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 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 . 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 ^nd 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' 
