



SECT. 213.] TISSUES OF BLOOD-VESSELS. 483 



muscular fibres of the vessels; in the veins, however, it contains 

 numerous longitudinal fibres, and possesses in all the larger vessels 

 more or less elastic elements and connective tissue. The tunica 

 adventitia, lastly, has its fibres again arranged for the most part 

 longitudinally; it is as thick, or even thicker than the middle 

 coat; and consists of little else than connective tissue and elastic 

 networks. 



On a closer examination of the several tissues composing the 

 Avails of the vessels, there are several points Fig. 195. 



worthy of note. The connective tissue is al- 

 most everywhere fully developed, consisting of 

 fine and coarse bundles and distinct fibres. 

 Onlv on the smallest arteries and veins, it 

 becomes a nucleated .indistinctly fibrous tissue, 

 passing at last into a delicate homogeneous \\\v \ 



membrane, in which nuclei may still be seen / \ \\ 



here and there. The clastic tissue appears {^ 

 nowhere in the body in such multifarious forms 

 as in the vessels. AVc meet with wide-meshed 

 loose networks of fibres, from the finest to the 

 coarsest (fig. 17, p. 50), and again with the / 

 narrowest closest web of fibres which form \ 

 almost a continuous membrane ; and between ,- 

 these two forms we meet with all grades of \ 

 transition; and we also observe every degree 

 of transformation between the elastic web just thf^nl^T^To/™™ 

 described and the true elastic membranes, whose ^"aT p ^^ \ 

 origin is often indicated by an elastic network ^^ wovks - Magl 

 of fibres, more or less indistinct, traversing 

 them and exhibiting here and there an aperture. In other cases, 

 the elastic membranes are perfectly homogeneous plates, either in 

 the whole or in part of their extent, and arc perforated by an 

 uncertain number of holes (fig. 19, p. 51). — Transversely striated 

 muscular fibre is found only at the openings of the largest 

 veins into the heart, while the unstriped muscular elements arc 

 very widely distributed, especially in the middle-sized vessels, but 

 also in the larger ones. Their contractile fibre-cells exhibit no 

 peculiarity in the majority of vessels, except that their length 

 hardly ever exceeds o'04'" to o , o6'", and that they are united 

 together, cither directly or by connective tissue and elastic fibrils, 

 into flat bundles and membranes of muscle, more rarely into net- 

 works. In the larger arteries, and in the smallest arteries and 



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