NORMAL HISTOLOGY. 



FIG. 119. 



Muscle-cells isolated from 

 the media of human artery. 



being present ; with the increase in the size of the vessel, however, 

 the quantity of such tissue becomes greater, in addition to which 

 bands of elastic tissue also make their appearance between the 

 muscle-bundles. 



In the large vessels the fibro-elastic tissue forms a considerable 

 portion of the media ; in the aorta the elastic 

 tissue occurs as robust circularly arranged bands, 

 supplemented by oblique and longitudinal tra- 

 beculae of similar nature ; these elastic fibres, 

 together with the accompanying fibrous tissue, 

 constitute the predominating structure, the 

 muscle being less conspicuous in places than 

 the intermuscular fibro-elastic strata. 



Owing to this generous admixture of fibrous 

 tissue, the large arteries, while possessed abso- 

 lutely of a greater amount of elastic tissue, have 

 walls relatively less contractile than those of the 

 smaller arteries, whose media is composed of 

 almost pure muscular tissue. 



The external coat, or adventitia, is the most resistant tunic of 

 the vessel, its characteristic strength being due to the generous 



amount of component fibro-elastic tissue. 

 The fibrous tissue is arranged as closely- 

 felted bundles, irregularly placed and 

 intermingled with longitudinal bands of 

 elastic tissue ; numerous flattened con- 

 nective-tissue cells lie between the bundles 

 applied to the fibrous trabeculae. The 

 mesh-work is closer and the amount 

 of elastic tissue greater next the media 

 than towards the outer surrounding con- 

 nective tissue into which the adventitia 

 insensibly blends. In the larger arteries 

 the middle and outer coats are separated 

 by a band of condensed elastic tissue 

 the external elastic membrane. Cer- 

 tain arteries present peculiarities in 

 their coats ; as examples of such varia- 

 tions may be noted the slight develop- 

 ment of the sub-endothelial tissue of the 

 intima of the external iliac, renal, mesen- 

 teric, and cceliac arteries, the appear- 

 ance of longitudinal muscle-cells within 

 the intima of the aorta, and the presence of longitudinally dis- 



Section of aorta of child : /, M, 

 and A, respectively intima, media, 

 and adventitia. The thick stratum 

 of sub-endothelial tissue and the layer 

 of longitudinally disposed bundles of 

 muscle (t) are peculiarities of the 

 inner coat. 



