MOTIVE POWER 45 
front to back, without which the bird would not 
make headway, is obtained first by the raising of 
the back part of the wing relatively to the front, 
and later, as the stroke advances, by the forward 
movement of the descending wing, which brings it 
about that the extremity occupies a position lower 
than and in advance of the base. For a moment 
let us consider the working of the wings in combi- 
nation. With the body they form a kind of funnel 
—obviously one side of the funnel is missing, but 
this is unimportant. Caught in this funnel and 
deflected from the wing-surfaces, the air impinges 
upon the body and lifts it. When the wing has 
strained forward and downward till it can strain 
no further, the muscles at length relax. The wing 
is no longer rigidly extended, but slightly bent at the 
elbow-joint, and soon at the wrist also. If the bird 
is rising and has little onward momentum, the 
Klevator muscle does the work of lifting. The great 
flight-feathers, which during the down-stroke have 
been pressed close against one another and so have 
made the wing impervious to air, are now slightly 
rotated, so that interspaces are left which allow the 
air to pass, and thus the raising is effected without 
much opposition (see Chap. vu, fig. 24). Hf, on 
the other hand, the bird has much way on, the air 
itself effects the lifting and little work on the part of 
the musclesis required. After the strain of the down- 
stroke, the Depressor muscle ceases its contraction, 
and, perhaps, the Elevator gets to work. In any 
case the front margin of the wing is no longer de- 
pressed relatively to the hinder margin, but is lifted. 
The wing lets the wind have its way, and is carried 
