THE BLOODVESSELS. 



G79 



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- 

 verse-ri^av- which give it, although perfectly homogeneous, a peculiar lon- 

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

 tratcd me?nlrane, as it is termed, with various sized, distinctly marked reti- 



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 lamellae of the 

 t. media of the larger arteries. The m iddletMnc<tf. tne small arteries is 

 purely muscular, without the si igh t es t admix lure of connective tissue 

 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, arid in still smaller ones by boiling and 

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

 fibrc-cells-0-02-0-03 of a line long, and 0-002-0-0025 of a line broad. 

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

 is usuaHy^asTnTck as the t. media or even a little thicker. 



Fia. 279. An artery, a, 0*002, and vein, 6, 0-OG7 of a line in diameter; from the mesen- 

 tery of a child ; treated with acetic acid, and magnified 350 diameters: fl, 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; y>, nuclei of the epithelial cells; <f, 

 elastic longitudinal fibrous membrane. 



