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." [Illustration: Fig. 48.] [Illustration: Fig. 49.] [Illustration: Fig. 50.] The wings of birds are acute or obtuse. The more angular the wing of birds--that is to say, the longer the feathers on the edge of the wing--the more rapidly does it propel itself through the air. The tail consists of a number of feathers, to which are attached a series of small muscles, one for each vertebra, which are capable of depressing and elevating the tail in various degrees; while a series of connections, whose fibres invest the base of the quills, curve round the edge of the tail. Their action is to spread out the tail-feathers, and incline them to the right or left; thus enabling it to perform the part of a helm or rudder as it cleaves the atmosphere. Besides flight, birds possess other means of locomotion. They are formed for walking or for swimming as well as for flying, according as their habits are aërial, terrestrial, or aquatic. Their general form, though possessing all the characteristics of the class, is modified and adapted to the kind of life they are intended to lead. Where the skin of a bird is covered with feathers, it is observed that the true skin, or derma, is thin and transparent; while the cuticle is thicker, and even covered with scales, in those parts where feathers are absent. Before addressing ourselves to the physiological functions of birds, a few words descriptive of their feathers, beaks, and claws will not be out of place.