A Bird's skeleton has characters both peculiar and well-marked. The bones undergo extensive anchylosis especially in the skull, pelvic region, hand and foot The bone-substance is dense, and is stated to contain a large proportion of lime phosphate. The cancellated tissue with the marrow is frequently absorbed and its place taken by air derived in the skull from the nasal passages and tympanic cavities: in the rest of the skeleton from extensions of the air-sacs connected with the lungs. All the bones in the Pigeon contain air, i. e. are pneumatic, save the caudal vertebrae, the fore-arm and hand and hind-limb.
The skull presents the following general features: - It has, like the Reptilian skull, a single condyle. The cranial surface is smooth, polished, and its sutures obliterated. The orbital cavities are large and separated one from another by a thin vertical inter-orbital septum, formed chiefly by the mesethmoid. The orbit is bounded in front by the lacrymal bone and the homologue of the lateral mass of the ethmoid in man. The beak consists mainly of the praemaxillae, which are continued backwards below the orbit by a slender bony rod. The anterior part of this rod is the maxilla, the posterior which articulates with the quadrate, a jugo-quadrato-jugal bone. A moveable quadrate articulates with the skull on the one hand, and the lower jaw on the other. The two rami of the mandible are anchylosed at the symphysis. A ring of bony plates developed in the sclerotic of the eye is seen suspended in the left orbit. A similar ring is found in many Lacertilia, the Chelonia, and extinct Ichthyosauria.
The vertebral column is divisible into a cervical, dorsal, so called sacral, and a caudal region. The articular surfaces of the centra are typically procoelous and cylindroidal, i. e. concave from side to side and convex from above downwards anteriorly, curvatures which are reversed posteriorly. The cervical and dorsal vertebrae have synovial joints. A ring of fibres binds together the edges of the opposing surfaces. Between them is interposed a fibro-cartilaginous meniscus thick at its circumference, thin centrally where it is perforated for the passage of the 'suspensory ligament' which unites the vertebral centra. The free caudal vertebrae have usually flattish centra. They articulate as in Mammalia by intervertebral discs (annuli fibrosi), which have centrally a 'nucleus pulposus,' the homologue of the suspensory ligament and formed as it is from notochordal cartilage.
The length of the neck in every Bird is at least equal to the height from the ground at which the legs carry the body and to the distance from the root of the neck to the last caudal vertebra. The actual length varies, and depends chiefly on the number of vertebrae present, and not on the length of their centra. In Mammalia while the number of vertebrae is nearly invariable, the length of their centra is very variable. The atlas is ring-like and articulates with the occipital condyle by a deep cup completed as in Reptilia by the odontoid process of the axis. This latter vertebrae has a neural spine, and an inferior spine, a structure present also in the two next vertebrae. The odontoid is anchylosed to its centrum. The third vertebra has its neural arch deeply emarginated before and behind, a peculiarity repeated in the seven following vertebrae. It has also, like the next nine vertebrae, cervical ribs anchylosed to the superior and inferior transverse processes, and inclosing a canal which lodges the vertebral artery and vein with the main trunk of the sympathetic. The fourth vertebra has no neural spine, and its centrum has ventrally a pair of down-growths which form a demi-canal for the protection of the common carotid arteries.
These features are repeated in the succeeding vertebrae to the tenth inclusive. The eleventh and twelfth have inferior spines; the two following - the thirteenth and fourteenth - have neural spines; their ribs are free but not connected to the sternum, and the last pair carry recurrent or uncinate processes like the first four pairs of dorsal ribs. By some anatomists the thirteenth and fourteenth vertebrae are counted as dorsal. Five vertebrae make up the dorsal region defined by the presence of free ribs connected to the sternum. They have large neural spines and transverse processes with keel-shaped centra. The three first have their centra anchylosed, and the ligaments connecting their neural spines, inferior spines, and transverse processes ossified. The fourth dorsal is free: the fifth unites with the sacrum. The sacrum contains, as it always does in Birds, vertebrae derived from four regions. The first is a dorsal vertebra with ribs. The six vertebrae succeeding it are lumbar, of which the three last retain only the upper division of the transverse processes, which are present also in all the remaining sacral vertebrae. The ligaments uniting these processes ossify, and a flat subcutaneous area (absent in Ratite birds) is thus formed.
To the lumbar succeed two sacrals, homologues of the vertebrae so named in Lizards, etc. In some specimens of the Pigeon, both of them carry a pair of stout bony rods or ribs visible only from below. These ribs are not free. They swell at their distal ends which fuse to the outer ends of the transverse processes (upper division) of their own vertebrae and coincide with the widest part of the area mentioned above, and lie therefore just internal to the acetabulum. Either the first or the second pair of these ribs may be absent. Behind the sacral vertebrae comes a variable number of caudal vertebrae, termed for distinction's sake 'uro-sacrals.' The free caudal region contains seven to eight vertebrae. They have no articulating processes. The last is thin, compressed, and up-turned. It is known as 'pygostyle' or ploughshare bone (os en soc de charrue), and represents four to six fused vertebrae.
The five pairs of ribs consist of ossified vertebral and sternal sections. Each vertebral section articulates only with its own vertebra: the first four bear uncinate processes united to their posterior borders. These processes, as in Hatteria, Iguana and the Crocodile, are pre-formed in cartilage. The last sternal section unites with its predecessor, not with the sternum. The sternum covers nearly the whole abdomen and has a deep concave internal surface. There are four borders: - an anterior bearing the rostrum in its centre with a coracoid groove on either side and ending laterally in a costal process; a right and left costal border deeply concave and bearing the ribs; and a posterior or xiphisternal border. This border is convex, of great extent, and interrupted, as in some other Birds, on either side the median line by an outer xiphisternal notch and an inner xiphisternal fontanelle - the inner notch of the Fowl tribe. Notch and fontanelle are in the living bird closed by membrane probably substituted for original cartilage. The border presents accordingly five processes - two outer, two intermediate, and one median.