The feathers vary in different parts of the bird, and are generally divided into those which cover the body - "clothing feathers" - and those which occur in the wings and tail - "quill-feathers." As regards the great quill-feathers of the wings, the longest are those which arise from the bones of the hand, and they are called the "primaries." Those which arise from the distal end of the fore-arm (radius and ulna) are termed the "secondaries," and those which are attached to the proximal end of the fore-arm are the "tertiaries." The feathers which lie over the humerus and scapula are the " scapulars." The rudimentary "thumb" also carries some quills, which form what is known as the "alula," or " bastard-wing." The smaller feathers, which cover the bases of the quill-feathers above and below, are the "wing-coverts "-"greater," "lesser," and "under." The great quill-feathers of the tail ("rectrices") form a kind of fan, of great use in steering the bird in flight; and their bases are covered by a series of feathers which constitute the "tail-coverts." Generally there are ten or twelve "rectrices ;" but there may be as many as twenty-four (as in the Pelican), or rarely more ; and they do not carry "accessory plumules." In addition to the "clothing-feathers " and the quill-feathers of the wings and tail, the body is protected by a more or less abundantly developed coating of " down-feathers" (" plumulae "), in which the barbules are not hooked, and the barbs are therefore free. In some cases there is no shaft to the down-feathers, and the barbs are attached in a tuft to the end of the quill. In other cases, the feathers closely approximate to hairs in form, being very long, slender, and flexible. These "filoplumae " consist of a delicate shaft, either destitute of vanes, or carrying a few barbs at the extremity.
Fig. 320. - Quill-feather (Stenopsis). a Quill or barrel; b Shaft; - c Webs, composed of the barbs, and together forming the "vane."
Though apparently completely covered with feathers, these appendages are really almost always confined to certain special tracts ("pterylae ") in the body of a bird, the intervening spaces ("apteria") being, with few exceptions, naked. These feathered and unfeathered regions are definite in form, size, and arrangement in many great groups of birds, and can thus be used as an important aid to classification.
The entire skeleton of the Birds is singularly compact, and at the same time singularly light. The compactness is due to the presence of an unusual amount of phosphate of lime; and the lightness, to the absence in many of the bones of the ordinary marrow, and its replacement by air.
As regards the vertebral column, birds exhibit some very interesting peculiarities. The cervical region of the spine is unusually long and flexible, since the fore-limbs are useless as organs of prehension, and all acts of grasping must be exercised either by the beak or by the hind-feet, or by both acting in conjunction. In all birds alike, the neck is sufficiently long and flexible to allow of the application of the beak to an oil-gland placed at the base of the tail, this act being necessary for the due performance of the operation of "preening" - that is, of lubricating and cleaning the plumage. The cervical vertebrae vary in number from eight to twenty-three. The front faces of their centra are cylindroidal (spheroidal in Penguins), convex from above downwards, and concave from side to side, the posterior faces being saddle-shaped, concave from above downwards and convex from side to side. Hence in vertical section, the vertebrae appear to be opisthocoelous, and in horizontal section procoelous. This structure of the cervical vertebrae is highly characteristic of Birds. The dorsal vertebrae vary from six to ten in number, and of these the anterior four or five are generally anchylosed with one another, so as to give a base of resistance to the wings. In the Cursorial Birds, however (such as the Ostrich and Emeu), and in some others (such as the Penguin), in which the power of flight is wanting, the dorsal vertebrae are all more or less freely movable one upon another. There are no lumbar vertebrae, but all the vertebrae between the last dorsal and the first caudal (varying from nine to twenty) are anchylosed together to form a bone which is ordinarily known as the " sacrum." To this, in turn, the iliac bones are anchylosed along their whole length, giving perfect immobility to this region of the spine and to the pelvis.
The coccygeal or caudal vertebrae vary in number from eight to ten, and are movable upon one another. In reality, however, the number of caudal vertebrae is much greater than the above, since some of the vertebrae of the anchylosed "sacrum " properly fall to be counted in this region, and the "ploughshare-bone " consists of more than one vertebra. The most noticeable feature about this part of the spinal column is what is known as the "ploughshare-bone." This is the last joint of the tail, and is a long, slender, ploughshare-shaped bone, destitute of lateral processes, and without any medullary canal (fig. 325, B). In reality it consists of two or more of the caudal vertebrae, completely anchylosed, and fused into a single mass. It is usually set on to the extremity of the spine at an angle more or less nearly perpendicular to the axis of the body; and it affords a firm basis for the support of the great quill-feathers of the tail ("rectrices"). It also supports the coccygeal oil-gland, and can be raised at pleasure, so as to meet the bill, when the operation of preening is in progress. In the Cursorial Birds, which do not fly, the terminal joint of the tail is not ploughshare-shaped. In the extraordinary Mesozoic bird, the Archaeopteryx macrura, there is no ploughshare-bone, and the tail consists of twenty separate vertebrae, all distinct from one another, and each carrying a pair of quill-feathers, one on each side (fig. 351). As the vertebrae of the ploughshare-bone are distinct from one another in the embryos of existing birds, the tail of the Archaeopteryx is to be regarded as a case of the permanent retention in the adult of an embryonic character. In the increased number of caudal vertebrae, however, and in some other characters, the tail of the Archaeopteryx makes a decided approach to the true Reptiles.
Fig. 321. - Skull of Spur-winged Goose (Plectropterus Gambensis).
The various bones which compose the skull of Birds are amalgamated in the adult so as to form a single piece, and the sutures even are obliterated, the lower and upper jaws alone remaining movable. The occipital bone carries a single occipital condyle only, and this is hemispherical or nearly globular in shape. The nasal bones are short, so that the nostrils (except in Apteryx) are placed far backwards. The "beak" (fig. 321), which forms such a conspicuous feature in all birds, consists of an upper and lower half, or a "superior" and "inferior mandible." The upper mandible is composed principally of the greatly elongated and coalescent intermaxillary bones, which give off long " frontal processes," and are flanked by the comparatively small superior maxillae. The inferior mandible is primitively composed of twelve pieces, six on each side; but in the adult these are all indistinguishably amalgamated with one another, and the lower jaw forms a single piece. As in the Reptiles, the lower jaw articulates with the skull, not directly, but through the intervention of a distinct bone - the quadrate bone - which always remains permanently movable, and is never anchylosed with the skull. In no living bird are teeth ever developed in either jaw, but both mandibles are encased in horn, forming the beak, and the margins of the bill are sometimes serrated. The quadrate bone is movable, and has articulated to it in front the slender rod-like "jugal" bone or "quadrato-jugal," which is, in turn, immovably united with the slender maxilla on each side. When the mandible is depressed, the quadrate bone is thrust forward, and the rod formed by the coalescent jugal and maxilla on each side elevates the upper half of the beak, which is usually articulated in a more or less movable manner with the front of the skull. The Parrots possess this movable articulation of the upper jaw with the skull in its greatest perfection; but it exists in a less complete form in most birds. The maxillae of birds, as before remarked, are comparatively slender bones; but they send inwards extensively developed horizontal processes ("maxillo-palatine processes"), which form a large portion of the hard palate. The extent, however, to which these processes are developed varies much in different birds, and on these variations, combined with the structure of the vomer, Prof. Huxley has proposed to found the following divisions of the Carinate Birds: