414 



ZOOLOGY 



Bird flight. — The difficulties which must be overcome in 

 order to fit birds for fiiglit arc, fii'st, tliat of sustaimng the heavy 

 body in a medium of such low specific gravity as the air ; anrl, 

 secondly, that of progression in this medium. To diminish the 

 chfficult}' of sustaining the iDody, the specific gravity is reduced 



Fig. .384. — White-breasted nuthatch {Silta 

 carolincnsiti) . 



Fig. 3.S.5. — Brown rreepcr 

 {C'erith ia fant iliaris) . 



to a minimum In' great air spaces in the bod>', which exist even 

 in the hollow bones. The body is k(>pt from l^eing overturned 

 in the air 1 ly the position of the wings, which are placed high up 

 on the trunk, while the digestive organs, breastbone, and breast 

 muscles arc plac(>d low. To aid locomotion, the general form 

 of the body is made conical, so as to offer little resistance to the 

 air, while, l)y varying the position of the head, wings, and tail, 



