276 THE ANATOMY OF VERTEBEATED ANIMALS. 



the vertebrae wliich enter into the composition of this com- 

 plex bone, however, not more than from three to five can 

 be regarded as the homologues of the sacral vertebrge 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 twenty-three, 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 ankylosed with the second ver- 

 tebra, and constitutes a peg-like odontoid process. 



The spines of the succeeding cei-vical 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 down- 

 wards, and convex from side to side; while the posterior 

 faces are convex from above downwards, and concave from 

 side to side. Hence, in vertical section, the centra appear 

 proccelous ; in horizontal section, opisthocoelous ; and this 

 structure is exceedingly characteristic of bii'ds. The under 

 surfaces of the centra frequently give off median inferior 

 processes. In the Ratitce, it is obvious that the cei-vical 

 vertebra have short transverse processes and ribs, disposed 

 very much as in the Crocodilia. For, in young birds, the ante- 

 rior end of the lateral face of each vertebra bears two small 

 processes, an upper and a lower ; and the expanded head of 

 a stylif orm rib is articulated with these by two facets which 

 represent the capitulum and the tuberculum. With age, 

 the cervical ribs may become completely ankylosed; 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 sympa- 

 thetic nei-ve. The cerdcal ribs and transverse processes 

 are similarly disposed in very young CarinatcB ; but in these 



