THE BIRDS 205 
ing to note that the knee has been drawn far up into the 
body, and that the joint above the foot is in reality the 
ankle. 
We thus see that the bird’s skeleton presents the same 
general plan as that of the lizard, for example ; but in order 
to combine the elements of strength, lightness, and com- 
pactness essential to successful flight, it has been necessary 
to remodel it to a considerable degree. 
192. Other internal structures.-_The lungs of birds con- 
sist of two dark-red organs buried in the spaces between the 
ribs along the back. Each communicates with extensive 
thin-walled air-sacs extending into the space between the 
Fie. 120.—Anatomy of a bird. avw., auricle; cbJ. and crb.h., cerebellum and cerebral 
hemispheres (divisions of the brain); duo., intestine (with portion removed) ; 
giz., gizzard ; kd., kidney ; 7.Jng., lung; ¢., trachea or windpipe ; vent., ven- 
tricle. 
various organs, and in many birds of flight they even extend 
into the bones of the body, and thus decrease their weight. 
“The enormous importance of this feature to creatures 
destined to inhabit the air will be readily understood when 
we learn that a bird with a specific gravity of 1.30 may 
have this reduced to only 1.05 by pumping itself full of air.” 
As we know, air is taken into the body in order that the 
oxygen it contains may combine with the tissues of the 
body to liberate the energy necessary for the work of its 
