ELEMENTARY SKETCH OF THE OSTEOLOGY OF BIRDS. 15 



a caudal region (c. vt. and ex.) which includes the remaining 

 vertebrse. The cervical vertebras (cv.) are always numerous, 

 being never less than eight, and sometimes exceeding twenty in 

 number. The first vertebra or atlas* is a small ring-shaped 

 bone, which is very frequently divided into two segments by the 

 ossification of the transverse ligament. With the exception of 

 this vertebra, all the vertebrse of birds consist of two parts, — g, 

 body or centrum, which forms the inferior portion, and an arch, 

 called the neural arch, which surrounds the spinal chord. This 

 arch in the later vertebrae generally carries a ridge or spine 

 (shown in the dorsal vertebrge of the figured skeleton) and which 

 is known as the neural spine. The vertebras articulate with one 

 another by the apposition of their centra, and also by two 

 other pairs of oblique joints known as prezygapopliyses^ 

 (pi I"V., fig 4, pz.) which look downwards, and postzygapophy- 

 sesX pt. z) which look upwai'ds. We have already said that 

 the first vertebra has no centrum. This centrum has in fact 

 become united, as in mammals, with the centrum of the second 

 or axis vertebra, of which it forms the odontoid§ process, and 

 on which as a pivot the atlas turns, carrying with it the skull. 

 The neural II spines of the succeeding cervical vertebrae are 

 either very small, or are entirely wanting. The articular sur- 

 faces of their centra are formed on a kind of ball and socket 

 plan, the anterior surface being convex vertically, and concave 

 transversely, while the posterior surface presents an exactly 

 opposite arrangement This form of the articular surfaces of 

 the centra of the cervical vertebrge is very characteristic of birds. 

 The latter cervical vertebrae bear a median spine on their in- 

 ferior surfaces, which is developed to the greatest extent 

 anteriorly. 



The dorsal vertebrae (pi. I. d.v.) are characterized by having 

 large and square neural spines, which in old individuals may be- 

 come anchylosed together to a considerable extent. They also 

 develope from their lateral surfaces other flat plates of bone, or 

 transverse processes with which the tuherculum^ of the rib 

 articulates. This transverse process arises solely from the neural 

 arch. At the anterior border of the lateral surface of the 



* Atlas, applied to the first cervical vertebra, as bearing the head. 



f Prezygapophyses, a barbarous vrord, derived from prce, before (anterior) c,vyo f , 

 a union, and airo(pvsi;, the anatomical word for a process. Applied to the anterior 

 articulating oblique facets of vertebrae. 



J Postzygapohyses, a similarly-formed word to the last, substituting post (behind) 

 {oi prcB. 



§ Odontoid, from o^ovs 6^0V9 os, a tooth, and f't'^os like. 



II Neural, from vtvpov a nerve ; applied to the spine, which is situated over 

 the spinal chord or nerve. 



^ Tuberculum, a little t'lbercle, applied to the second vertebral articulation of 

 a rib. 



