Fore-limb of a bird

The fore-limb of a bird, when in a state of rest, exhibits a great change of position if it be compared with that of an ordinary reptile; and this change is of a character similar to, but in some respects greater than, that which the arm of a man presents, when compared with the fore-limb of a quadrupedal mammal. The humerus lies parallel with the axis of the body, its proper ventral surface looking outward. The forearm is in a position midway between pronation and supination, and the manus is bent back upon the ulnar side of the forearm, in a position, not of flexion, but of abduction.

In ordinary birds, the proximal end of the humerus is expanded, and its articular head transversely elongated; its ventral face is convex, and provided with a strong preaxial ridge, which gives attachment to the pectoral muscle. The proper dorsal face is concave from side to side, especially toward the postaxial margin, where the pneumatic aperture occurs in those birds which have the humerus hollow. The distal end is expanded, and the articular surface for the radius is a convex facet, directed obliquely inward, on its ventral face. In this respect the bird's humerus exaggerates a feature of that of the Lizards.

In the Ratitae these peculiarities are very feebly, or not at all, marked, the humerus being a slender, cylindrical, slightly curved, bone. In the Casuaridae, Dinornithidoe, and Apterygidae, the fore-limb is extraordinarily reduced, and may become rudimentary. In the Penguins and, to a less degree, in the great Auk, the humerus becomes flattened from side to side; the proximal end is singularly modified, and, at the narrow distal end, the articular surface for the radius lies completely in front of, and rather above, that for the ulna.