THE BLOODVESSELS. 



679 



thick, and in the living subject is smooth!}^ 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 ruga', which give it, although perfectly homogeneous, a peculiar lon- 

 gitudinallystriated aspect ; in addition, it appears almost always as a, 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 tunic of the small arteries is 

 purely muscular, without the slightest admixture of connective tissue 

 and elastic elements, and is strono;er or weaker accordino; 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 nnited 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 

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

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

 is usually as thick as the t. media or even a little thicker. 



Fig. 279. — An artery, o, 0-0G2, and vein, b, 0-0G7 of a line in diainoter; from the mesen- 

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

 with elongated nurlei ; 0, 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; iT, 

 elastic longitudinal fibrous membrane. 



