382 THE MECHANICAL FUNCTIONS. 



paratively long, and the fibula is a very slender bone, yet 

 quite distinct from the tibia (t.) The ^slight degree of mo- 

 tion which is thus allowed between them is useful to the ani- 

 mal, in enabling the feet to lay hold of cornices or other pro- 

 jecting parts of the roofs of buildings, on which the animal 

 fastens itself, and hangs with the head downwards. It is 

 probably with the intention of facilitating this action that the 

 toes are turned completely backwards; and that they are of 

 a curved shape, and generally armed with sharp claws. A 

 bony appendix (a) projects outwards from the heel, for the 

 purpose of supporting the hinder prolongation of the mem- 

 brane, which often extends between the hind feet, and is far- 

 ther sustained by the tail, in those species which have the 

 spine prolonged to form one. 



Bats are also provided with another instrument for sus- 

 pending themselves to projecting objects, formed by the 

 thumb (b,) which is, apparently for this express purpose, 

 detached from the fingers that support the wing, and is ter-, 

 minated 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 adapta- 

 tion 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. In their formation, a new, and in as far as relates 

 to the organs of progressive motion, a more developed type 

 is adopted; still preserving a conformity with the general 

 plan of the vertebral organization, and with the general 

 laws of its development. 



The skeleton of birds has the same constituent parts as 

 that of other vertebrated classes: the bones of the anterior 



