Vertebral Column.
The vertebral column of birds is readily divisible into a very mobile cervical region, and an extremely rigid post-cervical region. In most birds the vertebral centra are without terminal epiphyses, but these structures are found in Parrots. The cervical vertebrae are generally large and vary in number from eight or nine to twenty-three in Swans. Except in some extinct forms, such as Ichthyornis and Apatornis, in which they are biconcave, the centra are characterised by having saddle-shaped articulating surfaces, which in front are concave from side to side and slightly convex from above downwards, while posteriorly they are convex from side to side and concave from above downwards. The atlas is small and ring-like, and its centrum is fused with the axis forming the odontoid process. Cervical ribs are often well developed, and in some of the Ratitae they remain for a long time distinct from the vertebrae.
The thoracic vertebrae are distinguished from the cervical by the fact that their true ribs are united to the sternum by means of sternal ribs. This distinction, however, though convenient, is somewhat arbitrary, as it has been shown that in the fowl and gannet, two pairs of ribs which in the adult are free from the sternum, are connected with it in the embryo. When, as in the Swans, the thoracic vertebrae are not all fused together, they generally have saddle-shaped articulating surfaces, but sometimes, as in the Penguins, Auks and Plovers, the centra are convex in front and concave behind. The trunk vertebrae generally have well-marked neural spines, while in the Divers the anterior ones have peculiar bifurcating hypapophyses.
The trunk vertebrae are not readily divisible into thoracic and lumbar. There are two true sacral vertebrae, but as development proceeds a number of other vertebrae become fused with the true sacrals, the whole forming a large compound sacrum. These pseudosacral vertebrae generally include the lumbar, and some of the thoracic and caudal vertebrae. Sixteen to twenty vertebrae or even more may be included in the compound sacrum, and sometimes the whole of the trunk vertebrae are fused together. In Archaeopteryx however but five vertebrae take part in the formation of the sacrum.
In Archaeopteryx there are twenty long caudal vertebrae, of which the last sixteen carry a pair of feathers apiece, but in all other birds the tail is short and in the great majority of cases the posterior vertebrae are fused together, forming the pygostyle. In the Ratitae and Tinamidae a pygostyle is rarely or imperfectly developed. In Hesperornis there are twelve caudal vertebrae, six or seven of which are united by their centra only, forming an imperfect pygostyle.
The free caudal vertebrae are generally amphicoelous.
The Skull.
The skull of all birds from Archaeopteryx onwards is essentially similar, differing from the skull of reptiles mainly in the extent to which the cranium is arched, and its greater size in proportion to the jaws.
Most of the bones of the cranium are pneumatic, and all show a marked tendency to fuse together, and have their outlines obliterated by the disappearance of the sutures. The several bones remain longest distinguishable in the Ratitae and to a less extent in the Penguins. The orbits are very large and lie almost entirely in front of the cranium; they are separated by an interorbital septum which is sometimes, as in Chauna and Scythrops, very complete, sometimes, as in Hornbills and the Common Heron, very slightly developed. As a general rule the sclerotic is cartilaginous.
The anterior nares are almost always situated far back at the base of the beak near the orbits, but in Apteryx they are placed right at its extremity. In Phororhacos they are placed very high up on the enormous beak and are not separated by any bony partition.