THE WING OF A BIRD. 77 



The problems come under two heads, — how the wing is 

 made and how it is managed, — which we will take up 

 separately. 



Let us study the wing as it looks in life, and see what we 

 can discover. The one here pictured is the same from which 

 the bones figured on page 69 were drawn and lies with the bones 

 in the same position as in that cut. 



We notice first that when the wing is spread the bones are 

 not stretched out as straight as those in our arms when they 

 are fully extended, but that there is a permanent crook at the 

 elbow which is filled in with skin covered with feathers. A 

 plucked chicken shows us that this extension is a fold of skin 

 with a stout tendon running along the double of it like the 

 drawstring of a bag. When the wing is closed this tendon 

 puckers up and holds the wing neatly folded by the bird's 

 side. When the wing is extended this skinny flap greatly 

 increases its area, and the tendon makes a firm selvedge along 

 the margin. Even the bat has such a membrane along the 

 front edge of the wing, and undoubtedly it assists both bird 

 and bat in steering their flight up or down, while it probably 

 aids, as a jib aids the mainsail of a vessel, in equalizing the 

 pressure of the wind against the after part of the wing. 



In examining the covering of feathers we see that they are 

 of different lengths, differently attached. There are the short 

 ones which cover the skinny portions of the wing in over- 

 lapping layers, and the long ones which are attached to the 

 back edge from tip to body in a single line of strong, wide, 

 long quills whose use is to increase the area of the wing by 

 adding the least possible weight. These quills are arranged in 

 series according to the place where they grow. Those that 

 spring from the tip of the wing, or hand, are called primaries ; 

 those that are attached to the forearm are the secondaries, and 



