326 



COMPARATIVE ZOOLOGY 



begins in the back part of the mouth, opening into the 

 pharynx by a slit, called the glottis, which, in mammals, 

 is protected by the valvelike epiglottis. The trachea 

 passes along the neck in front of the esophagus, and 

 divides into two branches, or bronchi, one for each lu-ng. 

 In birds and mammals, the bronchial tubes, after enter- 

 ing the lungs, subdivide again into minute ramifications. 

 Vertebrates are the only animals that breathe through 

 the mouth or nostrils. Frogs, having no ribs, and tur- 

 tles, whose ribs are soldered together into a shield, 



are compelled to 

 swallow the air. 

 Snakes, lizards, 

 and crocodiles 

 draw it into the 

 lungs by the play 

 of the ribs. 121 

 Birds, unlike other 

 animals, do not 

 inhale the air by 

 an active effort ; 

 for that is done by 

 the springing back 

 of the breastbone and ribs to their natural position. 

 To expel the air, the breastbone is drawn down toward 

 the backbone by muscles, which movement compresses 

 the lungs. 



Mammals alone have a perfect thorax i.e., a closed 

 cavity for the heart and lungs, with movable walls 

 (breastbone and ribs) and the diaphragm, or muscular 

 partition, separating it from the abdomen. 122 Inspira- 

 tion (or filling the lungs) and expiration (or emptying 

 the lungs) are both accomplished by muscular exertion ; 

 the former, by raising the ribs and lowering the di- 

 aphragm, thus enlarging the capacity of the chest, in 



FIG. 284. Skeleton of a Frog. 



