: FIG. 6.]

It is broadly composed of two fans, A and B. The out-most fan, A, is
carried by the bird's hand; of which I rudely sketch the contour of the
bones at _a_. The innermost fan, B, is carried by the bird's forearm,
from wrist to elbow, _b_.

The strong humerus, _c_, corresponding to our arm from shoulder to
elbow, has command of the whole instrument. No feathers are attached to
this bone; but covering and protecting ones are set in the skin of it,
completely filling, when the active wing is open, the space between it
and the body. But the plumes of the two great fans, A and B, are set
into the bones; in Fig. 8, farther on, are shown the projecting knobs
on the main arm bone, set for the reception of the quills, which make
it look like the club of Hercules. The connection of the still more
powerful quills of the outer fan with the bones of the hand is quite
beyond all my poor anatomical perceptions, and, happily for me, also
beyond needs of artistic investigation.

73. The feathers of the fan A are called the primaries. Those of the
fan B, secondaries. Effective actions of flight, whether for support or
forward motion, are, I believe, all executed with the primaries, every
one of which may be briefly described as the strongest cimeter that can
be made of quill substance; flexible within limits, and elastic at its
edges--carried by an elastic central shaft--twisted like a windmill
sail--striking with the flat, and recovering with the edge.

The secondary feathers are more rounded at the ends, and frequently
notched; their curvature is reversed to that of the primaries; they are
arranged, when expanded, somewhat in the shape of a shallow cup, with
the hollow of it downwards, holding the air therefore, and aiding in
all the pause and buoyancy of flight, but little in the activity of it.
Essentially they are the brooding and covering feathers of the wing;
exquisitely beautiful--as far as I have yet seen, _most_ beautiful--in
the bird whose brooding is of most use to us; and which has become the
image of all tenderness. "How often would I have gathered thy children
... and ye would not."

74. Over these two chief masses of the plume are set others which
partly complete their power, partly adorn and protect them; but of
these I can take no notice at present. All that I want you to
understand is the action of the two main masses, as the wing is opened
and closed.

Fig. 7 roughly represents the upper surface of th