118 COMPARATIVE ZOOLOGY. 
by the valve-like ep7glottis. The trachea passes along 
the neck in front of" 
the cesophagus, 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. 
Fic. 87.—Skeleton of a Frog. Vertebrates are 
the only animals that breathe through the mouth or nos- 
trils. Fishes inspire only. 
Turtles, whose ribs are sol- 
dered 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 animals, do not in- 
hale 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 to- 
ward the backbone by 
muscles, which compresses 
the lungs. 
Mammals alone have a 
perfect thorax, z.¢., a closed 
Frogs, having no ribs, and 
Fra. 88.—Human Thorax: a, vertebral col- 
umn; B, b’, ribs, the lower ones false; ec, 
clavicle; e, intercostal muscles, removed 
on the left side to show the diaphragm, d; 
J, pillars of the diaphragm attached to the 
lumbar vertebre ; g, muscles for elevating 
the ribs; h, sternum. 
