198 VERTEBRATE ZOOLOGY 



Each vertebra is composed essentially of the following parts: 

 the centrum, which forms its body ; the neural arch, which rises 

 from the dorsal surface of the centrum and is composed of the two 

 neural processes and the neural spine ; the paired transverse proc- 

 esses, which project laterally or ventrolateral^ from its sides. 



The cervical vertebrae are fourteen in number. Study these 

 and note carefully their variations of structure. The centrum is 

 elongated. Its ends are saddle-shaped, the anterior end being con- 

 vex dorsoventrally and concave laterally, and the posterior end 

 concave dorsoventrally and convex laterally. The transverse 

 process projects back from the anterior portion of the centrum on 

 each side and is prolonged into a short spine. At the base of the 

 process is a prominent foramen, one of the series of foramina in 

 the successive cervical vertebrae which form the vertebrarterial 

 canal; in this canal lie the vertebral artery and vein and the 

 sympathetic nerve. The transverse process and its spine are not 

 strictly homologous to the transverse processes of the other verte- 

 brae but are equivalent to a rib plus the process. The foramen 

 lies between the head of the rib and the process. Besides these 

 rudimentary cervical ribs two pairs of free ribs are present on the 

 last two cervical vertebrae ; they do not reach the sternum. 



Projecting from both the anterior and the posterior end of the 

 neural arches are two small processes by which the successive 

 vertebrae articulate with one another. These are called the prezy- 

 gapophyses and postzygapophyses, the former projecting from the 

 anterior end of the vertebra and articulating with the latter on 

 the vertebra in front of it. 



The first two cervical vertebrae are called the atlas and the axis ; 

 they support the skull and differ in structure from the others. The 

 atlas is a ring-shaped bone in which the centrum and the neural 

 spine are absent, and the transverse processes are broad, winglike 

 projections. On its anterior face is a median groove; into this 

 fits the condyle by which the skull articulates with the spinal 

 column. On its posterior face is also a median groove in which 

 lies the odontoid process of the axis. These grooves are bounded 

 dorsally by a transverse ligament which separates them from the 

 neural canal. 



