THE BLOODVESSELS. 



G81 



Fv'. 2S1. 



arc accompanied by a certain amount of connective tissue, and here and 

 there exhibit a disposition to form 

 special layers alternating with the 

 muscular, though not presenting the 

 characters of a continuous network 

 throughout the t. media. Thus, al- 

 though the t. media is deprived of 

 its eminently contractile structure, 

 it must be allowed that tlie mus- 

 cular fibres here also still retain a considerable preponderance. The 

 t. intima in the medium-sized arteries has, not unfrequently, between 

 the elastic inner membrane and the epithelium, several other layers, 

 among which the above described striped lamellce are the most remark- 

 able. These lamella', composed of fine elastic networks, wider towards 

 the exterior, and lying in a homogeneous, granular or fibrillated con- 

 nective substance, constitute the middle lamina of the t. intima, the 

 elements of which also all run longitudinally, and are for that reason 

 readily distinguishable from the muscular layers of the t. media, to 

 which in some respects they bear a resemblance. The t. adventitia, 

 lastly, in almost all these arteries, exceeds the t. media in thickness, at- 

 taining 0-05-0-16of a line. Its elastic fibres at the same time become 

 stronger and stronger, and even in the vessels 1 line 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 belong- 

 ing to the class now under consideration, as in the external and internal 

 carotids, the crural, brachial, |?ro/^(n'^a/e7?^om, mesenteric, and creliac, 

 in which it measures from 0-013 to as much as 0-04 of a line, and more, 

 and is to some extent very distinctly laminated, the structure of the 

 lamellce very often closely approaching that of the true elastic mem- 

 branes. Besides this, the external layers of the t. adventitia also con- 

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

 what finer and constitute minute lamella', but are conjoined with less 

 regularity. TJie largest of the medium-sized arteries cxhxh'xt an approach 

 to the largest arteries, inasmuch as, in their f. media certain portions 

 of the clastic networks constitute somewhat stronger elastic lamella;, 

 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 



Fig. 281. — Transverse section of the art. profunda fcmoris of ^^an,magmCled 30 diameters: 

 a, t. intima, whh the elastic layer (the cpilhelium is not i^erceptible) ; b, t. media, v.' iihout 

 elastic la7tieUcB, hut whh fine elastic fibres; c,t. adventitia, wiih elastic networks and con- 

 nective tissue. 



