54 



THE BIOLOGY OF BIRDS 



I, II, and III) backwards. The three parts of the wing — 

 the upper arm (humerus), the fore-arm (radius and ulna), 

 and the hand (carpo-metacarpus and digits) — move in one 

 plane in relation to one another, as the blade of a pocket- 

 knife in relation to the sheath. Running along the front 

 of the wing from the upper arm to the lower arm there is a 



Fig. 9. — Comparison of bird's wing and monkey's arm. H., humerus ; 

 R., radius; u., ulna; r., in bird, radiale, a free wrist or carpal bone; 

 H., in bird, ulnare, the other free wrist or carpal bone; CA., the carpal 

 region or wrist ; I., the phalanx or joint of the bird's thumb ; MCii., meta- 

 carpal of second digit, forming part of the peculiar bone, the carpo- 

 metacarpus, which is characteristic of birds ; 11., the second digit of the 

 bird, with two joints or phalanges ; in., the third digit of the bird with 

 one joint ; the metacarpal part of the carpo-metacarpus is equivalent to 

 three palm bones and is marked solid black, the distal carpals are fused 

 to it, and are touched by the radiale and ulnare ; OL., on the monkey's 

 arm is the olecranon or elbow process of the ulna ; mc.i., the metacarpal 

 of the thumb (th) ; MC.5, the metacarpal of the little finger (5). 



slight web or patagium with an elastic band (tensor patagii 

 longus) which is shorter than the bones of the wings and 

 thus prevents their complete extension. The whole adjust- 

 ment is complicated, but the net result is to reduce the 



