202 THE STRUCTURE AND LIFE OF BIRDS chap. 



branes, one of which, called the anterior membrane, 

 stretches from the head of the clavicle to the hand. 

 In the Gannet it is of great breadth, and it is so placed 

 that it makes an almost upright wall along the front 

 margin of the wing, which thus presents a deep 

 umbrella-like hollow, such as a parachute requires. 

 In most birds the membrane is slung more horizontally. 

 In all it is at once stretched, when the wing opens. 

 The other membrane lies behind, in the armpit, and 

 fastens the wing to the side. 



The great feathers make some movement, without 

 assistance except from the air. From below they are 

 concave and during the down stroke the concavity is 

 much lessened owing to the pressure upon them. When 

 the muscles cease to lower the wings, the feathers 

 regain their full curve owing to their own elasticity, 

 and their bending thus is equivalent to a slight 

 prolongation of the stroke. Up till this moment they 

 have yielded to the air, they now strike down against 

 it. The result of their action is that the stroke is 

 longer and less violent, and that the strain upon the 

 wing is lessened. 



The air lends very material assistance in another 

 movement, to effect which there is, as I have shown, 

 muscular machinery provided. The outer webs of the 

 feathers are very narrow compared with the inner ones. 

 The result is that the air acts much more strongly 

 upon the latter, forcing them upward, so that each 

 feather has the inner side of its vane pressed closely 

 against the one which lies next to it and above it on 

 the side nearer to the body. 1 Thus the shape of the 



1 This interesting fact was first pointed out by Professor Roy. 



