The Breath of a Bird eas 
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 
Fic. 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 which the air-cavities 
