THE MACHINERY OF FLIGHT 



79 



of all birds. The huge hollow beak is characteristic 

 of the whole skeleton (see fig. 22). 



Stiffness of Wing. Expanse of Bone. 



It is highly important that the bird's wing when 

 extended should be rigid — its bony framework, that 

 is. It has elasticity where it is wanted, the elas- 

 ticity of the great feathers, which tends, as I have 

 shown, to make the bird automatically stable during 

 flight. The stiffness of the scaffolding of the bird's 

 extended wing is most remarkable when we compare 

 it with the human arm. If we extend one of our 

 arms horizontally to its full length with the palm 

 of the hand downward, we can, while still keeping 

 the upper arm lifted, bend the elbow- joint and 

 point the fore-arm vertically upward. As the 

 bird's wing descends with great velocity through 

 the air, it does not give at the elbow, but remains 

 rigid. But the stiffness of the bird's wrist is still 

 more striking ; when the wing is extended hardly 

 any up-and-down movement at that point can 



