12 THE STORY OF THE BIRDS. 
modified. There being small use in flexing the fingers, 
there are very slight muscles on the forearm. The 
hand is not flexed to any great extent by tendons, but 
by a peculiar mechanical union of the two bones of 
the forearm. We have already noticed the use of the 
muscle and tendon which spreads the patagzwm, or 
skin fold, but the modern use of this tendon is more 
apparent, since, by its elasticity, it holds the wing 
when at rest automatically folded. This it can more 
easily do, since it stretches directly across the front 
angle and furnishes one of the few instances in Na- 
ture where a muscle pulls at the long end of a lever. 
This ligament and its membrane has another use— 
that of forming and maintaining a comparatively 
straight edge to the wing in all its positions in flight, 
and it is just probable that this has always been its 
sole use. If we conceive ligaments at C binding 7rd, 
sc, and the end of me at 7 together, and that wl, cu 
are similarly bound to mc near 0, it will be seen that 
when (C’ is drawn toward A, the point will auto- 
matically approach 4 without any muscular effort 
exercised on CD. 
There are many instances in Nature, however, 
where an old instrument, after ceasing to be useful in 
the old capacity, is put at new work under new con- 
ditions. ‘The bird’s fore leg is itself, as a whole, one 
of the most striking. 
Many of these changes have come about at the 
demand for lightness at the wing’s extremity, but 
other changes have been wrought for the sake of 
proper balance during flight. In quadrupeds and 
