202 THE STRUCTURE AND LIFE OF BIRDS cuap, 
branes, one of which, called the anterior membrarie, 
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, 
