Page:Natural History, Birds.djvu/17

4 than the secondaries. Attached to the little bone which represents the thumb are two or three short and very stiff feathers, called the winglet; they lie upon the basal part of the first primaries at the very edge of the wing. Corresponding to these series of feathers there are, both on the outer and inner surface of the wing, several rows of smaller ones, called coverts, from their office of covering the basal part of the quills.

The feathers of the wing, overlapping each other, present a continuous surface of great breadth, with which the repeated strokes upon the air are performed, which constitute flight. Each feather is concave, whether we regard it transversely or longitudinally; its stem, or midrib, is remarkably strong, though very light, and the beards, which present their edges in the direction of the stroke, are linked to each other by a series of minute hooks. All of these provisions increase the power of the wing in its downward strokes upon the resisting air.

To use these broad fans with sufficient force to impel the bird through the air, large and vigorous muscles are required. Accordingly, in Birds, particularly those of long and powerful flight, the greatest portion of the whole muscular force of the animal is concentrated upon these organs. The muscles which produce the downward stroke of the wing are enormous; and, for their attachment, the breast-bone is not only