THE VASCULAR SYSTEM. 



219 



made up of a few circularly disposed muscular fibers. The adven- 

 titia is composed of a small quantity of connective tissue, and con- 

 tains no vasa vasorum. 



(<) Veins. In the foregoing account of the structure of the 

 arteries we have described the structure of their walls according to 

 the caliber of the vessels. Such a differentiation in the case of the 

 veins would be impossible, since sometimes veins of the same cali- 

 ber present decided differences in structure in various parts of the 

 body. 



For the sake of convenience, we will commence with the de- 

 scription of a vein of medium size. Its intima consists of three 

 layers : (i) Of an inner layer of endothelium ; (2) of an underly- 

 ing layer of muscle-cells, interrupted here and there by connective 

 tissue ; and (3) of a fibrous connective-tissue layer containing fewer 

 elastic but more white fibrous connective -tissue fibers than is the 

 case in the arteries. Externally, the intima is limited by an in- 



,-'Intima. 



T Elastica interna. 



Media. 



Fenestrated elastic 

 membrane. 



Inner layer of the 

 adventitia with 



longitudinally ar- 

 ranged muscle- 

 cells. 



Connective tissue 

 of the adventitia. 



'".:" ;-''-V" Nerve. 



' 



Fig. 174. Cross-section of human internal jugular vein. At the left of the nerve are 

 two large blood-vessels with a smaller one between them (vasa vasorum) ; X I 5- 



ternal elastic layer. The media is in general less highly developed 

 than that of a corresponding artery, and contains muscle-cells which 

 have a circular arrangement and in some veins form a continuous 



