The Breath of a Bird 173 
of song is not solved. The marvel of the Canyon Wren’s 
melody becomes but the more wonderful; the voice of 
the Serlema, carrying over a mile, and the never-to-be- 
forgotten evening song of the Solitaire only impress us 
with the failure of the scalpel and microscope to explain 
more than superficially the varied expressions of life. 
Lungs and Air-sacs 
At the beginning of this chapter a bird was compared 
to an insect, and the reason will now be apparent. The 
body of an insect is aerated by means of an intricate sys- 
tem of tubes ramifying throughout the body, which in 
many instances are connected with air-sacs. The com- 
parison with a bird is not to its lungs, which are small 
and compact, but to a series of nine air-sacs, distributed 
through much of the body,—four pairs, and two which 
have coalesced into one. 
When a bird is dissected, the thin membranous walls 
of these air-cavities are collapsed and rather difficult 
to make out, being very similar in appearance to other 
connective tissues of the body. But if we insert a small 
blowpipe into the trachea of a dead bird, tie it tightly 
about with a piece of string and blow into it, all the air- 
sacs will become distended and_ bladder-like and can 
easily be made out. It is remarkable how closely these 
sacs fit around the viscera and muscles, occupying every 
crevice and filling the whole body of the bird with air, 
thus reducing its specific gravity, and making it a crea- 
ture literally “of the air.” There is sometimes a layer of 
