130 



HISTOLOGY. 



Endothelium. 



Connective 

 tissue. 





Z~ ] 3 



Smooth 

 muscle fibers. 



Elastic fibers. 



-~L --"~ 4 ) "*" 



The externa consists of connective tissue, which is denser and contains 

 more elastic fibers in its inner portion. A prominent layer of elastic tissue 

 near the media is called the outer elastic membranes, and is especially well 

 developed in the carotid, brachial, femoral, coeliac, and mesenteric arteries. 

 It is absent from the basilar artery and most of those within the skull. 

 Sometimes the externa contains scattered bundles of longitudinal muscle. 

 In the larger vessels it contains small nutrient blood vessels, the vasa 



vasorum. These may 

 penetrate the outer 

 part of the media. 

 Lymphatic vessels 

 often accompany the 

 blood vessels and have 

 branches in the ex- 

 terna* Their deeper 

 penetration is doubt- 

 ful, although they 

 have been reported in 

 the intima of certain 

 large v es s e 1 s . Sen- 

 sory nerves may ter- 

 minate in the externa 

 with free endings or 

 in lamellar corpus- 

 cles, the latter being 

 numerous in the ab- 

 dominal aorta ; free 

 sensory endings are 

 also found in the in- 

 tima. The vaso-motor 

 nerves are non-medul- 

 lated sympathetic 

 fibers which form plex- 

 with the muscle fibers. 



Elastic fibers. 



Connective 

 tissue. 



FIG. 155. FROM A CROSS SECTION OF THE THORACIC AORTA OF 

 MAN. X 100. 



uses in the media and terminate in contact 

 These plexuses are said not to contain ganglion cells. 



The largest arteries, the pulmonary and the aorta (Fig. 155), have 

 a broad intima which increases in thickness with age. It consists of an 

 endothelium of cells less elongated than those of smaller arteries, resting 

 on fibrillar connective tissue with flattened round or stellate cells. Its 

 elastic fibers are broader toward the media, but there is no distinct inner 

 elastic membrane. The media consists of very many concentric elastic 



