THE BLOODVESSELS. 



679 



thick, and in the living subject is smoothly stretched beneath the epithe- 

 lium, whilst in empty arteries it almost always presents a greater or less 

 number of usually strong folds, and frequently also, numerous, fine trans- 

 verJS.e.rwgwBy, which give it, although perfectly homogeneous, a peculiar lon- 

 gitudinally striated aspect ; in addition, it appears almost always as &fenes- 

 trated membrane, as it is termed, with various sized, distinctly marked reti- 



Fig. 279. 



culated fibres, and usually minute elongated openings; more rarely as a 

 true but very close network, of chiefly longitudinal elastic fibres, with nar- 

 row, elongated fissures, and completely corresponds in aspect, in its 

 great elasticity, and its chemical reactions, with the elastic lamellce of the 

 t. media of the larger arteries. The \ middle fojwgof the small arteries is 

 purely muscular, without the sligntest admixture of connective tissue i 

 and elastic elements, and is stronger or weaker according to the size of 

 the vessel (down to 0-03 of a line). In vessels of 1-10 of a line in dia- 

 meter, the fibre-cells, which are united into lamellce^ may be pretty 

 readily isolated by dissection, and in still smaller ones by boiling and 

 maceration in nitric acid of 20 per cent., when they appear as delicate 

 fibre-cells-0-02-0-03 of a line long, arid 0-002-0-0025 of a line broad. 

 The t.^adventitia consists of connective tissue and fine elastic fibres, and 

 is usuaHyliJTnick as the t. media or even a little thicker. 



FIG. 279. An artery, a, 0-002, and vein, 6, 0-007 of a line in diameter; from the mesen- 

 tery of a child ; treated with acetic acid, and magnified 350 diameters: a, tunica advenlitia, 

 with elongated nuclei; $, nuclei of the contractile fibre-cells of the t. media, viewed in part 

 on the flat surface, in part in apparent transverse section ; ^, nuclei of the epithelial cells ; f, 

 elastic longitudinal fibrous membrane. 



