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. 



(b) 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- 



_ Connective tissue 

 of the adventitie 



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) ; ^< 150. 



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 



