THE BLOOD-VESSELS. 297 



than 001"' or even of 0007'" ; and it may at the same time 

 be remarked, that its cells can no longer be isolated, and its 

 presence only be recognised from the closely placed elliptical 

 nuclei. 



Medium-sized arteries above §'" or V" , up to those of 2"' and 

 3'" in diameter, at first, present no great alterations in the 

 external and internal tunics, whilst the t. media is not only 

 always increased in thickness in 

 proportion to the size of the 

 vessel (from 0'05— 0'12 ,,/ ), but 

 is also changed in structure. 

 For now, in addition to muscu- 

 lar layers, the number of which 

 constantly increases, but the 

 elements of which are precisely 

 the same as before, we observe fine elastic fibres, which, united 

 into wide-meshed networks, run, at first singly and with little 

 regularity among the muscular elements ; but in larger vessels 

 of this category are accompanied by a certain amount of con- 

 nective tissue, and here and there exhibit a disposition to form 

 special layers alternating with the muscular, though not pre- 

 senting the characters of a continuous network throughout the 

 t. media. Thus, although the t. media is deprived of its 

 eminently contractile structure, it must be allowed that the 

 muscular fibres here also still retain a considerable pre- 

 ponderance. The t. intima in the medium-sized arteries has, 

 not unfrequently, betw r een the elastic inner membrane and the 

 epithelium, several other layers, among which the above-de- 

 scribed striped lamellae are the most remarkable. These lamellae, 

 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 /. 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 



Fig. 281. Transverse section of the art. profunda femoris of Man, x 30 diam.: 

 a, t. intima, with the elastic layer (the epithelium is not perceptible) ; b, t. media, 

 without elastic lamella, but with fine elastic fibres ; c, t. adventitia, with elastic net- 

 works and connective tissue. 



