HOW ANIMALS BREATHE. 119 



pipe be tied, and an opening be made in tiie wing-bone, 

 the bird will continue to respire. The right lung is usu- 

 ally the larger; in some Snakes, the left is wanting en- 

 tirely. In most Vertebrates, lungs are freely suspended ; 

 in Birds, they are fastened to the back. 



The lungs communicate with the atmosphere by means 

 of the trachea, or windpipe, formed of a series of cartilag- 

 inous rings, which keep it constantly open. It begins in 

 the back part of the mouth, opening into the pharynx by 

 a slit, called the glottis, wliich, in Mammals, is protected 

 by the valve -like epiglottis. The trachea passes along 

 the neck in front of 

 the oesophagus, and 

 divides into two 

 branches, or hronchi, 

 one for each lung. 

 In Birds and Mam- 

 mals, the bronchial 

 tubes, after entering 

 the lungs, subdivide 

 again into minute 

 ramifications.' 



Vertebrates are the ^"'- sT-skeieton of a Frog. ^ 



only animals that breathe through the mouth or nos- 

 trils. Frogs, having no ribs, and Turtles, 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." Birds, unlike other ani- 

 mals, do not inhale the air by an active effort ; for that is 

 done by the springing-baek of the breast-bone and ribs to 

 their natural position. To expel the air, the breast-bone 

 is drawn down towards the back-bone 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 (breast- 



