Physiology. 



vein 



Artery 



Fig. 40 



whereas the larger arteries stand open, after they are 

 emptied of blood. There are many cross-branches unit- 



ing veins, so that if the flow 

 is stopped in one vein, the 

 blood can take a " cross- 

 road" into another large 



. , . 



vein. Inis cross-branching 

 may usually be seen on the 

 back of the hand. 



The Valves in the Veins. 

 The only valves in the 

 arteries are those at the be- 

 Small Artery ginning of the aorta and pul- 

 monary artery. Many of the 



veins have similar pocket-like valves though less strong 

 than those of the arteries. They are usually in pairs, but 

 sometimes single or in threes. They all have the mouths 

 of the pockets toward the heart, so that the blood flows 

 freely toward the heart, but is kept from flowing the other 

 way on account of the filling 

 of the valves by the back 

 pressure of the blood. When 

 the blood is flowing through 

 the veins toward the heart, 

 the valves lie against the walls 

 of the veins (see Fig. 41). 



Illustration of Valves in the Veins. 

 Make a cloth tube (or take the O pen Shut 



lining of a boy's coat sleeve) and Fig . 41 . V aives of the Veins. 



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. This shows how the valves fill 

 and block the passage when there is back pressure of the blood. 



