£g;j° ry (124) THE BIRD WORLD. 
tion bring about the downward wing-beat. 
About the other muscles which aid in this work, 
and those which raise the wings at the end of 
the stroke, we need not worry here. 
Birds Without Keeled Breast-bones. 
How intimately the keel is associated with 
flight may be seen by an examination of birds 
which fly but little. In them the keel is always 
shallow, while in those which have lost the 
power of flight altogether it is reduced to a mere 
ridge of bone, as in the Owl Parrot (Stringops), 
or has vanished altogether, as in the flightless 
Ostrich tribe. 
Structure of the Wing. 
The wing itself is no less profoundly modi¬ 
fied, as we pointed out in an earlier chapter. 
Suffice it to say here that of the original five 
fingers but three remain. Of these the thumb 
and third finger are reduced, and little more than 
stumps, while the second finger has been elon¬ 
gated to form a long, rigid rod, strengthened at 
its base by that portion of the third finger which 
in ourselves contributes to form the “ palm ” of 
the hand. In the bird’s palm there are but two 
bones, forming the base of the second and third 
fingers respectively. (See Fig. 2.) 
The bird’s wing folds in a peculiar manner, so 
as to form a more or less Z-shaped rod, the 
humerus or upper arm forming the top of the 
Z, the fore-arm the down-stroke, and the hand 
the base of the Z. 
{To be continued?) 
Skeleton and Quills of the Wing. 
bast aT 31 c^uiWs 
1 1 1 
Pr\/v\ATC\ES secondaries 
Fig. 2.—The Wing of a bird, showing the skeleton and flight-feathers or quills attached thereto. The anterior and posterior 
patagial membranes are.folds of skin which help to increase the surface of the wing ; the posterior membrane serves more especially 
to strengthen the holdlof the quills. The vinculum elasticum is a band of tendon running along the edge of this hinder or posterior 
fold. It is elastic, and serves to keep the quills in position during flight. The reduced skeleton of the end of the third finger 
supports, and is concealed by, the seventh primary. 
