294 > SPECIAL HISTOLOGY. 



§215. 



For more easy description, the arteries may be divided, 

 according as the middle tunic is purely muscular, or composed 

 of muscular and elastic fibres intermixed, or else chiefly elastic, 

 into small, medium sized, and large arteries; and the more 

 properly so, because, concomitantly with the variations in the 

 structure of the middle tunic, the external and internal coats 

 present a different conformation, at all events, in many respects. 

 A general characteristic of the arteries is presented in the 

 circumstance, that the middle tunic is very strong, and consists 

 of numerous, regularly disposed lamina, the elements of which 

 observe a transverse direction. In the largest arteries the 

 t. media is yellow, highly elastic, and of great thickness; 

 towards the periphery of the body it gradually diminishes in 

 thickness, and becomes redder and more contractile, until, just 

 before the capillaries are reached, it appears quite thin and sub- 

 sequently inapparent. The whitish /. intima is always much 

 thinner, and varies in thickness within narrower limits, but this 

 is also regulated by the size of the vessel ; whilst on the con- 

 trary, the t. adventitia of the largest arteries is absolutely con- 

 siderably thinner than in those of a medium calibre, in which 

 it often equals the t. media in thickness, or may even exceed 

 it. In a special exposition of these points, it is best to begin 

 with the smallest arteries as the simplest in structure ; with 

 these the others may be afterwards readily compared. 



Arteries under § or V" in diameter, with few exceptions, pre- 

 sent, until close to the capillaries, the following structure (fig. 279). 

 The t. intima consists of only two lamina, an epithelium, and a 

 peculiar, glistening, less transparent membrane, which I shall 

 term the elastic internal tunic. The former contains well 

 marked, fusiform, pale cells with long-oval nuclei, which are 

 readily separated in connexion, in entire fragments, or even in 

 the form of perfect tubes ; but may also be isolated, and then 

 present no small resemblance, on the one hand, with the fusi- 

 form cells of pathologists (also with the formative cells of the 

 elastic fibres and of connective tissue), and on the other with 

 contractile fib re- cells ; from the former, however, they are dis- 

 tinguished by the less attenuation of their extremities and their 

 paleness; and from the latter by their rigidity, by the nuclei never 



