Page:Natural History, Birds.djvu/16

Rh last named organs are likewise proportionally very large, arranged along on each side of the spine, and occupying the hollows between the bases of the ribs. By this great development of the respiratory system, the blood is more rapidly and effectually oxygenised, and muscular energy greatly increased for the action of flight; while by the animal heat thus evolved the air contained in the complex apparatus described is rarified, and so the body is increased in bulk, but relatively diminished in weight.

The wings of a Bird correspond to the arms and hands of man; but the hand is composed of but two fingers and a thumb, all of which are rudimentary. From the bones of the hand arise the primaries, or quill-feathers properly so called, which are ten in number; these are the largest and strongest feathers of the wing, and the character and power of flight is indicated by their form, stiffness, and relative length. From the principal bone of the fore-arm arise the secondaries, the number of which varies in different species; they are usually shorter, broader, and more flexible than the primaries; and are less removed in form from the general clothing feathers of the body. The bone of the upper-arm (humerus) gives rise to another series of feathers, called tertiaries: these, in some birds, particularly the Plovers, Curlews, &c., are greatly lengthened; they are, however, still weaker in their structure