154 NATURE-STUDY 



the chest cavity by raising the ribs with the muscles attached 

 to them. The atmospheric pressure then forces the air into 

 the lungs, which thus fill the expanding chest cavity. Ex- 

 haling requires no eilort. We simply relax the muscles and 

 let the ribs descend, their weight forcing out the air. But 

 frogs have no ribs. There are simply stumps or rudimentary 

 ribs on each side of the backbone. So a frog cannot breathe 

 as we do, but has to force the air in by swallowing. As 

 before mentioned, frogs breathe somewhat through their 

 skin. The croaking is made by inflating the throat and 

 cheeks. Obser\'e this in a frog or a toad. 



The feet of the frog are webbed so as to enable him to 

 swim well. The toes of the hind feet are much elongated, and 

 with the web make an excellent paddle. Place a frog in the 

 water and see what an expert swimmer and diver he is. The 

 hind limbs are long and strong, and are well adapted for 

 leaping. Notice how the frog at rest squats with his legs 

 all ready for the jump. Try to make him jump. Measure 

 some of the longest jumps and compare this distance with 

 the frog's length. Can a boy jump as far in proportion? 



Hold a frog under water a short time. Note that he pulls 

 a thin, transparent film over the eyes. What is its use? It 

 is the lower eyelid. 



Just back of the eye is a circular area with a smooth, 

 flat surface. This is the ear, and the membrane is the ear- 

 drum. Frogs have a keen sense of hearing. Any one who 

 has approached a swamp where the frogs were holding a 

 concert will know how quickly they stop at the snapping of 

 a twig, the rustling of the grass under foot, or any other sound 

 made on coming toward the swamp. The fact that frogs 

 have "a song" implies an ear to hear it. 



