Two transverse processes are ordinarily con tinued from the anterior part of the bodies of the cervical vertebrae : the inter-space of these is filled up externally to the vertebral artery by a rudimentary styliform rib, which is sepa rated in the young bird, but afterwards anchy losed, and directed backwards parallel to the body of the vertebrae. These processes give attachment to numerous muscles of the neck, and being, with the transverse processes, more strongly developed in the rapacious birds, give a greater breadth to the cervical region in that order.
The superior spinous processes are but feebly developed ; they are most distinct on the vertebrae at the two extremities of the cervical portion of the spine. Inferior spinous processes are also found on the vertebrae at the commencement and termination of the neck, but are wanting in a great proportion of the intermediate cervical vertebrae.
The atlas is a simple ring. In general it is articulated with the occipital tubercle by a single concave facet on the body ; but in the Penguin and Ostrich there are two other facets, continuous with the middle one, but corresponding with the anterior articulating pro cesses of the rest of the vertebrae and applied to the condyloid portions of the occipital bone, while the middle facet is articulated to the ba silar portion as in other birds. The body of the dentata is joined to the atlas by a single synovial capsule, its odontoid process is tied down by a strong transverse ligament stretched above it, and by a longitudinal one extending from its extremity to the posterior part of the occipital condyle. In the articulations of the bodies of the remaining cervical vertebrae a moveable inter-articular cartilage is found in closed between reduplications of the synovial membrane, as in the joint of the lower jaw in mammalia. The articulations of the oblique processes have no peculiarities worthy of no tice.
A remarkable difference is found in the diameter of the spinal canal contained in the cervical vertebrae. If, e. g. the sixth cervical vertebra of a Stork be sawed down verti cally, the antero-posterior diameter is greatest in the middle, least at the ends ; but if it be sawed lengthwise horizontally, the transverse diameter is the reverse, being narrowest at the centre and widest at the ends. In the Ostrich, the Swan, and many other birds the spinal canal is widened in every direction at the extremities of the vertebrae; and on the dor sal or posterior aspect of the spine, the canal remains open for some extent in the intervals of the vertebra!, the cord being there protected only by membrane and the elastic ligaments which connect the roots of the spinous pro cesses together. The final purpose of this structure has been ably illustrated by Mr. Earle in the Philosophical Transactions, (1822, p. 276.) where he skews that it is adapted to pre vent a compression of the spinal cord during the varied and extensive inflections of the neck.
The vertebrae of the different regions of the spine bear a different proportion to each other in respect to number among birds, from what we observe in the mammalia and reptilia. The cervical portion in this class is generally com posed of a much greater number of vertebrae than any of the other divisions of the spine ; in this respect the fossil reptilian genus called Plesiosaurus alone resembles the bird. This singular animal was an inhabitant of the waters, and it is interesting to observe that the peculiarity which distinguishes it, viz. the great length of neck, is chiefly characteristic of the Ayes aquaticce of Nitzsch. In the Gral latores the length of the neck is determined by the height of the legs : in the Natatores it is necessary for the purpose of obtaining their food while swimming the waters. The dorsal vertebrae are usually less numerous than in mammalia. The caudal vertebrae are subject to few variations; they never project in the form of a tail, but are most numerous in those birds which make the greatest use of the tail feathers, as in the Swallows, to direct their rapid flight, and in the Woodpeckers, where they serve as a prop or climbing pole.
The following table, which, with some cor rections, is extracted from Cuvier's Lecons d'Anatomie Comparee, exhibits the variety that exists with respect to the number of ver tebrae in different species of birds.
The skull in all the Vertebrated Classes is composed of a considerable number of osseous pieces, which, in the Mammalia, unite in defi nite numbers and proportions, so as to form the bones termed occipital, temporal, sphenoidal, &c. In the cold-blooded Vertebrate the com ponent parts of these bones generally remain separated throughout life, an appear ance of great complexity to the skull, and occa sioning much difficulty in tracing their cor respondence with the cranial bones of the higher classes. Equal difficulty is experi enced in determining the component parts of the head in Birds, but from a very different cause. In the cold-blooded Crocodile, and Fish, this difficulty is caused by the tardiness of ossification, which prevents the coalition of the several elements of the cranial bones into their determinate groups ; while, in Birds, the energetic respiratory and circulating functions occasion so rapid an evolution of the osseous system, that the bones of the cranium become at an early period anchylosed into one piece, with a total obliteration of the original har monioo ; it is necessary, therefore, to examine the skull of the Bird at an early period of ex istence, and to compare, it with the foetal con dition of the skUll of the Mammal, when it will be found to be ossified from analogous centres, which, in their expansion and subsequent union, obey the same laws of, as it were, elective attraction.