Desmognathae. Maxillae sending inwards largely developed maxillo-palatine processes, which unite with one another to form a bony roof to the palate. The vomer truncated in front, small or obsolete. Ex. Birds of Prey, Parrots, Cuckoos, Kingfishers, Trogons, Anserine birds, Storks, Cormorants.
Schizognathae. Maxillo-palatine processes of the maxillae separated by a wider or narrower cleft. Vomer long and pointed in front, narrow behind. Ex. Plovers, Gulls, Penguins, Cranes, Fowls, Sand - grouse, Pigeons.
Aegithognathae. Maxillo-palatine processes separated by a cleft. Vomer truncated in front, narrow behind. Ex. Perching Birds, Swifts, Woodpeckers.
Dromaeognathae. Vomer broad behind, interposing between the pterygoids, the palatine bones, and the basi-sphenoid rostrum. This division includes only the Tinamous (Tinamomorphae).
Fig. 322. - Skull of young Ostrich, viewed from above (A), and from below (B). (After Owen.) of Occipital foramen; so Supra-occipital: eo Exoccipital; q Quadrate; pa Parietal; pp Pterygoid process ; f Frontal; e Ethmoid ; n Nasal; pm Maxillary process of praemaxilla; m Malar or jugal bone; im Praemaxilla; p Palatine bone ; v Vomer; l Lachrymal bone. The skull being that of a young bird, the sutures are not yet obliterated.
The thoracic cavity is bounded behind by the dorsal vertebrae, which are usually, as before said, anchylosed to one another to a greater or less extent. Laterally, the thorax is bounded by the ribs, which vary in number from six to ten pairs. In most birds, each rib carries a peculiar process - the "uncinate process" - which arises from its posterior margin, is directed upwards and backwards, and passes over the rib next in succession behind, where it is bound down by ligament. The first and last dorsal ribs carry no uncinate processes, and in some cases the processes continue throughout life as separate pieces (fig. 323, B). Anteriorly, the ribs articulate with a series of straight bones, which are called the "sternal ribs," and which in reality are to be looked upon as the ossified "costal cartilages." These sternal ribs (fig. 323, B) are in turn movably articulated to the sternum in front, and "they are the centres upon which the respiratory movements hinge" (Owen). In front the thoracic cavity is completed by an enormously-expanded sternum or breast-bone, which in some birds of great powers of flight extends over the abdominal cavity as well, in some cases even reaching the pelvis. The sternum of all birds which fly, is characterised by the presence of a greatly-developed median ridge or keel (fig. 323, A), to which are attached the great pectoral muscles which move the wings. As a general rule, the size of this sternal crest allows a very tolerable estimate to be formed of the flying powers of the bird to which it may have belonged ; and in the Ostriches and other birds which do not fly, there is no sternal keel. At its anterior angles the sternum exhibits two pits for the attachment of the coracoid bones.
The scapular or pectoral arch consists of the shoulder-blade or scapula, the collar-bone or clavicle, and the coracoid bone, on each side. The scapula, as a rule (fig. 323, A, s s), is a simple elongated bone, not flattened out into a broad plate, and carrying no transverse ridge, or spinous process. Only a portion of the glenoid cavity for the articulation with the nead of the humerus is formed by the scapula, the remainder being formed by the coracoid. The coracoid bones (fig. 323, A, k k) correspond with the coracoid processes of man; but in birds they are distinct bones, and are not anchylosed with the scapula. The coracoid bone on each side is always the strongest of the bones forming the scapular arch. Superiorly it articulates with the clavicle and scapula, and forms part of the glenoid cavity for the humerus. Inferiorly each coracoid bone articulates with the upper angle of the sternum. The position of the coracoids is more or less nearly vertical, so that they form fixed points for the action of the wings in their downward stroke. The clavicles (fig. 323, A, c) are rarely rudimentary or absent, and are in some few cases separate bones. In the great majority, however, of birds, the clavicles are anchylosed together at their anterior extremities, so as to form a single bone, somewhat V-shaped, popularly known as the "merry-thought," and technically called the "furculum" ("four-chette" of the French). The outer extremities of the furculum articulate with the scapula and coracoid; and the anchylosed angle is commonly united by ligament to the top of the sternum. The function of the clavicular or furcular arch is "to oppose the forces which tend to press the humeri inwards towards the mesial plane, during the downward stroke of the wing" (Owen). Consequently the clavicles are stronger, and their angle of union is more open, in proportion to the powers of flight possessed by each bird. The furculum is rudimentary in the Ostrich, Emeu, and Cassowary, and it is absent in the Apteryx and some Parrots.