Both jaws are completely destitute of true teeth, the want of which is, as we shall presently see, amply compensated. The upper jaw is either formed of one piece distinct from the skull and articulated with it, as in the parrots; or it is connected with it by means of yielding elastic bony plates, as in most other birds. These elastic plates admirably protect the bill (the upper part of which may be considered as an elongation of the intermaxillary bones) and the skull from the shocks of the former organ when used in pecking violently against hard substances.
In a few instances the upper jaw is entirely immovable. Blumen bach gives the Rhinoceros Bird and the Cock of the Wood (Tefrao Urogallus) as instances.
Bones of the Neck and Trunk.—The upper limbs, or, to speak more correctly, the anterior extremities of birds are calculated for flight, and entirely useless as prehensile organs, because the bird depends principally upon its bill to gather its food. To give a greater freedom of action to this organ, it was necessary, as the bones of the back have hardly any motion (the dorsal vertebra; being often anchylosed or immovably fixed by a continuation of bony secretion), that the neck should be long and flexible; and eminently flexible it is. In the mammifers the number of cervical vertebra, (neck-bones) is seven ; the Cameleopard (Giraffe) has no more, and the Elephant and Whale have no less. Cuvier indeed gives the Sloth nine. Professor Thomas Bell however has satisfactorily made out that the additional two are bones of the back, not of the neck. But, in Birds, nature has made up for the deficiency of motion in the back (a deficiency absolutely necessary to the comfortable existence of the animal, inasmuch as the back is the point of support to the wings) by the free grant of cervical vertebra', according to the wants which the peculiar habits of parti cular birds require. Thus the Raven has 12 neck-bones, the Domestic Cock 13, the Ostrich 18, the Stork 19, and the Swan 23, the largest number it is believed yet detected, while the smallest amounts to 10. The articulation is so contrived as to produce the greatest mobility, and that the contrivance is complete is proved by the ability of a bird to touch every point of its body with its bilL The vertebrae of the back are from 7 to 11 in number. There are no true lumbar vertebrre, for they are consolidated into one piece with the pelvis (os innominatum) which is elongated, broad, and simple ; and does not unite below, as in mammifcrs, to form what is called the symphysis pubis, but has the lateral portion& distinct from each other. This is the general rule. The pelvis of the ostrich forms an exception ; for it is joined below like that of most quadrupeds. In most of the quadrupeds the rump-bone (os coccygis) is prolonged into a true jointed taiL In birds it never is, but is very short, although it supports the large tail-feathers (rectrices).
Ten pairs of ribs are said to form the maximum among birds ; these, the true ribs, are joined to the breast-bone (sternum) by small inter vening bones. The false ribs (those which do not reach the breast bone) have a forward direction. There is a peculiar flat process directed upwards and backwards attached to the middle pairs of the true ribs.
The breastbone (sternum), a part of the greatest consequence, being the point of attachment for the most powerful of the muscles which set the wings in action, is composed of fire pieces strongly joined together, and prolonged below into a crest (crista) for that purpose.
The greater or less development of this crest or keel, and the greater or less ossification of the component parts of the breastbone, depend upon the wants of the bird. Those birds whose flight is strongest and most continuous have the crest very large, and the breastbone pieces very firmly cemented together, as any one may see who will examine the breast-bone of a hawk, or eagle, or that of a humming-bird ; while in the ostrich and cassowary this crest is entirely absent, and the breastbone presents a uniformly arched surface, somewhat like that of a Highlander's target In the crane and in the male wild-swan there is a cavity in the anterior part of the breast-bone for the reception of the involuted wind-pipe (trachea). The connection of the wings with the trunk is managed by means of the two clavicles, and of that peculiar fork-like elastic bone commonly called the merry-thought (furcula). This apparatus operates as an antagonist power to the action which would bring the wings together in flight, did not these bones, especially the merry-thought, keep the shoulders asunder. The greater or less development of this bone dependFron the exigencies of each particular cane. In birds whose flight is long and rapid it is strong, with the branches widely arched and carried forwards on the body; in birds which do not fly at all, in the ostrich, cassowary, and emu, for instance, the bone becomes a mere rudiment "In the ontrieh," as Macartney observes, "the two branches are very short, and never united, but anchylosed with the scapula (shoulder-blade) and clavicle (collar-bone). In the cassowary there are merely two little processes from the side of the clavicle which are the rudiments of the branches of the fork. In the emu there are two very small thin bones attached to the ante rior edge of the dorsal ends of the clavicles by ligaments ; they are directed upwards towards the neck, where they are fastened to each other by means of a ligament, and have no connection whatever with the sternum." The wing-bones are the homologues or representatives of the arms or upper extremities of man and of the monkeys. The following are the bones composing the wing of a bird :—The arm (os humeri); the fore-arm, consisting of two bones (ulna and radius); the wrist (carpus), formed by two bones ; the metacarpus, also made up of two bones ; a thumb, or rather the rudiment of one, there being but a single bone ; and two fingers, the finger next the thumb consisting of two portions, and the other only of one. To this hand are attached the primaries, or greater quill-feathers; the secondaries are affixed to the fore-arms ; and the arm supports feathers of inferior strength and development, called tertiaries and scapulars. The bone which represents the thumb gives rise to the bastard quills, and along the base of the quills are ranged the largest of those feathers which are denominated wing coverts Such is the structure of the 'sail-broad vans' which waft the condor over the Andes.