STRUCTURE OF BLOOD-VESSELS 



675 



tined particularly for the muscular tissue and, therefore, are directed to the media, 

 although vessels in which muscle is wanting, as in certain veins and the capillaries, 

 are not without nerves. From the plexus that surrounds the vessel, notably rich 

 about the arteries, nerve-fibrillae penetrate the media and end among the muscle- 

 fibres in the manner usual in such tissue (page 1015). Special sensory nerve- 

 endings have been described in both the external and internal tunics. 



The Arteries. — Since the arrangement of the component tissues is most 

 typical in arteries of medium size (from 4-6 mm. in diameter), the radial artery may 

 appropriately serve for description. Seen in cross-section (Fig. 635 j, after the usual 

 methods of preservation and staining, the intima presents a plicated contour as it 

 follows the foldings of the internal elastic membrane that appears as a conspicuous 

 corrugated light band marking the outer boundary of the inner tunic. The lining 

 endothelial cells are so thin that in profile their presence is indicated chiefly by the 

 slightly projecting nuclei. Between the endothelium and the elastic membrane the 



Endothelium 



^^^^^^ Internal elastic 

 '{,r-,-^-^r^^ membrane 



Involuntary muscle 



Elastica 

 Vasa vasorum 



Transverse section of artery of medium size. X 150. 



intima includes a thin layer of fibrous and elastic fibrillae. The media, thick and 

 conspicuous, consists of circularly disposed flat bundles of involuntary muscle sepa- 

 rated by membranous plates of elastic tissue, that in the section appear light and 

 unstained. After the action of selective dyes, as orcein, the elastica is very con- 

 spicuous (Fig, 636). Delicate fibrillae of fibrous tissue course among the musculo- 

 elastic strands. Beneath the outer coat, the elastica becomes condensed into a more 

 or less distinct external elastic membrane that marks the outer boundary of the media. 

 The adventitia varies in thickness, in the medium-sized arteries being relatively better 

 developed than in the larger ones. It consists of bundles of fibrous tissue intermingled 

 with elastic fibres of varying thickness. The adventitia contains the vasa vasorum 

 and chief lymph-channels of the vascular wall. 



Followed towards the capillaries, the coats of the artery gradually diminish in 

 thickness, the endothelium resting directly upon the internal elastic membrane so 

 long as the latter persists, and afterwards upon the rapidly attenuating media. The 

 elastica becomes progressively reduced until it entirely disappears from the middle 



