THE BLOOD-VESSELS. 



157 



rioles. In the latter, the endothelial cells are more nearly 



fusiform, longer, and somewhat narrower than in the venules. 



In the capillaries, their form and dimensions hold an interme- 



diate position between the arterial and venous types. The 



middle coat is entirely wanting in capillaries, and is much less 



conspicuous in the small veins than in the arterioles. In fact, 



under ordinary circumstances, the muscle-coat forms by far the 



most characteristic distinguishing feature between these ves- 



sels. Venules quite frequently have only a few sparsely scat- 



tered muscle-cells, in place of the continuous muscular layer 



which exists in minute 



arteries. The former 



also are either altoge- 



ther deficient in the in- 



ternal elastic coat, or the 



presence of this struc- 



ture is barely indicat- 



ed by delicate elastic 



fibres ; these latter usu- 



ally have a longitudinal 



direction. On the other 



hand, arteries of corre- 



sponding calibre are 



mostly furnished with 



a distinct elastic inner 



coat. Finally, with re- 



S*ard tO the adventitia 



we find it more highly 

 developed proportionally in venous than in arterial vessels, 

 whereas capillaries commonly have only a few faint fibres to 

 denote the presence, in them also, of this coat. 



The internal elastic coat of the larger and largest veins is 

 very feebly developed in comparison with that of the arteries. 

 Distinct fenestrated membranes are scarcely ever encountered. 

 Veins are likewise possessed of an internal fibrous layer, but 

 here again we observe that comparatively feeble development 

 of a coat which in the arteries is quite conspicuous. 



Among the many special characteristics of the various 

 veins in different regions, we will only mention the following : 

 the jugular veins show well-marked elastic reticula, the meshes 

 of which contain sparse muscular elements. In the femoral, 



FIG. 72. Longitudinal section of popliteal vein : r/, tritium ; 



