Page:The American Cyclopædia (1879) Volume VII.djvu/113

 FEATHERS 105 of the skin, and growing by nutrient vessels ; when fully developed, the vessels become atro- phied, 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 FIGS. 8 and 4. Structure of the Bulb. FIG. 8. 1, 1, 1. Bulb. 2. Part FIG. 4. 1. The medulla or of the bulb in process of dry- Ing up as the shaft forms. 8. Part of the completed shaft. 4,4. Growing barbs. bulb. 2 2, 8 8, 4 4, 5 5. Membranous cones, indi- cating stages of growth of the medullary matter. horns of the deer. The matrix which pro- duces the feather, according to Owen, has the form of an elongated cylindrical cone, and con- sists of a capsule, a bulb, and intermediate membranes which give proper form to the se- cretion 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 du- ring this period ; the capsule is made up of several lay- ers, the outermost consisting of epi- dermic cells, and its centre is oc- cupied by a soft fibrous bulb freely supplied with blood vessels from below and a nerve ; between the bulb and the capsule are two parallel mem- branes, 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 FIG. 5. Section of the Shaft and Vane magnified. 1. The pith. 2. Horny external surface of shaft. 3. Concave internal surface. 4. Flat side of shaft. 5. 5. Bases of barbs. 6, 6. Barbules. 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 med- ullary matter are indicated by a series of mem- branous 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 origi- nally connected together by a central tube, and the last remains of the bulb are seen in the lig- ament 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. The plumage is generally changed sev- eral times before the bird is adult; but some of the falcons are said to assume the mature plumage after the first moult, as the Greenland and Iceland falcons. Feathers serve to protect birds from injurious external influences, such as extremes of cold and heat, rain, &c., for which their texture and imbricated arrange- ment admirably adapt them ; and they also furnish their principal means of locomotion, in the latter case being stronger, more compact, and longer than those which cover the body. They generally increase in size from the head backward, and have received special names ac- cording to the region of the body, which are important aids in describing and recognizing species. Some of these names, constantly used in the ornithological articles of this Cyclopae- dia, not readily understood from the words themselves, are as follows : the scapulars, above the shoulder blade and humerus, apparently on the back when the wing is closed ; axillaries, long and straight feathers at the upper end of the humerus, under the wing ; tibials, covering the leg ; lesser wing coverts, the small feathers in rows upon the forearm ; under coverts, lining the lower side of the wings ; the longest quill feathers, arising from the bones of the hand, are the primaries ; the secondaries arise from the outer portion of the ulna, and the tertiaries from its inner portion and the humerus ; the bastard wing consists of the quills growing from the rudimentary thumb; greater wing coverts, the feathers over the quills ; tail coverts, upper and under, those above and below the base of the tail feathers. The relative size of the quills on the hand and forearm, and the con- sequent form of the wings, are characteristic of