] 82 ORDER CHEIROPTERA ; GENERAL CHARACTERS. 



true wings ; being capable of motion in such a manner, as to 

 strike the air, and thus to raise or propel the body by the resist- 

 ance which it affords to their extended surface. In their function, 

 therefore, they are completely analogous to the wings of Birds ; 

 which class may be considered as represented by the Bat tribe 

 among the Mammalia. And in their structure they only differ 

 in this respect, that the expanded surface, which is given in 

 Bats by the extension of the skin itself over a complex bony 

 framework, is afforded in Birds by the feathery appendages, 

 which are supported upon a framework of much simpler con- 

 struction. On looking at the skeleton of the Bat, we observe, 

 in the first place, that the humerus or arm-bone (k, Fig. 85,) is 

 long and large in proportion to the body ; but that the lengthen- 

 ing is still more remarkable in the bones of the fore- arm, CM, r. 



, 



FIG. 85. SKELETON OF BAT. 



cl, clavicle; ft, humerus; CM, ulna; c, radius; ca, carpus; po, thumb; me, metacar- 

 pus ; ph, phalanges ; o, scapula ; /, femur ; ti, tibia. 



The bones of the carpus, or wrist, ca, do not show any consider- 

 able increase ; but the most extraordinary extension is seen in 

 the metacarpal bones of the four fingers, which spread out widely 

 from each other when the wing is extended, but are capable of 

 being closed together, like the stretchers of an umbrella. Upon 

 these are situated the phalangeal or true finger-bones, ph ; the 



