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HANDBOOK OF PHYSIOLOGY. 



which make it smooth and polished, and furnish a nearly impermeable 

 surface, along which the hlood may flow with the smallest possible 

 amount of resistance from friction. 



Immediately external to the endothelial lining of the artery is fine 

 connective tissue, the sul-cndothelial layer, with branched corpuscles. 

 Thus the internal coat consists of three parts, (a) an endothelial lining, 

 () the sub-endothelial layer, and (c) elastic layers. 



Vasa Vasorum. The walls of the arteries, with the exception of 



Endothelium. 

 Sub-endothelial layer, 

 i Elastic intima. 





Middle coat. 



Fig. 155. Transverse section of aorta through internal and about half the middle coat. 



the endothelial lining and the layers of the internal coat immediately 

 outside it, are not nourished by the blood which they convey, but 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 sympa- 

 thetic nerves, which twine around the vessel very much like ivy round 

 a tree: and ganglia are found at frequent intervals. The smaller arter- 

 ies are also surrounded by a very delicate network of similar nerve-fibres, 

 many of which appear to end near the nuclei of the transverse muscular 

 fibres (fig. 156). 



