THE CIRCULATORY OR VASCULAR SYSTEM 9$ 



The Structure of Arteries. The arteries are com- 

 posed of three coats an external, a middle, and an 

 internal. 



The external coat is firm and strong, and is made up of 

 elastic and areolar tissues containing longitudinal and 

 oblique fibers. It is of tougher texture than the middle 

 coat. 



The middle coat is a thick, circular coat which contains 

 muscular tissue, the fibers of which are arranged in rings 

 that vary in number according to the size of the artery. 

 They contain some elastic tissue, but this is lost in the 

 smaller arteries; in the larger ones the elastic tissue is 

 blended with the areolar tissue. 



Endothelium of the 



intima. 

 ' Intima. 



Media. 



A dven titia with 

 non-striated mus- 

 cle-fibers in cross- 

 section. 



FIG. 50. Section through human artery, one of the smaller of the medium- 

 sized (X 640) (Bohm and Davidoff). 



The internal coat is a thin, endothelial layer on the in- 

 side, and has an elastic exterior. In the large arteries 

 there is an intermediate layer composed of areolar tissue. 



From what has been said it will be seen that, considered 

 from without inward, the external coat consists of areolar 

 and elastic tissues; the middle coat, of smooth muscular 

 fiber and areolar and elastic tissues; the internal coat, of 

 elastic tissue and endothelium. 



The walls of the capillaries are composed of only one 

 coat, which is thin, transparent, and composed of tissue 

 of light structure, which is lost in the elastic membrane of 



