46 THE FLIGHT OF BIRDS 



back, almost unresisting, but occasionally, as photo- 

 graphs show, it resists enough to cause a backward 

 bending of the flight- feathers. It looks as if the 

 bird, when his wings are being lifted by the rush of 

 air into position for a fresh stroke, checked them for 

 a fraction of a second in order to save himself from 

 loss of altitude. It is possible this check in the 

 course of the up -stroke may be the normal thing, 

 or it may be only occasional (see PI. v). When 

 the wing is thus raised by the rush of air — more 

 strictly by the resistance of the air to its momentum 

 — the flight- feathers are pressed against one another 

 and there are no gaps to make the lifting easy. But 

 no such help is required. The wing is blown back 

 as far as it has freedom to go, and at the end of its 

 rearward movement it is no longer facing as it was ; 

 its under- surface is facing outward and its anterior 

 margin is looking towards the bird's head. When 

 the wing is in this position it is easy to raise it com- 

 pletely and bring it forward ; in fact it moves edge- 

 ways. Anatomy supplies very remarkable evidence 

 that the raising of the wing requires little effort. 

 Whereas the muscle which lowers the wing is red, 

 ridged, and granulated, the Elevator muscle is paler 

 and exposes a smooth surface when it is cut. The 

 former is by far the better class of muscle, capable 

 of long, unflagging effort. 



In ordinary horizontal flight most birds take a 

 much shorter stroke than the one I have just 

 described, nor is the wing pointed much forward 

 (see PI. vn ). The upward incline from front to 

 back that is needed is obtained by a slight lowering 

 of the front margin relatively to the back, mainly 



