THE VALVES OF THE VEINS 



35 



This is one great difference between the arteries and the 

 veins ; the other is the presence of what are termed 

 valves in a great many of the veins, especially in those 

 which lie in muscular parts of the body. They are 

 absent in the largest trunks such as the superior and 

 inferior vena cava, and in the smallest branches, as also 

 in the portal, pulmonary, and cerebral veins, and in those 

 of the bones. 



FiQ. 9. 



-Transverse Section of an Artery and of a corrk- 

 spoNDiNO Vein. 



A, artery ; V, vein ; cc, endothelial cells ; m, muscular (middle) coat ; 

 c, connective tissue (outer) coat ; n, nuclei of endothelial cells. 



These valves are pouch-like folds of the inner wall of 

 the vein. The bottom of the pouch is turned towards 

 those capillaries from which the vein springs. The free 

 edge of the pouch is directed the other way, or towards 

 the heart. The action of these pouches is to impede the 

 passage of any fluid from the heart towards the capillaries, 

 while they do not interfere with fluid passing in the oppo- 

 site direction (Fig. 10). The working of some of these 

 valves may be very easily demonstrated in the living body. 

 When the arm is bared, blue veins may be seen running 

 from the hand, under the skin, to the upper arm. The 

 diameter of these veins is pretty even, and diminishes 



D 2 



