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Life and Health 



passing onwards toward the heart, at length form the two 



great vessels which open into the right auricle. 



These two great venous trunks are the inferior vena cava, 



which brings the blood from the trunk and the lower limbs, 



and the superior vena cava, which brings the blood from the 



head and the upper limbs. 



The four pulmonary veins, as we have learned, carry the 



arterial blood from the lungs to the left auricle. 



Many of the veins lie near the surface of the body. 



Some of them may be easily seen under the skin of the 

 hand and forearm, especially if the 

 arm is allowed to hang down a few 

 moments. 



222. How the Blood flows in the 

 Veins. The walls of the larger veins 

 are thin, and when empty they col- 

 lapse. The inner surfaces of many 

 of the veins are supplied with pouch- 

 like folds, or pockets, which act as 

 valves to prevent the backward flow 

 of the blood. These valves can be 



FIG. 74 . 



A, part of a vein laid open, 



with two pairs of valves; shown b letting the forearm hang 



B, longitudinal section of J 5 



a vein, showing the valves down and sliding the finger upwards 

 over the veins. 



The onward flow of the blood in the veins is due to 

 various causes, the chief being the pressure behind of the 

 blood pumped into the capillaries. Then as the pocket- 

 like valves prevent the backward flow of the blood, the 

 pressure of the various muscles of the body urges along 

 the blood and thus promotes the onward flow. 



223. The Capillaries. The capillaries are minute tubes, 

 with very thin walls, which form the connection between 

 the ending of the finest arteries and the beginning of the 



