218 



THE CIRCULATORY SYSTEM 



[CH. XIX. 



amount of resistance from friction. Immediately external to the 

 endothelial lining of the artery is fine connective tissue (sub- 

 endothelial layer) with branched corpuscles. Thus the internal coat 

 consists of three parts, (a) an endothelial lining, (5) the subendo- 

 thelial layer, and (c) elastic layer. 



Vasa Vasorum. The walls of the arteries are, like other parts 

 of the body, supplied with little arteries, ending in capillaries and 

 veins, which, branching throughout the external coat, extend for 

 some distance into the middle, but do not reach the internal coat. 

 These nutrient vessels are called vasa vasorum. 



Nerves. Most of the arteries are surrounded by a plexus of 

 sympathetic nerves, which terminate in a plexus between the 

 muscular fibres. 



Endothelium. 

 Subendothelial layer. 

 Elastic layer. 



Middle coat. 



FIG. 199 Transverse section of aorta through the internal and about half the middle coat. 



The Veins. 



The venous system begins in small vessels which are slightly 

 larger than the capillaries from which they spring. These vessels 

 are gathered up into larger and larger trunks until they terminate 

 (as regards the systemic circulation) in the two venae cavae and the 



