THE VERTEBRATE BODY 335 



and is supplemented with oxygen absorption through the skin. How- 

 ever, oxygen cannot be absorbed from a dry surface and the frog will 

 die if his skin dries out, for the lungs alone cannot supply sufficient 

 oxygen for the body. 



If you look at a frog from the side you can see the throat expanding 

 and contracting at regular intervals. This is the respiratory movement 

 of the frog ; it is pumping air down into the lungs, by a method that is 

 similar to swallowing water. First, the air is sucked into the oral cavity 

 through the nostrils as the throat is expanded. Then the throat is con- 

 tracted and valves close the nostrils so that the air is forced through the 

 glottis, into the larynx, and to the lungs. The lungs are elastic and con- 

 tract to force the air back out when the pressure is released. The larynx 

 contains two narrow bands across it ; these are the vocal cords and can 



Fig. 23.5. Method of inhaling air in the frog. The floor of the mouth is lowered 



causing air to come in through the nostrils to fill the mouth cavity. Then, the nostrils 



are closed and the floor of the mouth is raised. This forces air into the lungs. The 



elasticity of the lungs then expels the air when the nostrils are opened. 



be stretched so as to vibrate as air passes over them to make sound. 

 They can produce various sounds with a definite meaning. Frogs have 

 a cry of alarm, a cry of pain, and a sex call, in addition to the croaking 

 that seems to have no particular meaning. The bullfrog has very heavy 

 vocal cords, which produce deep booming sounds that echo through the 

 surrounding countryside with a force all out of proportion to the size 

 of the frog. 



The lungs are two small sacs which are divided internally into many 

 smaller sacs, the alveoli. This greatly increases the absorptive surface. 

 These alveoli are lined with a network of fine capillaries so that the air 

 comes in close contact with the blood. The oxygen, being more abun- 

 dant in the air than in the blood, diffuses through the thin membranes 

 between and is absorbed by the hemoglobin in the red blood corpuscles. 

 On the other hand, there is more carbon dioxide in the blood than in the 

 air, so it diffuses in the opposite direction and is expelled when the air 

 is exhaled. The same principle applies to the absorption of oxygen and 

 the release of carbon dioxide from the skin. 



