208 THE BLOOD-VESSELS IN GENEUAL. 



superior parts of the body, is called the Superior, or Descending 

 Vena Cava. The other, which comes from the lower parts of 

 the body, is called the Inferior, or Ascending Vena Cava. 



The veins which correspond with the branches of the Pul- 

 monary Artery, and return to the heart the blood of the lungs, 

 are four in number ; two of them proceeding from each lung. 

 They are called Pulmonary Veins. 



Fig. 172.* 



In many of the veins there are valves which prevent the 

 blood they contain from moving towards the surface and 

 extremities of the body, but allow it to pass towards the heart 

 without impediment. 



From the construction of the cavities of the heart, and the 

 position of the valves which are in them, as well as the situation 

 of the valves at the commencement of the great arteries, and 

 the above mentioned valves of the veins, it is evident, that 



* Fig. 172. Trunk of a large vein, laid open in order to exhibit the valves, 

 formed by doublings of the internal lining membrane, a, Superior portion of 

 the vein, or that towards the heart, b, Valves, the concavity of which is di- 

 rected towards the heart, c, Branches of the veins, anastomosing together, 

 and uniting to form a large branch d, which opens into the principal trunk 

 at e. — r. 



