SECT. 215.] STRUCTURE OF SMALL VEINS. 493 



adventitia, lastly, is generally the thickest layer, and, indeed, its 

 relative and absolute thickness increases with that of the vessels. 

 It completely agrees in its composition with that of the arteries, 

 except that in many veins, those of the abdominal cavity especially, 

 we find in it very well-developed, longitudinal, muscular fibres, 

 which give the whole Avail of the vein a peculiar character. 



The smallest veins (fig. 198, b) may be said to consist only of 

 nucleated connective tissue, indistinctly fibrous or homogeneous, 

 together with an epithelium. The component cells of the latter 

 are oblong or round, with oval or even roundish nuclei ; whilst the 

 former tissue forms an outer coat, which is relatively thick, and a 

 thinner layer (fig. 198, e), representing the tunica media, the fibres 

 of both coats running in the longitudinal direction. Below 001'", 

 the veins gradually lose the outer connective tissue and the epi- 

 thelium, and the middle coat passes, to all appearance, into the 

 structureless coat of the capillaries. A muscular coat, and usually, 

 also, a layer of circular fibres, appear first in veins above 0-02'" in 

 diameter; when first seen, these elements appear as transversely 

 oval cells, at first widely apart from each other, with short oval, or 

 even roundish transverse nuclei. These cells gradually become 

 longer and more numerous, and at last, on vessels o - o6'" to o'o8'" 

 in diameter, form a continuous layer (fig. 197), which, however, is 

 never so well developed as that of the corresponding arteries. The 

 structure of the veins remains the same as far as those of o*i'" in 

 diameter ; but their fine elastic networks gradually appear exter- 

 nally to the epithelium in the muscular coat and tunica adventitia, 

 while the muscular layers themselves increase, and connective 

 tissue and fine elastic fibres become developed between their 

 elements. 



Veins of medium size, from one to four lines in diameter, com- 

 prise the cutaneous and deep veins of the extremities, as far as the 

 brachial and popliteal, and the visceral and cerebral veins, with 

 the exception of their main trunks. Such veins are distinguished 

 (more especially those of the lower extremities) by the greater 

 development of their circular fibrous coat, which, as in the arteries, 

 is reddish-yellow in colour, and arranged transversely. Still, even 

 where it possesses the greatest thickness, this coat never equals 

 that of the corresponding arterial vessels, and does not exceed 

 cro6"' to o - 07'" in thickness. It is distinguished also from the 

 similar structure in the arteries by possessing not only transverse, 

 but also longitudinal layers. The former are represented by or- 

 dinary, wavy connective tissue, with fine, loose, elastic fibres, 



