he piece formed by the two
united bones, the limb is continued by two joints, forming a second
finger, inside of which there is usually a single small bone,
representing a third finger. But all these parts are stiffly attached
to one another, admitting of very little motion, so that the whole
hand forms as it were a single piece. The bony structure of the bird's
wing is in point of fact a rod hinged in two places, at the elbow and
the wrist, for the convenience of being folded into a small compass.
The flight of the bird is effected by the agency of a number of stiff
feathers implanted in the skin covering the bones and muscles of the
arm and hand; these fold together like the sticks of a fan when the
wing is folded, and are spread into an elastic instrument for striking
the air when the different sections of the bony framework are extended
by the action of their respective muscles.

In the Bat the structure is very different. Of course, as in the
Vertebrata generally, we find in the Bat's fore-limb the same three
main sections as in birds; and as the function of the limb is the
same, and a certain stiffness is necessary in the extended organ, the
movements of the joints at the elbow and wrists are hinge-like. But
the bones of the arm and fore-arm are longer and more slender,
especially the latter; and in this part, in place of the two parallel
bones of the bird's wing, we find in the Bat only a single long bone
representing the smaller bone of the bird, the larger one being
usually reduced to very small dimensions, and firmly united with the
other into a single piece, although it still forms the elbow-joint. At
the other end of this long fore-arm we find some small wrist-bones and
to these the fingers are articulated. In birds, as we have seen, only
two or three fingers are represented, and these are more or less
reduced in size, and the most important of them soldered together;
Bats, on the contrary, show the whole five fingers as distinctly as in
the hand of man or any other mammals. The first of them, or the
_thumb_, is short, slender, and flexible, and composed of three
joints; the other four are very long and slender, but chiefly
composed of the metacarpal bones, corresponding to those of the palm
of the human hand. The first, or index finger, indeed, in many Bats,
consists of this bone alone; but in the others it is followed by two
or three slender joints, gradually tapering to the extremity, the
second finger,