298 SPECIAL HISTOLOGY. 



/. media in thickness, attaining 0*05 — 0*1 6'". Its elastic 

 fibres at the same time become stronger and stronger, and 

 even in vessels 1"' in diameter, a considerable aggregation 

 of them, where it adjoins the t. media may be perceived ; the 

 line of demarcation between the two tunics, being in all these 

 arteries extremely well defined. This elastic membrane of the 

 t. adventitia is extremely well marked in the largest of the 

 vessels belonging to the class now under consideration, as in 

 the external and internal carotids, the crural, brachial, profunda 

 femoris, mesenteric, and cseliac, in which it measures from 

 0*01 3'" to as much as O'Oi'", and more, and is to some extent 

 very distinctly laminated, the structure of the lamella very 

 often closely approaching that of the true elastic membranes. 

 Besides this, the external layers of the t. adventitia also contain 

 elastic networks, only that the elements of the latter are some- 

 what finer and constitute minute lamellae, but are conjoined 

 with less regularity. The largest of the medium-sized arteries 

 exhibit an approach to the largest arteries, inasmuch as, in 

 their t. media certain portions of the elastic networks con- 

 stitute somewhat stronger elastic lamellae, which, however, are 

 continuous through the entire thickness of the tunic, and also, 

 more rarely, form true elastic membranes, by which they 

 are distinguished from the elastic plates of the annular fibrous 

 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. cruralis, mesenterica superior, caeliaca, iliaca 

 externa, brachialis, and in the external and internal carotids ; 

 whilst they occur in a remarkable manner in the commence- 

 ment of the tibialis antica and postica, and in the popliteal 

 artery throughout the entire middle tunic; they are particularly 

 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 respects also, the transition from the medium-sized to 

 the largest arteries is rendered extremely gradual. With re- 

 spect to the /. intima, its epithelial 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'". The rest of this tissue does not necessarily in- 

 crease in amount with the size of the vessel, although it 



