[Illustration: FIG. 31.--SKELETON OF THE SUPERIOR LIMB OF A BIRD (VULTURE): LEFT SIDE, EXTERNAL SURFACE.
1, Humerus; 2, radius; 3, ulna; 4, radial bone of the carpus; 5, ulnar bone of the carpus; 6, first metacarpal; 7, second metacarpal; 8, third metacarpal; 9, first digit, the homologue of the thumb; 10, first phalanx of the second digit; 11, second phalanx of the second digit; 12, third digit.]
For the rest, in order to be able to distinguish readily the corresponding parts in the hand of a bird and that of a man, we merely have to place the human forearm obliquely, in a direction upwards and forwards (Fig. 32), the radius being above; this position we can obtain by semi-pronation; then, to incline strongly the hand downwards and backwards, moving the ulnar border of the hand towards the ulna; the thumb is then anterior, the little finger posterior, and the palm of the hand is turned towards the trunk.
The carpus in birds is formed by two bones only, with which the skeleton of the forearm articulates. That which is in contact with the radius is called the _radial bone of the carpus_; and that with which the ulna articulates is named the _ulnar bone_.
[Illustration: FIG. 32.--SUPERIOR LIMB OF THE HUMAN BEING, THE DIFFERENT SEGMENTS BEING PLACED IN THE ATTITUDE WHICH THE CORRESPONDING PARTS OCCUPY IN BIRDS: LEFT SIDE, EXTERNAL SURFACE.]
The metacarpus is formed of three bones; the first, which is very short, is fused at its superior extremity with the adjoining one; this latter and the third, both longer than the first, but of unequal size, are fused at their extremities. The metacarpal, which articulates with the radial bone of the carpus, is larger than the one which is in line with the ulna. To the metacarpus succeed three digits, of which the central is the longest, and is formed of two phalanges; the other two are formed each by a small, stylet-shaped bone. The middle finger, situated on the prolongation of the metacarpal, which articulates with the radial bone of the carpus, has its first phalanx large and flattened transversely; this phalanx seems to have been formed by the union of two bones of unequal development; the second phalanx is styloid in form. As to the other two fingers, they are placed, one in front and the other behind; the first, which articulates with the short metacarpal, fused at its upper end with the principal bone of the metacarpus, in position represents the thumb. The other, which is the third finger, articulates with the inferior extremity of the thinnest bone of the metacarpus; it is sometimes closely united to the corresponding border of the first phalanx of the large--that is to say, of the median--digit.