THE BIRDS 213 



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. 



198. 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 betAveen the 



1'b.h 



FIG. 122. Anatomy of a bird. ., auricle ; cbl. and crb.h., cerebellum and cerebral 

 hemispheres (divisions of the brain) ; duo., intestine (with portion removed) ; 

 giz., gizzard; kd., kidney ; r.lng., lung; tr., 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 



