skit. 214.] MEDIUM-SIZED ARTERIES. 489 



taken along with the fine clastic networks situated further 

 outwards in a homogeneous, granular, or fihrillated connective 

 substance, form a middle layer in the tunica intima of o - oo6'" to 

 0-05' " in thickness, all of whose elements run together in the 

 longitudinal direction, and are thus easily distinguished from the 

 muscular layer of the tunica media, which, in some respects, has 

 a resemblance to them. — Lastly, the tunica adventitia of these 

 middle-sized arteries measures almost always more than the middle 

 coat, and increases from o"05'" to 016'" in thickness. Its elastic 

 fibres become, at the same time, thicker and thicker; even in 

 vessels of one line in diameter a thick collection of them can be 

 recognised, just at the line where the external coat forms its well- 

 deli ned boundary from the middle tunic. This elastic coat of the 

 tunica adventitia becomes very beautiful in the largest vessels of 

 this class, such as the external and internal carotids, the crural and 

 brachial arteries, the profunda femoris, the mesenteric and coeliac 

 arteries. In these, it measures CC013"' to o"04'" and upwards, and 

 is partly made up of very distinct lamellae, whose structure is often 

 closely allied to that of the true elastic membranes. Moreover, the 

 outer lavers of the tunica adventitia also contain elastic networks ; 

 but these elements are here somewhat finer and form no lamella?, but 

 are more irregularly connected with each other. — The larger of 

 the medium-sized arteries already present an approximation to the 

 largest arteries in the structure of their tunica media ; the elastic 

 network of this coat occasionally forms actual lamellce of some 

 thickness, and, more rarely, even two elastic membranes; but 

 these are still connected with each other throughout the whole 

 thickness of the middle coat, and thus they are best distinguished 

 from the elastic plates, which will be presently described as existing 

 among the circular fibres of the middle coat of the larger arteries. 

 The first indication of these lamellae appears in the inner portion 

 of the tunica media of the crural, superior mesenteric, casliac, 

 external iliac and brachial arteries, and in the external and internal 

 carotids; whilst at the commencement of the tibialis antica and 

 postica, and in the popliteal, they occur throughout the whole 

 middle coat; they are, indeed, very beautifully developed in the 

 latter artery, which has also usually somewhat thicker walls than 

 the crural. 



In the manner here described, then, and by intervening forms 

 of tissue occurring in the other coats, the transition from the 

 medium-sized to the largest arteries is very gradually effected. 

 We will now examine the structure of the main arterial trunks of 



