THE BLOOD-VESSELS. 



299 



exhibits, especially in the aorta, a great proneness to become 

 thickened, so that it is often difficult to determine its normal 



Fig. 282. 



thickness. As regards its structure, it consists chiefly of lamellae 

 of a clear, sometimes homogeneous, sometimes striated, or even 

 distinctly fibrillated substance, presenting, for the most part 

 the characters of connective tissue (Eulenberg obtained some 

 gelatin from the t. intima), and is pervaded by finer and coarser 

 longitudinal elastic networks. Usually these networks become 

 more and more close from within to without, and their elements 

 stronger, and on the side towards the t. media the inner mem- 

 brane ceases, either with an elastic close network or a true 

 fenestrated more or less fibrous membrane, obviously corres- 

 ponding with vhe elastic inner membrane of the small arteries. 

 Immediately beneath the epithelium the elastic networks are 

 either very fine, or are replaced by one or several clear layers 

 ■ — the striped lamella, — which, when nucleated, often appear to 

 consist of coalesced epithelial cells; and when homogeneous 

 and without nuclei, to approach pale elastic membranes. In 

 the annular fibrous layer, the largest arteries present, as a new 

 element, peculiar elastic membranes or plates, which, except in 

 the transverse disposition of their fibres, are constituted in all 

 essential respects in the same way as the elastic inner membrane, 

 particularly of the smaller arteries, and sometimes appear as very 

 thick networks of strong elastic fibres, sometimes as true fenes- 

 trated membranes with a less evident fibrous structure. These 



Fig. 282. Transverse section of the aorta below the superior mesenteric artery. 

 1, t. intima; 2, t. media; Z, t. adventitia ; a, epithelium; b, striped lamellae; c, 

 elastic membrane of the intima ; d, elastic lamellae of the t. media ; e, its muscles 

 and connective tissue ; f, elastic networks of the t. adventitia. From Man, x 30 

 diam., treated with acetic acid. 



