458 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



Fig. 315. — A magnified view of one barb (b b), 

 with its barbules {b') armed with hooks. 



we give a magnified view of one barb with its barbules 

 and hooks. The object of this structure is to make an 

 impermeable, light, and elastic plane. If these delicate 



parts are disarranged 

 or ruffled, they are 

 easily rearranged, 

 and hooked together 

 again by preening. 

 So much for the de- 

 vice for making an 

 impermeable plane. 

 Now observe how 

 the plane is set on the fore limb. It is not set on both 

 sides, but only on the backward side of the extended 

 arm. The downward stroke of the wing therefore tips 

 up the wing plane behind, so that the same stroke sus- 

 tains and also drives forward. In Fig. 316, which is a 

 cross section of the wing plane, if 1, 3 represent the whole 

 air pressure in the down- 

 stroke, then 1, 2 will be ^--^ 

 the force which sustains 

 the bird, and 3, 2 that 

 which drives it forward. 

 How Formed. — 

 We have seen that all 

 these structures thus far 

 are formed in or on a 

 mold, which determines their shape, and from which they 

 are pushed out. Now, the same is true of feathers also. 

 All this complex structure is secreted in an equally com- 

 plex mold, from which it is necessarily pushed out, and 

 the mold corresponding successively destroyed. 



Gradation to Hairs.— We have described the quill 

 feathers of the wings and tail, for these are most com- 

 plex and characteristic, but all gradations may be traced 



Fig. 316.— Cross section of the wing 

 plane of a bird, showing its action in 

 flying. 



