wing is closed, the forearm is the
segment that reaches obliquely forward. The wrist joint is the second
angle of the wing. In the wrist there are two small bones (the radiale
and ulnare) which serve an important purpose in joining the forearm
with what is known as the hand, and make possible the specialized
movement of the two parts upon each other. The hand is the terminal
segment of the wing, composed of the metacarpal bones and the digits or
fingers. Of the last-named organs there are ordinarily three, forming
a graceful tapering point to the wing, and giving to it the symmetry
and proportion that are required for effective use. When the wing is
folded, the hand extends obliquely downward and backward.

Now, these bones and their attendant ligatures are wonderfully and
wisely contrived. The humerus moves freely in its socket in the
shoulder, so that it can be swung in every required direction, and yet,
as should be the case, its principal movement is up and down in a
vertical line--the precise movement required for the effective
wingstrokes in flight. But note further. The elbow joint, unlike that
of the shoulder, is a rigid hinge, permitting motion in only one plane,
that of the wing itself, or nearly so. The same is true of the wrist
joint, which holds the hand firmly, allowing no motion save that which
opens and closes the wing. The wisdom of this arrangement will be seen
at a glance.

In the human arm the hand can be moved in every direction with the
greatest freedom, and, moreover, the wrist may be turned and the hand
laid on its back, its palm, its edge, or at almost any conceivable
angle. This is a very convenient contrivance for man, but it would be
a great misfortune for our avian friends if their wings would rotate so
readily; for in that case they would not have sufficient rigidity to
answer the purposes of flight, but would be twisted into every position
by the assaults of the air currents. Besides, even in ordinary flight
it would require a constant muscular effort to keep the wings in the
proper position. How wisely Nature has devised the bird's flying
apparatus! When outstretched, it is held firmly by the power of its
own mechanism, with its broad under surface lying horizontally, and no
breezy current can bend or twist it from its normal position.

The set of muscles that open the wing are called the extensors, and
those that close it, the flexors. The former lie upon the back of th