276 THE BLOODVESSELS, BY C. J. EBERTH. 



THE EPITHELIAL LAYER consists of cells that, when compared 

 with the corresponding structures in the arteries, present a more 

 polygonal and less distinctly fusiform shape, and are consequently 

 both shorter and broader. Their size varies in different regions. 



ELASTIC INTERNAL MEMBRANE. The veins, like the arteries, 

 possess an elastic membrane, situated immediately beneath the 

 epithelium, and apparent even in small vessels. This tunic 

 never acquires the size and strength it exhibits in the arteries, and 

 usually appears as a delicate and rather loose network of fibres, 

 which, for the most part, run in a longitudinal direction, and 

 but rarely, as in the larger trunks, undergo development into 

 a fenestrated elastic tunic. In the iliac and crural veins this 

 coat appears in some places to be split into two laminae, which 

 intercommunicate with one another by fine elastic fibrils. A 

 delicate indistinctly fibrous connective tissue containing lon- 

 gitudinally and transversely arranged short fusiform cells, 

 occupies the interspaces of this network. 



The internal longitudinal fibrous tunic is situated between the 

 epithelial layer and the internal elastic membrane, as in the 

 arteries, but is developed to a much less extent. In some veins 

 it is almost wholly absent, as in those of the neck, the axillary 

 vein, the vena cava, the mesenteric and portal veins, the vena 

 azygos, and the branches of the pulmonary vein. The thick- 

 ness of this layer by no means corresponds with the size of the 

 vessel. Thus it is absent in the vena cava inferior, both above 

 and below the liver, reappearing in the iliac vein, and increasing 

 gradually in strength until the popliteal is reached, where it 

 attains its greatest thickness. At this part the membrane often 

 forms thickenings, which appear even to the naked eye as small 

 elevations and transverse rugae. On tracing it further towards 

 the periphery its thickness will be found to undergo gradual 

 diminution. 



.Its structure is essentially similar to that of the same layer 

 in the arteries, with the exception that in many parts numerous 

 muscles are present which fail to appear in the corresponding 

 arteries. Thus the crural vein presents small bundles of longi- 

 tudinal muscular fibres between the laminae of its elastic inner 

 coat, and the popliteal possesses in the same layer an internal 



