290 



SPECIAL HISTOLOGY. 



extensively distributed tissues assume very different forms. 

 With respect to the arrangement and subdivision of these 

 tissues, they may be said to exhibit a very strong tendency to 

 lamination and, in the different layers, to the assumption of a 

 constant direction in the course of their constituent elements. 

 The former of these dispositions, however, rarely extends to the 

 actual isolation of the individual layers ; and to the latter, 

 though more rarely, there are also exceptions. The tunica 

 intima is the thinnest of the membranes of the vessels, and 

 always consists of a cellular layer, the epithelium; most usually 

 also of an elastic membrane in which a longitudinal direction of 

 the fibres predominates; to which again may be superadded 

 other layers of one kind or another, which also almost invari- 

 ably retain the longitudinal direction. The t. media is for the 

 most part a thick layer, and is especially the seat of the trans- 

 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 quantity. The t. adventitia, lastly, 

 again exhibits a preponderating longitudinal fibrillation, is as 

 thick as or thicker than the t. media, and 

 consists, for the most part, only of con- 

 nective tissue and elastic networks. 



If the separate tissues of the vascular 

 tunics are investigated somewhat more 

 closely, it will be seen that the connective 

 tissue appears almost universally fully de- 

 veloped, with fine and coarse bundles 

 and distinct fibrils. It is only in the smallest 

 arteries and veins that it is replaced by a 

 nucleated, indistinctly fibrous tissue, and 

 ultimately passes into a perfectly homo- 

 geneous, still occasionally nucleated, deli- 

 cate 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. 62), up to the nar- 

 rowest, closest, membraniform interlacements of these fibres, all 



Fig. 277. 



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

 with an indication of fibrous networks, x 350 diam. 



