that sends it 
through the 
great aorta in- 
to numerous 
branches _ that 
lead all over 
the body. 
Respiration. 
—The breath- 
ing organs are 
shown in Fig. 
269, 4,7. The 
lungs, 4,2, are 
two in number, 
spongy in con- 
sistency, and 
are attached to 
the walls of the 
cavity in which 
they are placed. 
BIRDS. 22g 
Fic. 269.—Parts of a bird. 1, digestive apparatus ; 
c, crop; g, gizzard; ¢, trachea; 4, 4, bronchial 
tubes ; /, 2, lungs; 2, bones of the wings, 
The large air-passages of the lungs end in air-sacs, that 
in turn connect with the cavities of the bones, so that air 
taken in passes down the trachea, 4, enters the lungs, /, /, 
aérating the blood there, also passing into the 
Fic. 270.—1, Brain of a 
bird ; 2, eye, showing nic- 
titating membrane. 
air-sacs and penetrating the hol- 
low bones in every part of the body ; 
thus the bird can alter its specific 
gravity at will. The air-sacs are 
nine in number ; two are placed in 
the abdomen, four in the thorax, 
and three near the wish-bone. 
Nervous System.— The brain 
(Fig. 270, 1) is larger than in the 
reptiles, and has no convolutions. 
Covering.—The birds differ from all other animals in 
being covered with feathers (Fig. 271), that are modified 
