THE VERTEBRA IN BIRDS. 237 



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 ex- 

 ceedingly characteristic of birds. The under surfaces of the 

 centra frequently give off median inferior processes. In the 

 Matitce, it is obvious that the cervical vertebrse have short 

 transverse processes and ribs, disposed very much as in the 

 Crocodilia. For, in young birds, the anterior end of the lat- 

 eral face of each vertebra bears two small processes, an upper 

 and a lower ; and the expanded head of a styliform rib is ar- 

 ticulated w^ith these by two facets which represent the capitu- 

 lum 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 Carinatce^' but in these birds their form frequently be- 

 comes much modified in the adult ; and they develop pro- 

 longations, 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 cervi- 

 cal vertebrse become elongated and freely movable in the 

 CarinatCB^ as in the JRatitoe. 



The first dorsal vertebra is defined as such, by the union 

 of its ribs with the sternum by means of a sternal rib ; which 

 not only, as in the Crocodilia, becomes articulated with the 

 vertebral rib, but is converted into complete bone, and is con- 

 nected by a true articulation with the margin of the sternum. 



The number of the dorsal vertebriB (reckoning under that 

 head all the vertebrae, after the first dorsal, which possess dis- 

 tinct ribs, whether they be fixed or free) varies. The centra 

 of the dorsal vertebrae either possess cylindroidal articular 

 faces, like those of the neck, as is usually the case ; or, more or 

 fewer of them may have these faces spheroidal, as in the Pen- 

 guins. In this case, the convex face is anterior, the concave, 

 posterior. They may, or may not, develop inferior median pro- 

 cesses. They usually possess well-marked spinous processes. 

 Sometimes they are slightly movable upon one another ; some- 

 times they become anchylosed together into a solid mass. 



It is characteristic of the dorsal vertebrae of Birds that the 

 posterior, no less than the anterior, vertebrae present a facet, 

 or small process, on the body, or the lower part of the arch, 



