Page:Natural History, Birds.djvu/18

Rh greatly enlarged, but its surface is still further increased by its medial portion rising into a high perpendicular keel or ridge, the two faces of which, from their direction, afford a point of resistance, or purchase, of peculiar advantage.

To resist the tendency of the shoulders to be drawn together by the powerful muscular action exerted during flight, there is inserted between the two bones (coracoids) to which the shoulder-blades are attached, a singular bone of an arched form, well known as the merry-thought. In the common fowl, which flies but little and weakly, this bone is feeble, but in birds of vigorous flight, as the Hawks, the Swallows, and the Humming birds, it is very strong and elastic. On the other hand, where the bird never rises upon the wing, as in the case of the Ostrich, Emu, and kindred birds, this bone is reduced to a mere rudiment. The posterior extremities also differ materially in structure from those of quadrupeds. The general number of toes is four, which is never exceeded; but a few birds have but three, and the Ostrich but two. For the most part, three toes are directed forwards, and one, answering to the great toe, backwards. The climbing birds, as the Parrots and Woodpeckers, have the outer toe also reversed; while the Swifts have all the four turned forwards. The feet are much more lengthened than in the Mammalia generally,