THE VASCULAR SYSTEM. 217 



structure. It is divided by means of elastic fibers and membranes 



Intima. 



: Elastica in- 

 terna. 



x Endothelium of 

 the intima. 



_ ;'-_7~ ] Vasa vasorum. 



Fig. 171. Cross-section of the human carotid artery ; 'X 150. 



(fenestrated membranes) into a large number of concentric layers 

 containing but few muscle-fibers. Here also the tunica media is 

 separated from the intima by an elastic limiting membrane, the 

 fenestrated membrane of Henle, or the lamina elastica interna. In 

 the aorta this membrane as such is not recognizable. The intima 

 presents three distinct layers the inner composed of flattened endo- 

 thelial cells, and the other two consisting chiefly of elastic tissue 

 (fibrous layers). Of these latter the inner is the richer in cellular 



Endothelium of the 



intima. 

 "Intima. 



Media. 



Adventitia with 

 nonstriated mus- 

 cle-fibers in cross- 

 section. 



Fig. 172. Section through human artery, one of the smaller of the medium-sized ; X 640. 

 elements and has a longitudinal arrangement of its fibers, while the 



