CH. XVIII. j 



THE VEINS. 



215 



Vasa Vasonun. 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 

 rsympathetic nerves, which twine around the vessel very much 



Endothclium. 

 Sub-endothelial layer. 

 Elastic intima. 





Middle coat. 



Fig. 217. Transverse 



ion uf norta tliruugh internal and about half 

 the middle coat. 



Hike ivy round a tree : and ganglia are found at frequent in- 

 tervals. They terminate in a plexus between the muscular 

 -fibres (fig. 218). 



The Veins. 



Distribution. The venous system begins in small vessels which 

 are slightly larger than the capillaries from which they spring. 

 These vessels arc gathered up into larger and larger trunks until 

 they terminate (as regards the systemic 'circulation) in the two 

 venro cavrc and the coronary veins, which enter the right auricle, 

 and (as regards the pulmonary circulation) in four pulmonary 



