THE CIRCULATORY SYSTEM 



161 



tissue in the media (Fig. 97), which makes indistinct the boundaries 

 between intima and media, and between media and adventitia. The 

 walls of the aorta are thin in proportion to the size of the vessel, in- 

 creased strength being obtained by the decided increase in the amount 

 of elastic tissue. Of the intima, the endothelial cells are short and 

 polygonal; the intermediary layer similar to that of a medium-sized 

 artery; the elastic layer less distinct and often broken up into several 





Fig. 97. — From Transverse Section of Dog's Aorta, to show Elastic Tissue. 

 X60. (Technic 7, p. 165.) Elastic tissue stained black, a, Intima; h, media; c, 

 adventitia. 



thin layers. The media consists mainly of elastic tissue arranged 

 in circular plates or fenestrated membranes. Between the elastic- 

 tissue plates are groups of smooth muscle cells and some fibrillated 

 connective tissue. The advejititia resembles that of the medium-sized 

 artery. There is no external elastic membrane. 



Certain arteries have structural peculiarities. The arteries of the brain and 



cord are thin-walled in proportion to their calibre, the inner elastic membrane 



is especially well defined, and there are few elastic fibres in the media. In the 



renal, coeliac, mesenteric, and external iliac arteries there is little or no connective 



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