HOW ANIMALS BREATHE. 



119 



pipe be tied, and an opening be made in the 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, which, 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 bronchi, 

 one for each lung. 

 In Birds and Mam- 

 mals, the bronchial 

 tubes, after entering 

 the lungs, subdivide 

 again into minute 

 ramifications. 



Vertebrates are the 

 only animals that breathe through 

 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. 67 Birds, unlike other ani- 

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

 done by the springing-back 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 

 compresses the lungs. 



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

 cavity for the heart and lungs, with movable walls (breast- 



Fig. 87.— Skeleton of a Frog. 



the mouth or nos- 



