SKELETON OF BIRDS.
The skull, which in the adult has no sutures, is articulated to the spine by a single rounded condyle. This structure gives the head a great freedom of motion, particularly in the horizontal direction. It enables the bird to place its bill be tween the wings when asleep ; a situation in which none of the mammalia can place the snout.
The lower jaw is articulated to the cra nium by means of a square bone on each side, called os quadratum. The superior mandible, which is completely immove able in mammalia, has, with a few excep tions, more or less motion in birds. It either constitutes a particular bone, dis tinct from the rest of the cranium, to which it is articulated, as in the psittaci (birds of the parrot kind ;) or it is con nected into one piece with the cranium, by means of yielding and elastic bony plates ; as is the case with birds in ge neral. It is quite immoveable in a very few instances, as the tetrao urogallus (cock of the woods) and the rhinoceros bird.
The jaws are entirely destitute of teeth. The bill may be considered, in some de gree, as supplying the place of teeth; yet as none of these animals masticate their food, but swallow it whole, the bill can only be compared to the incisors of such animals as use them for seizing and procuring their food.
It consists of a horny fibrous similar to that of the nail, or of proper horns ; and is moulded to the shape of the bones which constitute the two man dibles, being formed by a soft vascular substance, covering these bones. Its form and structure are as intimately con. nected with the habits and general cha racter of the animal, as those of the teeth are in the mammalia. Hence an enume ration of its different figures and consis tence belongs properly to the department of natural history, where it forms the foundation of classical distinctions.
The accipitres, or rapacious birds, have it very hard, hooked at the end, and furnished with a process on either side ; calculated, therefore, in all respects, for seizing and lacerating their prey. Those of the parrot kind have it also hard, for bruising the firmer vegeta ble fruits; and the wood-pecker, nut hatch, &c. for penetrating the bark of trees.
Those birds, which take a softer kind of food, and which require a sense of feeling in the part, for distinguishing their food in mud, water, &c. have it approach
ing to the softness of skin. Such are, the duck, snipe, woodcock, &c.
In several classes, particularly the ac cipitres and gallinx, the base of the bill is covered with a soft skin, called the cire, of unknown use.
The cervical vertebra of birds are very numerous,and have a very free motion on each other. This great mobility of the neck enables the animal to touch every point of its body with the bill ; and thus supplies the want of the pre hensile faculty of the anterior extremity. The sternum is prolonged below into a vertical process (crista)• for the attach ment of the strong pectoral muscles, which are the chief agents in the act of flying. In the male wild swan (anas cygnas) and in some species of the genus ardea, as the crane, this part forms a pe culiar cavity for the reception of a consi derable portion of the trachea. The crista is entirely wanting in the ostrich and cassowary ; where the sternum pre sents, on its anterior or tinder surface, an uniform convexity, and this peculiarity of structure is accounted for by observ ing that these birds have not the power of flying.
The wings are connected to the trunk by means of three remarkable bones. The clavicles, which are always strong, constitute straight cylindrical bones, ar ticulated to the sides of the front of the sternum, and extending straight forwards. Their anterior extremities are connected to the sternum, by means of a bone pe culiar to birds, viz. the fork-like bone, or, as it is more commonly termed, the mer rythought. (Furcula, in French, la hi nette or fourchette.) The scapula, which is flattened in form, but elongated, ex tends baCkwards from the front of the clavicle, parallel to the spine. The point of the fork-like bone is joined to the most prominent part of the keel of the ster num ; and the extremities of its two branches are tied to the humeral ends of the clavicles, and the front of the scapu la, just where these bones join each other, and are articulated with the hu merus. Hence it serves to keep the wings apart in the rapid motions of fly ing.