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- 

 gard 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 : a, iiitimu ; 

 6, media ; c, adventitia. 



