The Spinal Cord

The spinal column of birds contains numerous and well ossified vertebrae, a considerable number of which (more than six) are anohylosed together to form a sacrum. Of the vertebrae which enter into the composition of this complex bone, however, not more than from three to five can be regarded as the homologues of the sacral vertebrae of a Crocodilian or Lacertilian reptile. The rest are borrowed, in front, from the lumbar and dorsal regions; behind, from the tail. The cervical region of the spine is always long, and its vertebrae, which are never fewer than eight, and may be as many as twentythree, are, for the most part, large in proportion to those of the rest of the body.

The atlas is a relatively small ring-like bone; and the transverse ligament may become ossified and divide its aperture into two-an upper, for the spinal cord, and a lower for the odontoid process of the axis vertebra. The os odontoideum is always anchjdosed with the second vertebra, and constitutes a peg-like odontoid process.

The spines of the succeeding cervical vertebrae are often obsolete, and are never very prominent in the middle region of the neck.

The anterior faces of their elongated vertebral centra are cylindroidal, slightly excavated from above downward, and convex from side to side; while the posterior faces are convex from above downward, and concave from side to side. Hence, in vertical section, the centra appear procoelous; in horizontal section, opisthocoelous; and this structure is exceedingly characteristic of birds. The under surfaces of the centra frequently give off median inferior processes. In the Retitae, it is obvious that the cervical vertebrae have short transverse processes and ribs, disposed very much as in the Crocodilia. For, in young birds, the anterior end of the lateral face of each vertebra bears two small processes, an upper and a lower; and the expanded head of a styliform rib is articulated with these by two facets which represent the capitulum and the tuberculum. With age, the cervical ribs may become completely anchylosed; and then they appear like transverse processes, perforated at the base by a canal, which, as in the Crocodilia, contains the vertebral artery and vein, and the main trunk of the sympathetic nerve. The cervical ribs and transverse processes are similarly disposed in very young Carinatae; but in these birds their form frequently becomes much modified in the adult; and they develop prolongations, which extend downward and inward, and protect the carotid artery or arteries.

The neural arches have well-developed pre-and post-zygapophyses. The ribs of one or two of the posterior cervical vertebrae become elongated and freely movable in the Carinatae, as in the Ratitae.