FROGS AND THEIR RELATIVES 



113 



with its larger amount of carbon dioxid, to the lungs and the 

 skin. 



In the capillaries which connect the arteries and veins in 

 every part of the body (Fig. 82) all the changes in the com- 

 position of the blood take place, since their thin walls per- 

 mit the food materials and oxygen to enter the cells and the 

 wastes from the cells to enter the blood. 1 The capillaries 

 in the lungs likewise permit the interchange of oxygen and 

 carbon dioxid. 



FIG. 82. Network of cap- 

 illaries connecting an ar- 

 tery and a vein. 



FIG. 83. Capillaries in web of 

 frog's foot. 



85. Respiration and the liberation of energy. The walls 

 of the frog's lungs contain a network of capillaries, and in 

 these thin-walled tubes the red corpuscles absorb the oxygen 

 that is forced into these sacs by the upward movement of 

 the floor of the mouth. As we have seen, the blood with a 

 fresh supply of oxygen flows from capillaries of the lungs into 



1 A tadpole's tail is excellent for demonstrating the blood current. 

 Wrap a tadpole in wet cloth or cotton and support it so that the tail 

 can be placed between two glass slides on the stage of the micro- 

 scope. The space between the two slides should be kept filled 

 with water. The movement of the corpuscles through the margin 

 of the tail should be examined with the low power of the micro- 

 scope (Fig. 83). 



