THE ARTERIES. 



309 



their elasticity. In the arteries of medium size that is, those 

 between the carotids and those having a diameter of about 2 mm. 

 the amount of muscular tissue is very much increased, while the 



Endothelium of the 



intima. 

 " Intima. 



Media. 



Adventitia with 

 non-striated mug- 

 cle-fibers in cross- 

 section. 



FIG. 160. Section through human artery, one of the smaller of the medium-sized ; 

 X 640 (Bohm and Davidoff ). 



elastic tissue is also well represented. Such arteries possess, there- 

 fore, both contractility and elasticity. In the small arteries that 

 is, those less than 2 mm. in diameter the external coat gradually 



Intima. 

 Elastica in- 

 v terna. 



Endothelium of 

 the intima. 



m. > Media. 



Fenestrated 



?? | elastic mem- 

 brane. 



Elastica ex- 



terna. 



Inner layer of 

 adventitia. 



Outer layer of 

 adventitia. 



~- Vasa vasorum. 



FiG. 161. Cross-section of the human carotid artery ; X 150 (Bohm and Davidoff). 



disappears until in the arterioles there remains only muscular 

 tissue, representing the middle coat and the internal coat. These 

 vessels are endowed with the property of contractility. 



