170 



HISTOLOGY 



colored by resorcin-fuchsin and other elastic tissue stains (upper segment 

 in Fig. 165). In smaller arteries the endothelium appears to rest directly 

 upon the elastic network which replaces this 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 tissue 

 is said to be lacking. The inner elastic membrane is not a continuous 

 sheet of tissue, since it is perforated by elongated apertures; it forms a 



&% ;--3BWi- ! ' /: }!: :J& 



This portion is 

 shown enlarged 

 on the left. 



f Endothelium. 

 Intima \ Inner elastic 

 f membrane. 



Media 



Externa 



Fie. 165. A SECTION THROUGH A HUMAN ULNAR ARTERY AND VEIN, SHOWING THE WALL OF THE ARTERY 



ON THB LEFT AND OF THE VEIN ON THE RIGHT. THE UPPER PART OF THE FIGURE (a-d) is FROM A 



SECTION OF THE SAME VESSELS STAINED WITH RESORCIN-FUCHSIN, AN ELASTIC TISSUE STAIN. X 550. 



a, Circular, and b, radial elastic fibers of the media of the artery; c, external elastic membrane; d, elastic 



fibers in the media of the vein; e, circular, and g, longitudinal muscle fibers of the media; f, endothelium. 



fenestrated membrane and the development of such membranes from elastic 

 networks has already been described (cf. Fig. 54, p. 67). The membrane 

 is particularly thick in the larger arteries of the brain, and it is sometimes 

 double. 



The media, which consists of but a single layer of circular muscle 

 fibers in the pre-capillary vessels, becomes many-layered in larger arteries. 

 Generally the fibers are all circular or perhaps oblique, but in the loose 

 musculature of the umbilical arteries, longitudinal fibers are numerous. 

 Longitudinal fibers are said to occur in certain other vessels near theintima, 

 being especially well developed in the subclavian artery. The post-mor- 

 tem contraction of the circular fibers, which throws the intima into folds, 



