CIRCULATORY SYSTEM. 113 



Veins. These vessels convey the blood from the 

 capillaries back to the heart. The progressive in- 

 crease in size and the thickness of their walls is 

 accompanied by a relative increase in blood pressure 

 and rate of blood flow, yet nowhere is this equal to 

 what obtains in the large arteries. Structurally 

 we find the same layers in veins as in arteries, with 

 the chief difference that the vein wall is much thinner. 

 The endothelial layer of cells is supported by a 

 very thin layer of delicate connective-tissue fibers, 

 mostly non-elastic, while the fenestrated mem- 

 brane of Henle is incomplete and usually difficult 

 to demonstrate. The media, as in arteries, is the 

 most prominent layer, but, unlike arteries, the non- 

 elastic fibers prevail. Smooth muscle fibers, mostly 

 circular, are often significant in this layer, while 

 the other tissue elements are less conspicuous. 

 The adventitia resembles more closely that found 

 in arteries, with perhaps even less of the elastic ele- 

 ments and more of the smooth muscle cells. Com- 

 paring the different sizes of veins, we find an excess 

 of elastic and muscular tissue in large veins. In 

 the pulmonary vein the circular muscle fibers are 

 well developed, while in the large cranial veins, 

 such as the meningeal sinuses, muscle tissue is almost 

 entirely absent. Veins, like arteries, therefore, 

 show a structural variation, depending not only on 

 size, but on location. It should be mentioned that 

 in many superficial long veins, like those of the legs 

 and neck, valves are present in the form of crescentic 

 folds of the intima which function in overcoming 

 the pressure of blood due to gravity. Those of the 



