ARTERIES 



169 



B, the muscle fibers form a single but continuous layer, the media, out- 

 side of which the connective tissue is compressed to make the externa. 

 Its fibers tend to be parallel with the vessel. The walls of such an artery 

 are so thick that it is possible to focus on the layers separately; thus in A, 

 the endothelium, which with a delicate elastic membrane beneath it con- 

 stitutes the intima, is not seen, being out of focus. The nuclei of the 



. . .. 



"H^ft 



v- 



FIG. 163. FRAGMENTS OF HUMAN ARTERIOLES. X 240. 



i, Nuclei of endothelial cells; m, nuclei of circular muscle fibers; a, nuclei of connective tissue. 

 In A, since the endothelium is out of focus, its nuclei are not seen. 



media and externa are evident. A cross section of such a vessel is seen 

 in Fig. 177. 



The larger arteries are lined with endothelium similar to that of the 

 capillaries, as shown in silver nitrate preparations (Fig. 164). This endo- 

 thelium rests on a layer of connective tissue containing flattened cells 

 and a network of fine elastic fibers. The meshes of the fibrous and elastic 



Endothelial cell. 

 \ 



Indentations made by smooth muscle fiber. 



FIG 164. ENDOTHELIUM OF A MESENTERIC ARTERY OF A RABBIT. SURFACE VIEW. X 250. 



tissue are elongated lengthwise of the vessel, and on surface view they 

 present a longitudinally striped appearance, f In addition to this subendo- 

 thelial tissue and the endothelium, the intima includes the inner elastic 

 membrane (Fig. 165). This is usually a conspicuous layer thrown into 

 wavy folds by the post-mortem contraction of the vessel. It is easily 

 seen with ordinary stains, appearing as a refractive layer, and is deeply 



