THE BLOODVESSELS. 



675 



verse elements and of the muscles, although in the veins it also contains 

 numerous longitudinal fibres, and in all the larger vessels presents, in 

 addition, elastic elements, and connective tissue in greater or less quan- 

 tity. The t. adventitia, lastly, again exhibits a preponderating longi- 

 tudinal fibrillation, is as thick as or thicker than the t. media, and con- 

 sists, for the most part, only of connective tissue and elastic net- 

 works. 



If the separate tissues of the vascular tunics are investigated some- 

 what more closely, it will be seen that the connective tissue appears 

 almost universally fully developed, with fine and coarse bundles and 

 distinct fibrils. It is only in the smallest arteries and veins that it is 



Fig. 277. 



. 



replaced by a nucleated, indistinctly fibrous tissue, and ultimately 

 passes into a perfectly homogeneous, still occasionally nucleated, delicate 

 membrane. The elastic tissue nowhere presents such manifold forms as 

 it does in the vessels. From wide-meshed, lax networks of the finest, 

 middle-sized, and thickest fibres (Fig. 22, p. 81), up to the narrowest, 

 closest, membraniform interlacements of these fibres, all transitionary 

 forms are here met with ; and besides this, we may notice every degree of 

 transformation of the latter, or of the elastic reticulated membranes into 

 true elastic membranes, which either betray their derivation, in an elastic, 

 more or less indistinct, fibrous network pervading them, and distant 

 openings, or are transformed, either partially or entirely, into perfectly 

 homogeneous plates, exhibiting more or fewer openings (Fig. 25, p. 82). 



FIG. 277. Elastic membrane from the tunica media of the popliteal artery in Man, with 

 an indication of fibrous networks ; magnified 350 diameters. 



FIG. 278. Muscular fibre-cells from the human arteries, magnified 350 diameters. 1, 

 from the popliteal artery ; cr, without, 6, with acetic acid. 2, from a twig, a line in 

 diameter, of the anterior tibial artery : a, nuclei of the cells. 



