FEATHERS, a complicated modification of the tegumentary system forming the exter nal covering or plumage of birds, and peculiar to this class of animals. Notwithstand ing the varieties of size, strength, and color, all F. are composed of a quill or barrel; a shaft; and a vane, beard, or web, on either side of the shaft, the vane consisting of barbs and barbules.
The quill by which the feather is attached to the skin is wider but shorter than the shaft, and forms a semi-transparent, horny, cylindrical tube, which terminates below in an obtuse extremity, presenting an orifice termed the lower umbilicus. A second orifice, leading into the interior of the quill, and termed the upper umbilicus, is situated at the opposite end, where the two vanes meet and unite. The cavity of the quill contains a series of conical capsules fitted one upon another, and united by a central pedicle; and the whole structure presents a remarkable combination of strength and lightness.
The shaft is always of greater length than the quill, and tapers gradually to its free extremity; it is flattened at the sides, is more or less convex on the back, and presents a longitudinal groove inferiorly. It is composed of white, elastic, spongy structure, which is covered by a thin horny sheath.
At the point of junction of the shaft and quill, we usually observe—except on the F. of the wings and tail—a small supplementary shaft given off, which is furnished with barbs or fibers, and is termed the plumule or accessory plume. In the ostrich it is altogether absent; in the rhea, it is represented by a tuft of down ; in the emu, on the other hand, it equals the original F. in size, so that the quill supports two shafts; and in the cassowary there is a second plumule of considerable size, so that the quill presents three distinct shafts.
The vanes or webs are composed of numerous barbs or small fibers arranged in a single series along each side of the shaft. They are fine prolongations of the outer coat of the shaft, are of a flattened form, and lie inclined towards the apex of the feather, with their fiat sides toward each other, and their margins in the direction of the external and internal sides of the feather. The barbs are broader near the shaft than at the free
apex, and in the large wing-feathers the convexity of one is received into the concavity of another. They are, however, generally kept in position by the barbules, which are minute curved filaments arising from the upper edge of the barb, much as the latter arises from the shaft. There are two sets of these barbules, one curved upwards, and the other downwards, and those of one barb hook so firmly into those of the next, as to form a close and compact surface. In the ostrich, the barbules are well developed, but are loose and separate, and it is this arrangement which gives to the F. of this bird their soft, plumous appearance.
F. present numerous gradations of structure. In the cassowary, the wings, instead of being provided with ordinary F., are furnished with five cylindrical stalks destitute of barbs, so that here we have merely the quill and shaft. On the breast of the wild turkey there is a tuft of F. resembling long black hair. In the dasylopltus eumingii, the F. of the crest, breast, and throat are changed, at their extremities, into round, horny lamellae, looking like shining black spangles; and in the common waxwing or Bohemian chatterer, some of the wing-feathers present at their extremities small horny expansions, resembling red sealing-wax, both in color and consistence.
Besides the common F., the skin of many birds, especially of aquatic species—in which plumules rarely exist—is covered with a thick coating of down, which may be described as consisting of very minute F., each of which is composed of a very small soft tube lying in the skin, from the interior of which arises a minute tuft'of soft fila ments, without any central shaft. This downy covering secures warmth without weight, like the soft fiir at' the base of the hair of arctic Mammals. In most birds, the skin also bears a good many scattered hair-like appendages, which indicate their rela tions to the ordinary F. by the presence of a few minute barbs towards the apex.