CIRCULATORY SYSTEM. 



coarser vascular system of the outer layer, never 

 directly with the blood within the artery through 

 the intima. Of course, nerve endings are found, 

 which can only be demonstrated in specially pre- 

 pared sections. 



3. Tunica Adventitia. This is the outer layer, 



- Endolhelium. 

 Subendothelium. 

 Fenestraied mem- 

 brane oj Henle. 



Vasa vasorum. 



Fig- 77- Cross section of aorta. 



and is made up of a loose arrangement of tissues, and, 

 taken as a whole, is less definitely defined than the 

 media. It varies in thickness according to the lo- 

 cation of the artery, but, as a rule, it is not so wide 

 as the media; while in the media the connective- 

 tissue fibers are arranged in sheets which interlace 

 with any muscle that may be present, in the adven- 

 titia the fibers form diagonal bundles, mostly of the 



