128 HISTOLOGY. 



by occasional smooth muscle fibers. In Fig. 152, C, the oval nuclei of 

 the endothelium are seen to be elongated parallel with the course of the 

 vessel. As is usually the case, the walls of the endothelial cells are not 

 visible. The rod shaped nuclei of the muscle fibers are at right angles 

 with the axis of the vessel. In the somewhat larger artery, B, the muscle 

 fibers form a single but continuous layer, the media, outside of which the 

 connective tissue is compressed to make the externa. Its meshes 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 endothe- 

 lium which with a delicate elastic membrane beneath it constitutes the 

 intima, is not seen, being out of focus. The nuclei of the media and ex- 

 terna are evident. 



The structure of the larger arteries is illustrated by the cross section, 

 Fig. 154. The intima consists of endothelium resting on a layer of con- 

 nective tissue containing flattened cells and a network of fine elastic fibers. 



Endothelial cell. Indentations made by smooth muscle fibers. 



FIG. 153. ENDOTHELIUM OF THE MESENTERIC ARTERY OP A RABBIT. SURFACE VIEW. X 260. 



The meshes of the fibrous and elastic tissue are elongated lengthwise 

 of the vessel and on surface view they present a longitudinally striped 

 appearance. Toward the media, the intima contains a conspicuous inner 

 elastic membrane which is fenestrated and usually thrown into longitudinal 

 folds. (Fenestrated membranes have been described on page 42). In 

 the smaller arteries (those under 2.8 mm. in diameter) the endothelium 

 rests directly upon the inner elastic membrane; and in such large ones 

 as the external iliacs, the principal branches of the abdominal aorta, and 

 the uterine arteries in young persons, the subendothelial connective 

 tissue is said to be lacking. The inner elastic layer is very thick in the 

 larger arteries of the brain, and may be double. 



In the media the number of layers of circular smooth muscle fibers 

 increases from the precapillary vessels which have but one, to large arteries 

 like the brachial which have many. Sometimes the media near the intima 

 contains a few longitudinal fibers; these have been reported in the sub- 



