Fig. 48.
Fig. 49.
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.
The covering of birds is known by the general name of plumage. It is composed of many individual feathers. The feathers are horny productions, consisting of a hollow tube or barrel, and a stem rising from it. Chemically, this covering is of the same material as the hair on Mammals and the scales on reptiles and fishes, differing only in its mechanical structure. Besides the more conspicuous feathers, most birds have an underneath covering of smaller ones known as down-feathers. A feather of the ordinary kind consists of the tube, or barrel, by which it is attached to the skin, varying in length according to the species; the stem, or shaft, composed internally of a soft, compact, but elastic substance of a whitish colour, and in its buoyancy not unlike cork; the web, which is a lateral prolongation of the external coating of the shaft, and which assumes the form of a thin linear membrane springing from it at an angle more or less acute in different species: this is the barb. From the upper edge of each barb two sets of minute filaments proceed at an angle similar to that of the barb itself in respect to the shaft. These smaller filaments are the barbules, by means of which the barbs are retained in opposition—not by the barbules of one barb interlocking with those of another in the manner of dovetailing, but by the anterior series of one barb overlapping and hooking into the recurvate formation of the barb next to it (Figs. 51, 52). The barbules themselves frequently throw out filaments in the same manner, which are called barbicels, whose object is apparently the same—namely, that of connecting and retaining the barbules in position. These may be observed, by the aid of a small magnifying glass, in the quills of the Golden Eagle, Aquila chrysaëtus.