58 PHYSIOLOGY. 



of the blood stream. When the blood is flowing through 

 the veins toward the heart the valves lie against the walls 

 of the veins. 



The valves are most numerous in the medium-sized veins, 

 and especially in the veins of the extremities ; more abun- 

 dant in the leg than in the arm. Valves are absent from the 

 caval and some other veins, and from the very small veins. 



Illustration of Venous Valves. Make a cloth tube (or take the 

 lining of a boy's coat sleeve) and sew three patch-pockets on the in- 

 side, in a circle, i.e. with edges touching each other. Make the pockets 

 a little " full." Pour sand, shot, or grain through the sleeve first in 

 one direction and then in the other. 



Evidences of Valves in our Veins. 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. 



Stroke a vein toward the body, and the blood is pushed along with- 

 out resistance. 



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. 



Dissection of the Valves in a Vein. The valves may be seen by 

 dissecting out the jugular vein (or any other large superficial vein) of a 

 cat, dog, or rabbit. Split the vein and pin it out on a board. 



Effect of Pressure on the Veins. Since the valves in 

 the veins open toward the heart, any intermittent pressure 

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



