682 



SPECIAL HISTOLOGY. 



coat of the largest arteries, yet to be described. Lamellae of the former 

 character are found in the inner layers of the t. media of the aa. cru- 

 ralis, mesenterica superior, cceliaca, iliaca externa^bracJiialis, and in 

 the external and internal carotids ; whilst they occur in a remarkable 

 manner in the commencement of the tibialis antica and postiea, and in 

 the popliteal artery throughout the entire middle tunic ; they are par- 

 ticularly well developed in the last-named vessel, which has also usually 

 somewhat thicker walls than the crural. 



From the conditions of the t. media above stated, and in other re- 

 spects also, the transition from the medium-sized to the largest arteries 

 is rendered extremely gradual. With respect to the t. intima, its epi- 

 thelial cells in the latter are usually no longer so much elongated as in 

 the smaller vessels, though still retaining their fusiform figure and a 

 length of 0-006-0*01 of a line. The rest of this tissue does not neces- 

 sarily increase in amount with the size of the vessel, although it exhibits, 

 especially in the aorta, a great proneness to become thickened, so that 

 it is often difficult to determine its normal thickness. As regards its 



structure, it consists chiefly of lamella; of a clear, sometimes homogeneous, 

 sometimes striated, or even distinctly fibrillated substance, presenting, 

 for the most part, the characters of connective tissue (Eulenberg ob- 

 tained 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 membrane ceases, either 

 with an elastic close network or a true fenestrated more or less fibrous 

 membrane, obviously corresponding with the elastic inner membrane of 

 the small arteries. Immediately beneath the epithelium the elastic net- 

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



FIG. 282. Transverse section of the aorta below the superior mesenteric artery. 1, t. 

 intima : 2, t. media ; 3. t. adventitia : a, epithelium ; />, striped lamellce ; c, elastic membrane 

 of the intima; d, elastic lamellae of the t. media; e, its muscles and connective tissue; 

 /, elastic networks of the t. adventitia. From Man. Magnified 30 diameters, treated with 

 acetic acid. 



