HOW ANIMALS BREATHE, 117 
quickly die for lack of it. In Birds, respiration is not 
confined to the lungs; but, as in Insects, extends throngh 
a great part of the body. Air-sacs connected with the 
lungs exist in the abdomen and under the skin of the 
neck, wings, and legs. Even the bones are hollow for 
this purpose; so that if the windpipe be tied, and an 
opening be made in the wing-bone, the Bird will continue 
to respire. The right lung is usually the larger; in some 
Snakes, the left is wanting entirely. In Mammals, the 
lungs are freely suspended in the thorax; in other Verte- 
brates, 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 
SS 
a 
f 
1; 
i VOT a ra 
Fia. 85.—Lungs of a Frog: a, hyoid . ‘L 
apparatus; 5b, cartilaginous ring at Ee’ f 
root of the lungs; c¢, pulmonary as i : 
vessels ; d, pulmonary sacs having 
this peculiarity common to all cold- sae coe 
blooded air-breathers, that the tra- Fre. 86.— Distribution of Air-tubes in Mam- 
chea does not divide into bron- malian Lungs: a, larynx; B, trachea; e, d, 
chial branches, but terminates ab- _left and right bronchial tubes; e, f, g, the 
ruptly by orifices which open at ramifications. In Man the subdivision con- 
once into the general cavity. A  tinues until the ultimate tubes are one-twen- 
cartilaginous net-work divides the __ ty-fifth of an inch in diameter. Each lobule 
space into little sacs, on the walls represents in miniature the structure of the 
of which the capillaries are spread, entire lung of a Frog. 
the back part of the month, opening into the pharynx by 
a slit, called the glottis, which, in Mammals, is protected 
