THE BLOODVESSELS. 685 



of the arteries, except that in many veins, especially of the abdomen, 

 longitudinal muscles, occasionally very well developed, appear in it, 

 and give the entire venous wall a peculiar character. 



The smallest veins (Fig. 280 b), may be said to consist solely of a 

 nucleated, indistinctly fibrous, or homogeneous connective tissue and an 

 epithelium. The elements of the latter are elliptical or round, with 

 oval or even rounded nuclei, whilst the former constitutes a proportion- 

 ately strong t. adventitia, and besides that, a thinner layer, which sup- 

 plies the place of the t. media (Fig. 280 e), both having a longitudinal, 

 fibrous arrangement. Below the size of 0*01 of a line the veins gradu- 

 ally lose the external connective tissue and the epithelium, the t. media 

 appearing to pass into the structureless wall of the capillaries. A 

 muscular membrane, and in general, a layer of annular fibres are first 

 seen in veins more than 02 of a line in size, in the form, at first, of 

 widely separated, transversely oval cells, with short-oval, sometimes 

 even almost rounded, transverse nuclei. By degrees these cells become 

 more elongated, and more numerous, and ultimately, in vessels of 0-06- 

 0-08 of a line constitute a continuous layer (Fig. 279 ft), but which is 

 always less developed than the corresponding arterial tissue. This 

 continues to be the structure of veins up to the size of 0-1 of a line ; 

 when elastic networks, at first fine, begin gradually to make their ap- 

 pearance externally to the epithelium in the t.t. musculosa and adven- 

 titia, whilst at the same time the muscular layers multiply, and even 

 admit connective tissue and fine elastic fibres among their elements. 



Veins of the medium diameter of 1-3-4 lines, such as the cutaneous 

 veins, and deeper veins of the extremities up to the brachial and popli- 

 teal, the veins of the head and viscera, except the main trunks, are 

 characterized (particularly those of the lower extremity) by the not in- 

 considerable development of their annular fibrous membrane, which, as 

 in the arteries, is of a yellowish-red color, and striped transversely. 

 But even where thickest, it is far from equalling 

 that of the corresponding arterial vessels ; and 

 in thickness never exceeds 0-06 to 0-07 of a 

 line. It consists not only of transverse but also 

 of longitudinal layers, and in this respect like- 

 wise differs from that of the arteries. The 

 transverse layers are composed of common, undu- 

 lating connective tissue, with fine, loosely ar- 

 ranged, or rather isolated elastic fibres (nucleated fibres, as they are 



FIG. 284. Transverse section of the vena saphena magna, at the mallclus, magnified 30 

 diameters: a, striped lamella and epithelium of the t. intima ; 6, its elastic membrane; c, longi- 

 tudinal, internal connective-tissue layer of the t. media, with elastic fibres; d, transverse 

 muscles, and c, longitudinal elastic networks, disposed in a laminated manner ;/, t. adventitia. 



