46 THE FLIGHT OF BIRDS 
back, almost unresistif®, 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 Pl. 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 Pl. vit). 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 
