196 THE STRUCTURE AND LIFE OF BIRDS  cuap. 
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 fo 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. When 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 
