries of 

 the same size. It consists of fibrous and elastic tissue and circularly 



