WINGS OF BIRDS. 501 



finger. The degree of developement of these bones 

 varies in different tribes of birds. 



Feathers are attached to all these divisions of the 

 limb, namely, to the humerus, the fore arm, the 

 hand, and occasionally to the single phalanx of the 

 thumb. The structure of feathers is calculated in 

 an eminent degree to combine the qualities of 

 lightness and of strength, which we elsewhere 

 rarely find united ; and they also furnish the 

 warmest possible covering to the body. This latter 

 property results from the great multitude of minute 

 detached fibrils of which they are principally com- 

 posed : as is more especially observable in the down 

 which protects the chest of aquatic birds where 

 such provision is particularly needed. 



The horny materials of which the stem of the 

 quill is made are tough, pliant, and elastic ; and, 

 as we have already seen, are disposed in the most 

 advantageous manner for resisting flexion by being 

 formed into a hollow cylinder. But the vane of 

 the feather is still more artificially constructed ; 

 being composed of a number of flat threads, or 

 filaments, so arranged as to oppose a much greater 

 resistance to a force striking perpendicularly against 

 their surface, than to one which is directed late- 

 rally ; that is, in the plane of the stem. They 

 derive this power of resistance from their flattened 

 shape, which allows them to bend less easily in 

 the direction of their flat surfaces than in any other ; 

 in the same way that a slip of card cannot easily 

 be bent by a force acting in its own plane, though 

 it easily yields to one at right angles to it. Now 

 it is exactly in the direction in which they do not 

 bend that the filaments of the feather have to 



