Reptiles and Birds / A Popular Account of Their Various Orders, With a Description of the Habits and Economy of the Most Interesting

by Louis Figuier

Fig. 45.—Skeleton of the Swan.

Fig. 46.

Wings alone, then, would fail to support the bird in space. The position renders a double system of breathing necessary. Vital heat in animals is always in proportion to their respiration, for the oxygen of the air, which penetrates every cell and cavity of their bones, feathers, and body, warming and giving increased activity to their circulation, and specific lightness to their bodies, from its rich organisation enables birds to live in the coldest atmospheric regions. In [Fig. 46] the respiratory organs of a Pigeon are represented. The trachea, or windpipe, is composed of many bony rings, varying in different species. In the Falcons it is slightly flattened, and tapers in a small degree; but in many genera it presents dilatations and contractions, and in others it is variously curved, two slender muscles, which run along its sides towards the sternum, serving to contract it. In many of the song-birds several pairs of small muscles are attached to the lower larynx where the tube bifurcates, by which they are enabled to control this organ, which is the producer of their note.

Fig. 47.

The trachea carries the air to the lungs in a Pigeon, and separates into two branches in the breast, where it abuts on the aërial sacs, and on the two lungs ([Fig. 47]). The air carried by the windpipe acts upon the blood through the thin substance of the cells which constitute the pulmonary tissue, in which it traverses in an infinity of minute vessels, whose thin walls are permeable by the gas.

The lungs are small, and placed in the upper part of the thorax, where they are confined on each side by a cavity bounded above by the ribs and below by an imperfect diaphragm; but they are perforated by tubes which communicate with membranous cells distributed over the thoracic and abdominal cavities, between the muscles, beneath the skin, and in all parts of the body—the air even penetrating many of the bones when the species are peculiarly aërial in their habits.

The external form of birds is modified so as to be subservient to aërial progression. The vertebral column, or spine, along the centre of which runs the spinal cord, is divided into three regions—the cervical, dorsal, and sacral regions—terminating in the caudal extremities, the number of vertebræ, or pieces, varying much in different genera. The body consists of the dorsal, sacral, and caudal parts of the column; laterally, of the ribs and pelvis; and beneath, of the sternum and the soft parts contained in it. Its anterior part, containing heart, lungs, and liver, is named the thorax; the posterior, the pelvis. The sternum, with the clavicles and scapulæ, is perhaps the most curiously modified part of the skeleton of birds.

The sternum, then, is a large expanded plate extended over the whole anterior part of the thorax, and even covering more or less what may be considered the abdomen. It varies greatly in different genera; but in all it is more or less four-sided, and convex externally, forming the basis for the powerful muscles by which the wings are moved. These wings serve as arms by which the bird guides itself, ascending or descending according to the impulse given them. "That the anterior form of birds is modified so as to be subservient to the aërial progression for which these animals are intended," says McGillivray, "is obvious and intelligible. Their bodies are oval, with the more powerful muscles placed on the breast, so that, when the horizontal position is assumed, the centre of gravity comes between the wings, and is kept near the lower part by the weight of the pectoral muscles. The length and flexibility of the neck enable the bird to make the necessary changes in the centre of gravity, while the solidity of the dorsal spine gives advantage to the action of the muscles. The head is terminated by a pointed bill, which aids in cleaving the air; the feet, when short, are drawn up and concealed under the feathers; when long, they are stretched out beneath or behind the tail, which is more or less expanded, and helps to support the body in the air, as well as, by acting in the manner of a rudder, to change its direction, or, by being expanded, to break its descent."