302 SPECIAL HISTOLOGY. 



layers also. It is in general thin, but absolutely stronger in 

 the medium -sized veins than in the larger, and in these also 

 the muscular element is most vigorously developed. The t. ad- 

 ventitia, lastly, is usually the thickest of the three coats, its 

 relative and absolute thickness usually increasing with the size 

 of the vessels. In constitution it precisely resembles that of 

 the arteries, except that in many veins, especially of the abdo- 

 men, 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 ellip- 

 tical or round, with oval or even rounded nuclei, whilst the 

 former constitutes a proportionately strong t. adventitia and 

 besides that, a thinner layer, which supplies the place of the 

 t. media (fig. 280 e), both having a longitudinal, fibrous 

 arrangement. Below the size of 001'" the veins gradually 

 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 002"' 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'" 

 constitute a continuous layer (fig. 279 /3), 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'"; 

 when elastic networks, at first fine, begin gradually to make 

 their appearance externally to the epithelium in the t.t. muscu- 

 losa and adventitia, 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'", such as the cuta- 

 neous veins, and deeper veins of the extremities up to the brachial 

 and popliteal, the veins of the head and viscera, except the main 

 trunks, are characterised (particularly those of the lower extre- 

 mity), by the not inconsiderable development of their annular 

 fibrous membrane, which, as in the arteries, is of a yellowish red 



