BIRDS, THEIR STRUCTURE AND ADAPTATIONS 283 



Feathers. Feathers are modified forms of scales and develop 

 in the same way from the skin. Some unchanged scales are always 

 found on the feet and legs, which remind one of their relationship 

 to reptiles. They are not evenly distributed over the bird's body, 

 but are found in certain feather tracts, between which the skin is 

 nearly bare, though the over- 

 lapping feathers do not re- 

 veal it. There are three 

 kinds of feathers; the soft 

 down which retains bodily 

 heat, the ordinary body 

 feathers that give the smooth 

 and graceful outline to the 

 otherwise angular form, and 

 the large quill feathers of 

 the wing and tail. 



These latter are the ones 

 concerned in flight and con- 

 sist of a broad vane spreading 

 from an axis (the rachis) 

 terminating in a hollow quill. 

 The vane is made up of in- 

 numerable rays called barbs, 

 each like a tiny feather, 

 having projections called 

 barbules (little barbs) which 

 in turn are held together by 

 interlocking hooks of micro- 

 scopic size. This compli- 

 cated arrangement provides a vane which is very strong, light, 

 and elastic, and furthermore, if the barbules become unhooked 

 as when a feather is " split " by accident, the bird merely shakes 

 them or draws them through its beak, and the feather is whole 

 again. This is a great advantage over a wing membrane such as 

 is possessed by the bats which, if once injured, cannot be repaired. 



The rachis is grooved and the quill hollow, both being adapta- 



FIG. 96. Structure of quill feather. 



