408 THE MECHANICAL FUNCTIONS. 



walking, in consequence of the centre of gravity 

 being between the wings. On a level plane, in- 

 deed, the bat can advance only by a kind of crawl- 

 ing or hopping motion. The whole anterior half of 

 the trunk is much more fully developed than the 

 posterior half, which appears as if its growth had 

 been arrested. The pelvis (p) is of diminutive size, 

 compared v, ith the rest of the skeleton : the pubic 

 bones are lengthened backwards, and are joined 

 merely at a small point. The whole posterior limb 

 is short, the femur (f) comparatively long, and the 

 fibula is a very slender bone, yet quite distinct 

 from the tibia (t). The slight degree of motion 

 which is thus allowed between them is useful to the 

 bat, in enabling the feet to lay hold of cornices, 

 or other projecting parts of the roofs of buildings, 

 on which the animal fastens itself, and hangs with 

 the head downwards. It is probably with the in- 

 tention of facilitating this action that the toes are 

 turned completely backwards; and that they are of 

 a curved shape, and genei-ally armed with sharp 

 claws. A bony appendix (a) projects outwards 

 from the heel, for the purpose of supporting the 

 hinder prolongation of the membrane, which often 

 extends between the hind feet, and is farther sus- 

 tained by the tail, in those species which have the 

 spine prolonged to form one. 



Bats are also provided with another instrument 

 for suspending themselves to j)rojecting objects, 

 formed by the thumb (b), which is, apparently for 

 this express purpose, detached from the fingers that 

 support the wing, and is terminated by a strong 

 claw, which projects, even when the wings are 

 folded, and is useful in progression, by serving as a 

 point of supi)oi't. 



