Circulation of the Blood. 65 



Evidences of Valves in Our Veins. i. With the forefinger stroke 

 one of the veins on the hand or wrist toward the tips of the fingers. 

 The veins swell out. The blood meets resistance in the valves of the 

 vein. Their location may be determined by their bulging out during 

 the experiment. 



2. Stroke a vein toward the body, and the blood is pushed a/ong 

 without resistance. 



3. Let the left hand hang by the side. Note the large vein along 

 the thumb side of the wrist. Place the tip of the second finger on this 

 vein just above the base of the thumb. Now, while pressing firmly 

 with the tip of the second finger, let the forefinger, with moderate 

 pressure, stroke the vein up the wrist. It may be seen that the blood 

 is pushed on freely, but comes back only part way. It stops where it 

 reaches the valves, filling the vein full to this point, but leaving it col- 

 lapsed beyond, as shown by the groove. Remove the second finger, 

 and the vein immediately fills from the side nearer the tip of the fingers. 



These experiments show that the blood in the veins moves freely 

 toward the body, but cannot flow outward to the extremities. 



Effect of Pressure on the Veins. Since the valves in 

 the veins open toward the heart, any alternating pressure 

 on the veins helps to push the blood on toward the heart. 

 The valves are most numerous in the veins near the sur- 

 face and in the veins of the muscles. The pressure of the 

 muscles during their action (thickening while shortening) 

 produces pressure on the veins ; and as the muscles act 

 for a short time only, and then relax, this alternate com- 

 pression and release aids very much in moving the blood 

 on toward the heart. 



How the Muscles help the Heart. This effect is greater 

 at the time the muscles need the most active circulation ; 

 namely, when they are in action, and are using the most 

 blood. The heart has power enough to pump the blood 

 clear around from each ventricle to the auricle of the other 

 side of the heart ; but this outside aid comes in good play 

 to relieve the heart at a time when it has an unusual 



