Wings and Shoulder-Girdle.—The anterior limbs or wings of a bird are the fore limbs of other vertebrates modified for the purposes of flight. They are attached to the shoulder-girdle or pectoral arch of the skeleton, which in this class is of extraordinary strength and impor tance. It consists of the thoracic ease formed by the dorsal vertebra•, the true ribs and the breast-lmmes (sternum), which are connected by ligaments and ossifications. All the bones of birds are dense and hard. The sternum is re markably large and strong, serving for the at tachment of the muscles which depress the wings. It generally exhibits a projecting ridge, or keel, along the middle, which is proportionately largest in birds of most powerful night or swim ming ability. and is want ing only in the flightless birds (see above). This plate ossifies only as the bird imitates, being in early life a cartilage, the flexibility of which may easily be felt in young fowls, affording housekeepers a test for alleged'spring'ehickens. The variations in form of the sternum are of considerable taxonomic significance. Outside and attached to this case on each side, at a point as high and forward of the centre of gravity as feasible; is the frame work of the shoulder-joint, braced against the tremendous leverage of the wings: it consists of the scapula, or shoulder-blade, a coracoid and a clavicle, the junction of which forms a cup in which rests the head of the humerus. The scapula is a more or less sabre-shaped hone placed above the ribs, and lying parallel to the spine, imbedded in muscles and firmly attached (sometimes fused) at the forward extremity to the head of the coracoid. The coracoid is a pil lar-like bone which serves as the main support of the shoulder. "for while at one extremity it sustains the wing, at the opposite end it is firmly united to the front of the sternum. A third brace is afforded by the combined clavicles, which curve downward and backward from the shoul der-joint, imbedded in muscles, and unite under the neck, and jut in advance of the sternum, to form the furculum (fork), which we know as the 'wish-bone', and the English call 'merry thought.' " See FOLK-LORE.
The wing-bones are homologous with those of the arid in other vertebrates, but have be come modified in both form and relative position to adapt them to the requirements of flight.
"The fore limb of a bird, when in a state of rest," says Huxley, "exhibits a great change of position if it be compared with that of an ordinary reptile ; and the change is of a charac ter similar to. but in some respects greater than, that which the arm of a man presents, when compared with the fore limb of a quadruped mammal. The humerus lies parallel with the axis of the body, its proper ventral surface look ing outward. The forearm is in a position mid way between pronation and supination, and the minus is bent hack upon the nlnar side of the forearm, in a position not of flexion, but of ad duction." The humerus is usually short as compared with the radius and ulna (the latter is more curved and stouter than the former) which together form the forearm and main stretch of the visible wing; these remain separate, and at their outer extremities articulate with the two small bones of the wrist (carpus) which alone remain free. The other. distal, carpal bones "fuse in the embryo with the proximal end of the three first metacarpals, and all trace of their originally separate existence disappears." The metacarpus consists of three bones ( T., II., which are more or less flattened and fused. The first bears the pollex or 'thumb' (whence springs, in some birds, the bastard wing or alula), and the second the index finger, which is lung, straight, and has two, or sometimes three, phalanges. These bones complete the wing, except that a few species have a rudimentary third finger. Archa opteryx, however, had three well-developed fin gers, all arined with claws; and in embryo os triches a class sometimes appears on the third finger. Such is the bony framework of the wings, operated by muscles of immense power (the pectoral muscles art' said to make about one fourteenth of the weight of the whole body in birds of prey, and one-eleventh in a wild goose) ; hut their usefulness depends upon the external furniture of feathers, heretofore deseribed, which forms the expanse necessary for buoyancy and Slight (see following paragraph),