Birds, of all animal creation, can traverse distances with the greatest rapidity. The fleetest among the Mammifera cannot run over five or six leagues in an hour. Certain birds easily traverse their twenty leagues in the same interval of time. In less than three minutes we lose sight of a large bird, such as a Kite or an Eagle, whose body is more than a yard from wing to wing. It is assumed, from these facts, that these birds traverse more than fifteen hundred yards each minute, or more than fifty miles in an hour. A Falcon of Henri II. strayed from Fontainebleau in pursuit of a Bustard; it was taken the next day at Malta. Another Falcon, sent from the Canaries to the Duke de Lermes, in Spain, returned from Andalusia to the Peak of Teneriffe in six hours--the flight representing a distance of two hundred and fifty leagues. In short, the whole organisation gives to a bird that remarkable lightness which contributes so much to its velocity. Not to speak of the feathers with which it is covered, its bones are hollow and form large cells, called _aërial sacs_, which it is able to fill with air at will, and its sternum is furnished with a bony frame or breast-bone, formed somewhat like the keel of a ship, into which the pectoral muscles are inserted--which, besides being largely developed, in birds of flight possess remarkable contractile properties. The vocal apparatus in birds, represented in Figs. 70 and 71, is very complicated, and differs from the human larynx and trachea. It consists of a kind of osseous chamber; which, however, is only a swelling in the arterial trachea at the point where it bifurcates and enters the breast to form the bronchial tube. It is this formation, called the lower larynx, which constitutes the organ of song. Five pairs of muscles, attached to the walls of this chamber, stretch or relax the vocal chord, by which means they enlarge or diminish the cavity of the larynx. Whoever has watched any song-bird singing must have noted the swelling and contracting of its throat as it poured out its melody, modifying, in a thousand ways, the tension of the vocal chords and of the larynx, and producing those marvellous modulations whose perfection must always be a subject of astonishment and admiration. [Illustration: Fig. 70.] [Illustration: Fig. 71.]