Feathers, a complicated modification of the tegumentary system, forming the external covering or plumage of birds. Though chemically similar to and homologous with the hair of mammals, their anatomical structure is in some respects different. An ordinary feather is composed of a quill or barrel, a shaft, and a vane or beard consisting of barbs and barbules.
i The quill, the part attached to the skin, is a hollow cylinder, semi-transparent, composed of coagulated albumen, resembling horn both in appearance and chemical constitution. It is light, but strong, terminated below by an obtuse extremity pierced by an opening, the lower umbilicus, through which the primary nutritive vessels enter; above, it is continuous with the shaft, with which it communicates internally by an opening, the upper umbilicus;
' the cavity contains a series of conical shrivelled membranes, fitting one upon the other, that have formerly been subservient to the growth of the feather. The shaft is more or less quadrilateral, gradually diminishing in size to ! the tip; it is always slightly curved, convex above, and the concave lower surface, divided ' longitudinally by a groove, presents two inclined planes meeting at an obtuse angle; it is covered by a thin horny layer, and contains in its interior a white, soft, elastic substance, called the pith, which supplies strength and nourishment to the feather. The vane consists of two webs, one on each side of the shaft, each web being formed of a series of laminae or barbs, of varying thickness, width, and length, arranged obliquely on the shaft, and composed of the same material; their flat sides are placed close to each other, enabling them to resist any ordinary force acting in the direction of their plane, as the impulse of the air in the act of flight, though yielding readily to any force applied in the line of the shaft.
The barbs taper to a point, but are broad near the shaft, and in the large wing feathers the convexity of one is received into a concavity of another; but the barbs are kept in place chiefly by barbules, minute curved filaments arising from the upper edge of the barb, as the latter does from the shaft; there are two sets, one curved upward and the other downward, those of one barb hooking 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 long, loose, and separate, giving that soft character conveyed by the term plume. The barbules are sometimes provided with a similar apparatus on their sides called barbicels, as in the quills of the golden eagle and albatross; these serve to keep the barbules in position, but are less numerous than the latter. In most feathers there is an appendage near the upper umbilicus of a downy character, called the accessory plume; small in the quills of the wings and tail, in some body feathers of hawks, ducks, and gulls it is of large size, in some species as large as the feather which supports it; in the emu two plumy feathers arise from one quill, and sometimes three in the cassowary, the additional plumes being these accessory feathers; in the ostrich there is no such additional tuft.
There is, therefore, every gradation from a simple barrel and shaft, as in the cassowary's quills, to the feather,with barbs, barbules, and barbicels. Some feathers are all downy, like the abdominal ones of the eagle-owl; others have very little down, as the harsh plumage of the penguin; in the eider duck, and other arctic species, there is at the base of the common feathers a soft downy covering, securing warmth without weight, like the soft fur at the base of the hair of arctic mammals; young birds are covered with down before the development of feathers, the latter being guided through the skin by the former. In the chick the formation of down begins on the eighth day of incubation, and is continued until the hatching; 10 to 12 radiating filaments are formed at the same time in an epidermic sheath, which soon after birth dries and sets free the plumes, allowing them to spread out as a pencil of down; a stem is developed, and the downy filaments become the primary web of the feather. Feathers in some cases resemble stiff bristly hairs, as about the bill in most birds, and the tuft on the breast of the wild turkey. In the genus dasylophus, peculiar to the Philippine islands, we have remarkable instances of the modifications of the epidermic covering of birds.
In D. Cumingii (Fras.), the feathers of the crest, breast, and throat are changed at their extremities into ovoid horny lamellae, looking like shining black spangles, expansions of the true horny structure of the shaft; something of the kind is seen in the Bohemian chatterer or wax-wing (ampelis garrulus, Linn.), in which some of the secondary and tertial quill feathers end in small, oblong, flat appendages, in color and consistence resembling red sealing wax, which are also expanded horny prolongations of the shafts of the ordinary feathers. In D. superciliosus (Cuv.), the only other species of the genus, the feathers over each eye are changed for three fourths of their length. into red silky hairs or bristles, the base of the feather having the usual appearance; each shaft seems to divide into several of these hair-like filaments, which are finer and more silky than the appendage on the breast of the turkey, and directly continuous with ordinary feather structure, while in the turkey there is a complete transformation of feathers into hairs in the whole extent. In most birds there will be found a number of simple hair-like feathers scattered over the skin after they have been plucked; they arise from short bulbs or slender rounded shafts.
Feathers are developed in depressions in the skin lined by an inversion of the epidermis which surrounds the bulb; they grow by the addition of new cells from the bulb, which become modified into the horny and fibrous stem, and by the elongation and extension of previously formed cells; like the hair, they originate in follicles producing epidermic cells, though when fully formed the cellular structure is widely departed from except in the medullary portion. They are, when first formed, living organized parts, developed from a matrix connected with the vascular layer of the skin, and growing by nutrient vessels; when fully developed, the vessels become atrophied, and the feathers dry and gradually die from the summit to the base, so that at last they become dead foreign bodies, as completely incapable of vital modifications as the perfect horns of the deer. The matrix which produces the feather, according to Owen, has the form of an elongated cylindrical cone, and consists of a capsule, a bulb, and intermediate membranes which give proper form to the secretion of the bulb; as the conical matrix sinks into and becomes more intimately connected with the true skin, its apex protrudes above the surface, and the investing capsule drops off to give passage to the feather which has been growing during this period; the capsule is made up of several layers, the outermost consisting of epidermic cells, and its centre is occupied by a soft fibrous bulb freely supplied with blood vessels from below and a nerve; between the bulb and the capsule are two parallel membranes, in whose oblique septa or partitions the barbs and barbules are developed, nearly in the same way that the enamel of the teeth is formed between the membrane of the pulp and that of the capsule. The part to which the barbs are attached and the pith of the shaft are formed respectively from the outer and inner surfaces of the membranes of the compound capsule; the shaft and barbs at the apex of the cylinder become hardened first, and are softer the nearer the base of the matrix; the first formed parts are pushed forward by the cell growth at the base, the products of the bulb being moulded into shape by the membranes exterior to it; the successive stages of the growth of the medullary matter are indicated by a series of membranous cones or caps, the last formed of which cannot escape from the hardened and closed shaft, and constitute the light dry pith seen in the interior of the quill; these cones are originally connected together by a central tube, and the last remains of the bulb are seen in the ligament which passes from the pith through the lower umbilicus, attaching the quill to the skin. Feathers grow with great rapidity, and in some birds to a length of more than two feet; they are almost always renewed annually, and in many species twice a year; this amount of formative power demands a considerable increase of the cutaneous circulation, making the season of moulting always a critical period in the life of a bird.