on each side, at a point as high and forward
of the eontre of ^r-i^ity as feasible; is the frame-
work of the sliouUler-joint, braced against the
treraenjous leverage of the winjjs: 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 bone
placed above the ribs, and lying parallel to the
spine, imbedded in muscles and lirmly 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 do^vnward and backward from the shoul-
der-joint, imbedded in muscles, and unite under
the neck, and just in advance of the sternum, to
form the fnrculum (fork), which we know as
the 'wish-bone", and the English call 'merry-
thought.' " See Folk-lobe.
The wing-bones are homologous with those of the arm 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 tlie arm of a man presents, when compared with the fore limb of a quadrujied 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 Ix'tween jironation and supinati<m, and the manus is bent back upon the ulnar 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 tlian 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 jn'oximal end of the three first metacarpals, and iiU trace of their originally ■separate existence disappears." The metacarpvis consists of three bones (I., II., HI.), which are more or less flattened and fused. The first bears the pollex or 'thumb' (whence springs, in some hirds, the bastard wing or alula), and the second the index finger, which is long, straight, and has two, or sometimes three, ])halanges. These hones complete the wing, except that a few species have a rudimentary third finger. Archa-- optery.x, however, had three well-developed fin- gers, all armed with claws; and in embryo os- triches a claw sometimes ap])cars on the third finger. Such is the bony framework of the wings, operated by muscles of immen.se power (the pectoral muscles are said to make about one- fourteenth of the weight of the whole body in birds of prey, and one-eleventh in a wild goo.se) ; but their usefulness depends upon the e.xternal furnituri' of feathers, heretofore <leseribe(l, which forms llu> expanse necessary for buoyancy and flight (see following paragraph).
T*IuscuLAR System. The muscles of birds con- form, in general, with the vertebrate type, though the homologies, especially about the fore limbs, are sometimes obscure. For these homol- ogies, Newton's review in Bronn's Thier-licich (Vbgel, pp. 9-32.5) is complete, and is epito- mized in the former's Dictionary of Birds, pp. 002-620. Specified arrangements of muscles and tendons, especially the plantar tendons, have been much experimented with as a basis of classi- fication, but the results have not proved of great value when taken alone. Only a few general considerations need be presented here. The mus- cular system of birds is characterized b_v an extraordinary strengtli in those parts concerned with activity and endurance, and has certain interesting automatic functions. Thus the vast pectoral muscles attached by a broad fleshy base to the sternum and narrowing to a complicated tendinous attachment to the wing-bones, are mainly two, forming the thick whitish 'breast' of edible fowls; they depress the wing — the eil'ec- tive movement, in flight. An elastic band in front of the wing passes from the head of the humerus to the carpal joint ; when the wing is extended it is sti'etched about three times its ordinary length, and its natural contraction enables the bird to keep the wing closed without ert'ort. As it lies just beneath the skin, it is easily cut, and as such a severance prevents the wing from being firmly extended, it ofl'ers the best method of rendering captive birds incapable of flight; to disable one wing is quite enough. See Sluscu-L.R SY.STEM, EVOLT TION OF.
"In the hind limb of most birds there is a singular e.xtensor muscle, which arises from the pubis, ends in a tendon which passes to the outer side of the knee-joint, and terminates in the leg by uniting with the flexor digitorum perforatus. The result of this arrangement is, that the toes are flexed whenever the leg is bent upon the thigh, and, con.sequently, the roosting bird is held fast upon his perch by the weight of his own body." (Huxley. Anatomy of Yertebrated Animals, New Y(nk, 1878.) Vlsceral vSy.stem. The digestive apparatus of birds resembles that of mammals; exhibiting, however, various modifications, according to the dill'crent kinds of food — some birds feeding on flesh, others on fish, others exclusively on insects, others on seeds, others more indiscriminately on a variety of animal and vegetable substances. Few birds masticate their food in any degree; upon being swallowed, it enters tlu> crop or crair, an enlargement of the a?soi)hagus situated just before the l)reast-bone. where it is moistened by saliva and partly digested, so that its contents, easily regurgitated, form a suitable food for nestlings in some families, as, notably, the pigeons. The crop is wanting in the ostrich, and also generally in birds that feed on fish, and is of greatest size in those of which the food consists of seeds or grain. It is generally single, and on one side of the gullet; sometimes, as in pigeons, it is double. A second dilatation of the (esophagus, called the proroitriciilus, is generally largest in those birds in which the crop is wanting or small; and in this the food is further softened and chemically treated. The third and i)rineipal stonu.cli is the gizzard (q.v.), which in liirds of prey, fish-eating birds, etc., is a mere mend)ranous sac, but in birds which feed on grain or seeds is very thick and muscular.
