568 THE MECHANICAL FUNCTIONS. 



the body. The metacarpus (m) consists origi- 

 nally of two bones, which soon become united 

 into one at the upper part. On the radial side 

 it has a process, derived perhaps from a third 

 metacarpal bone, which is anchylosed at a still 

 earlier period of ossification ; and to this process 

 a small pointed bone is connected, corresponding 

 to a rudimental thumb (t). There are gene- 

 rally two fingers, of which the first exhibits 

 traces of having been originally two bones : the 

 inner finger consists of two or three long pha- 

 langes, and the outer one of a single phalanx : 

 there is sometimes also a rudimental bone cor- 

 responding to a little 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. The horny mate- 

 rials 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 advan- 

 tageous 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 



