THE BLOOD-VESSELS. 



295 



having the rod-like form and by their chemical reactions. The 

 elastic tunic is, on the mean 0001"' thick, and in the living 



subject is smoothly stretched beneath the epithelium, whilst in 

 empty arteries it almost always presents a greater or less number 

 of usually strong folds, and frequently also, numerous, fine 

 transverse rugae, which give it, although perfectly homogeneous, 

 a peculiar longitudinally striated aspect ; in addition, it appears 

 almost always as a fenestrated membrane, as it is termed, with 

 various sized, distinctly marked reticulated fibres, and usually 

 minute elongated openings; more rarely as a true but very 

 close network, of chiefly longitudinal elastic fibres, with narrow, 

 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 middle tunic 

 of the small arteries is purely muscular, without the slightest 

 admixture of connective tissue and elastic elements, and is 

 stronger or weaker according to the size of the vessel (down to 



Fig. 279. An artery, a, O062'", and vein, b, 0067'" in diam., from the mesentery 

 of a child, treated with acetic acid, x 350 diam.: a, tunica adventitia, with elongated 

 nuclei ; j3, 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 ; 

 9, elastic longitudinal fibrous membrane. 



