The Breath of a Bird 



175 



bone and ribs, and when carefully removed show fur- 

 rows made by these latter bones. There is still much 

 to be learned of the manner of a bird's breathing, but 

 it is probable that there is a sidewise or dorsal expan- 

 sion of the ribs, rather than of that portion corresponding 

 to our chest. In a bird the latter region is chiefly an im- 

 mense flight-muscle, which could hardly yield to the 

 action of breathing while carrying on the tremendous 

 work of keeping the wings in motion, and when a 



Fig. 132.— Cross-section of wing-bone of Ostrich and Black Swan. In life the 

 bone of the Ostrich is filled with marrow; that of the Swan with air. 



bird squats on a branch with its breast pressed close 

 to the perch, ''chest expansion" must be all but 

 impossible. 



We cannot help being surprised at first when we see 

 how small are the lungs of a bird in comparison with 

 the size of its body. 



The first thought that occurs to us is that the air- 

 sacs in birds, and the hollow cavities of the bones, must 

 function chiefly as aids to flight, and we should expect 

 to find as best flyers those birds in w^hich the air-cavities 



