172 



HISTOLOGY 



predominates. On section the wall of the fresh aorta consequently 

 appears yellow, and not reddish like the more muscular walls of smaller 

 arteries. The elastic tissue is arranged in a succession of circular fenes- 

 trated membranes connected with one another by oblique fibers. Be- 

 tween them are the muscle cells. According to Koelliker, in the inner 

 layers of the media, the muscle cells form an anastomosing syncytium 



of short, broad and flattened ele- 

 ments, somewhat resembling car- 

 diac muscle (Fig. 167), but in the 

 outer layers the fibers are of the 

 ordinary type. The externa con- 

 tains no outer elastic layer and is 

 relatively thin; its inner elastic 

 portion may have been taken over 

 into the media. 



VEINS. Since the artery to 

 any structure and the returning 

 vein are often side by side, they 

 are frequently included in a single 



section and may readily be com- 

 pared. In embryos the veins are 

 of much larger diameter than the 

 corresponding arteries, and they 

 have thinner walls. Although 



Nerv 



Vasa vasorum. 



FIG. 166. FROM A TRANSVERSE SECTION OF THE 

 HUMAN THORACIC AORTA, STAINED WITH RE- 

 SORCIN-FUCHSIN. X 80. 



a, Endothelium; b, subendothelial fibrous tissue; 

 c, d, elastic membranes of the media. 



FIG. 167. BRANCHED SMOOTH MUSCLE 

 CELLS FROM THE THORACIC AORTA OF 

 A CHILD AT BIRTH (a) AND AT FOUR 

 YEARS (b). (After Koelliker.) 



the difference in diameter is less marked in the adult, it generally remains 

 a distinctive feature (Fig. 177, p. 186), and the difference in the thickness 

 of the walls becomes accentuated (Fig. 165). In comparing the diameters 

 of the ulnar vein and artery in Fig. 165, it should be remembered that the 

 ulnar artery is usually accompanied by two returning veins, only one of 

 which is shown in the figure. Because of their thinner walls, which con- 

 tain relatively little elastic tissue, the veins are generally partly collapsed ; 

 the lumen is therefore irregular, whereas that of the arteries tends to be 



