554 THE MECHANICAL FUNCTIONS. 



wards ; and that they are of a curved shape, and 

 generally armed with sharp claws. A bony ap- 

 pendix (a) projects outwards from the heel, for 

 the purpose of supporting the hinder prolonga- 

 tion of the membrane, which often extends be- 

 tween the hind feet, and is further sustained by 

 the tail, in those species which have the spine 

 prolonged to form one. 



Bats are also provided with another instru- 

 ment for suspending themselves to projecting 

 objects, formed by the thumb (b), which is, ap- 

 parently for this express purpose, detached from 

 the fingers that support the wing, and is termi- 

 nated by a strong claw, which projects, even 

 when the wings are folded, and is useful in 

 progression, serving as a point of support. 



§ 2. Birds, 



It is in Birds alone that we find the most perfect 

 adaptation of structure to the purposes of rapid 

 and extensive flight : in them the frame of the 

 skeleton, the figure, position, and structure of 

 the wings, the size of the muscles, the pecu- 

 liar nature of their irritability, and even the 

 outward form of the body have all a direct and 

 beautiful relation to the properties of the element 

 in which Nature has intended them to move. 



