Fig. 323. - A, Breast-bone, shoulder-girdle, and fore-limb of Penguin (after Owen): b Sternum, with the sternal keel; s s Scapulae ; k k Coracoid bones ; c Furculum or merry-thought, composed of the united clavicles ; h Humerus : u Ulna; r Radius ; t Thumb; m Metacarpus ; p p Phalanges of the fingers. B, Ribs of the Golden Eagle: a a Ribs giving off (b b) uncinate processes ; c c Sternal ribs.
We have next to consider the structure of the bones which compose the fore-limb or "wing" of the bird; and as this organ is the one which chiefly conditions the peculiar life of the bird, it is in it that we find some of the most characteristic points of structure in the whole skeleton. Though considerably modified to suit its function as an organ of aerial progression, the wing of the bird is readily seen to be homologous with the arm of a man or the fore-limb of a Mammal (fig. 323, A, and fig. 324). The upper arm (brachium) is supported by a single bone, the humerus, which is short and strong, and articulates above with the articular cavity formed partly by the scapula and partly by the coracoid (fig. 324, h). The humerus is succeeded distally by the fore-arm (antibrachium), constituted by the normal two bones, the radius and ulna (fig. 324, r, u), of which the radius is the smaller and more slender, and the ulna the larger and stronger. The ulna and radius are followed inferiorly by the bones of the wrist or carpus; but these are reduced in number to two small bones, one radial and one ulnar, "so wedged in between the antibrachium and metacarpus as to limit the motions of the hand to those of abduction and adduction necessary for the folding up and expansion of the wing; the hand is thus fixed in a state of pronation; all power of flexion, extension, or of rotation, is removed from the wrist-joint, so that the wing strikes firmly, and with the full force of the contraction of the depressor muscles, upon the resisting air" (Owen). One other bone of the normal carpus (namely, the "os magnum") is present, but this is anchylosed with one of the metacarpals. There are thus really three carpal bones, though only two appear to be present. (According to Morse, there is a fourth carpal, which early anchyloses with the base of the metacarpal of the middle finger.) The carpus is followed by the metacarpus, the condition of which agrees with that of the carpal bones. The two outermost of the normal five metacarpals are absent, and the remaining three are anchylosed - together with the os magnum - so as to form a single bone (fig. 324, m). This bone, however, appears externally as if formed of two metacarpals united to one another at their extremities, but free in their median portion. The metacarpal bone which corresponds to the radius is always the larger of the two (as being really composed of two metacarpals), and it carries the digit which has the greatest number of phalanges. This digit corresponds with the "index" finger, and it is composed of two, or sometimes three, phalanges (fig. 324, p). At the proximal end of this metacarpal, at its outer side, there is generally attached a single phalanx, constituting the so-called "thumb" (fig. 324, /), which carries the "bastard-wing," and is sometimes furnished with a claw. The digit which is attached to the ulnar metacarpal corresponds to the middle finger, and never consists of more than a single phalanx (fig. 324). In the Apteryx and the Cassowary there is only one complete digit to the hand.
Fig. 324. - Fore-limb of the Jer-falcon. h Humerus ; r Radius ; u Ulna ; /"Thumb;" m Metacarpals, anchylosed at their extremities ; p p Phalanges of fingers.
As regards the structure of the posterior extremity or hind-limb, the pieces which compose the innominate bones (namely, the ilium, ischium, and pubes) are always anchylosed with one another; and the two innominate bones are also always anchylosed, by the medium of the greatly-elongated ilia, with the sacral region of the spine. In no living bird, however, with the single exception of the Ostrich, are the innominate bones united in the middle line in front by a symphysis pubis. The stability of the pelvic arch, necessary in animals which support the weight of the body on the hind-limbs alone, is amply secured in all ordinary cases by the anchylosis of the ilia with the sacrum.
As in the higher Vertebrates, the lower limb (fig. 325, A) consists of a femur, a tibia and fibula, a tarsus, metatarsus, and phalanges; but some of these parts are considerably obscured by anchylosis. The femur or thigh-bone (fig. 325, A, /) is generally very short, comparatively speaking. The chief bone of the leg is the tibia (t), to which a thin and tapering fibula (r) is anchylosed. The upper end of the fibula, however, articulates with the external condyle of the femur. The ankle-joint is placed, as in Reptiles, between the proximal and distal portions of the tarsus. The proximal portion of the tarsus, consisting of two bones, representing the astragalus and cal-caneum or the former only, is undistinguishably amalgamated with the lower end of the tibia. The distal portion of the tarsus is anchylosed with the second, third, and fourth metatarsals to constitute the most characteristic bone in the leg of the Bird - the "tarso-metatarsus" (m). In most of the long-legged birds, such as the Waders, the disproportionate length of the leg is given by an extraordinary elongation of the tarso-metatarsus.