196 THE STRUCTURE AND LIFE OF BIRDS chap. 



limb becomes rigid. I think this is due mainly to the 

 ligaments. When the wing is flexed there is much less 

 rigidity, for, to say nothing of the movements necessary 

 to flight, the different parts of the wing must face 

 different ways in order to fold neatly over the body, 

 the upper arm looking upwards and inwards, forearm 

 upwards and outwards, the hand outwards and only 

 very slightly upwards. 



The way in which the wrist joint has been modi- 

 fied is remarkable : the hand has very little of the up 

 and down movement that comes to ours so easily, and 

 when the wing is fully extended, none at all. Its only 

 free movement is away from the thumb and towards 

 where the little finger would be. Our wrists are very 

 stiff, if we try to move them thus. This peculiarity in 

 the bird's wrist may be traced to the radius. When 

 the wing is folded the bone slides forward and, extend- 

 ing beyond the ulna, forces the hand into the position 

 described. When the wing is straightened the radius 

 slides back and brings the hand into line with it. 



The wing presses with tremendous force upon the 

 bones that support it. W r hen it descends like a flail, 

 with its face during the first half of the stroke looking 

 not only downwards and backwards, but also outwards, 

 there must be great pressure inwards upon the pivot on 

 which it turns. I have in an earlier chapter shown 

 whence this pivot derives its strength. The shoulder 

 is the meeting-place of three bones (the coracoid, the 

 scapula, and clavicle), though only the two former 

 actually help to form the joint. The coracoid and 

 clavicle slope outwards, and it is this outward slope 

 that gives them their power to resist the pressure 



