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 

 Elevator 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, vn, fig. 24). If, on 

 the other hand, the bird has much way on, the air 

 itself effects the lifting and little work on the part of 

 the muscles is 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 



