THE CIRCULATION 



171 



40. But the chief point of distinction is in the presence of 

 the valves in the veins. These are little folds of membrane, 

 disposed in such a way that they open only to receive the 

 blood flowing toward the heart, and close against a current in 

 the opposite direction. Their position in the veins on the back 

 of the hand may be readily observed, if we first obstruct the 

 return of blood by a cord tied around the forearm or wrist. 



FIG. 43. VEIN WITH VALVES 

 OPEN 



FIG. 44. VEIN WITH VALVES CLOSED ; 

 THE STREAM OF BLOOD PASSING OFF 

 BY A LATERAL BRANCH 



fn a few moments the veins will appear swollen, and upon 

 them will be seen certain prominences, about an inch apart. 

 These latter indicate the location of the valves, or, rather, they 

 show that the vessels in front of the valves are distended by 

 the blood, which cannot force a passage back through them. 



41. This simple experiment proves that the true direction 

 of the venous blood is toward the heart. That the color of the 

 blood is dark will be evident, if we compare the hand thus 

 bound by a cord with the hand not so bound. It also proves 

 that the veins lie near the surface, while the arteries are 



40. Valves in the veins ? What are they ? Their position ? Experiment with the cord ? 



41. What will be proved by the experiment ? What inference is drawn ? 



