THE GENERAL CIECULATION. 93 



The arteries, in addition to their three coats, are supplied 

 with nutrient vessels, vasa vasorum, as well as nerves from 

 the neighboring parts. They are also surrounded by a cel- 

 lular covering called the sheath of the artery. 



The veins consist of the same number of coats as the ar- 

 teries, though much more delicate and weak in their struc- 

 ture, and readily collapse when empty or divided. The 

 middle coat is decidedly muscular at the entrance of the 

 larger veins into the heart, and the difference between this 

 coat and that of the arteries is, that while the fibres are 

 chiefly circular in the arteries, they are mainly longitudinal 

 in the veins, which distinction in the arrangement of the 

 muscular fibres of these two kinds of vessels, has been con- 

 sidered a fundamental and essential point in the physiology 

 of circulation. 



The inner coat of veins is also serous, and continuous 

 with that lining the cavity of the right heart. Its great 

 peculiarity is in having valves formed by its duplication 

 whose free edge looks towards the heart; they consequently 

 favor the onward circulation of the blood, while they as 

 effectually hinder its retrograding. 



Each valve has a semi-circular shape, is connected by its 

 convex edge to the interior surface of the vein, which, being 

 dilated at this point, presents a knotted appearance. These 

 valves are most numerous in the extremities, and more abun- 

 dant in the superficial than the deep-seated veins. They 

 exist generally in pairs three have been found together, 

 and sometimes there is only a single one, as at the 

 mouths of the coronary vein, vena azygos, and vena-cava 

 ascendens. They are absent in the large trunks, as the 

 venaB cavse, venaa innominatae, internal jugulars, iliacs, 

 portal veins, and the sinuses of the brain. 



The veins have their vasa vasorum, like the arteries. 

 They also possess the properties of contraction and expan- 

 sion, and are, in some measure, elastic. 



