EXTERIOR PARTS OF BIRDS 



159 



d. 2 ; that of the third finger is nearly as long as the main 

 metacar})a], but much slenderer, and usually fused only at its two 

 ends, leaving between itself and the main metacarpal a considerable 

 space, as seen opposite the letters mc in the figure. The wing is 

 finished off with three fingers or digits marked d 2, d 3, d 4. The 

 middle one of these, E to D in the figure, is much the largest, and 

 forms the main continuation of the hand. The digit, d 3, ordinarily 

 consists of two bones, called phalanges, })laced end to end, as in the 

 example before us ; but occasionally there is found a third })halanx. 

 The outer or radial digit, d 2, ordinarily consists of two bones, of 



2. — 



Mcui. 



ji — 



r—- f- 







Fig. 29 his. — Diagram of fore limbs of man, bat, horse, and bird, 

 cutting tlie limbs into morphologically equal parts, or isomeres. 



The lines 1-9 are isoiomes 



which the terminal one is small, and may be wanting. The inner 

 or ulnar digit, d 4, consists of a single small phalanx, closely bound 

 to the side of the middle finger. Corresponding to the compactness 

 and consolidation of these terminal segments, the digits enjoy 

 little individual motion. The outer or radial digit is the most 

 independent one. In the Arclwioptcryx the three metacarpals Avere 

 free bones, and the whole hand more like that of a lizard. No bird 

 now has free metacarpals in adult life ; none has more than three 

 digits. These three are supposed by some to correspond to the 

 thumb and fore and middle fingers of our hands ; by others, to the 



