56 ELEMENTARY BIOLOGY. [CHAP. 



a. Examine carefully and draw various aspects of a 

 detached vertebra, say the third. 



a. Its solid flattened ventral part (centrum), with 

 an anterior concave and a posterior convex 

 surface (procoelous). 



fi. The neural arch : an arch of bone arising from 

 the centrum dorso-laterally. It furnishes the 

 undermentioned. 



The transverse process : a bony bar arising on 

 each side from the base of the arch and passing 

 outwards and a little downwards. 



The articular processes (zygapophyses) ; an an- 

 terior and posterior pair, springing from the 

 sides of the arch; the anterior (pre-zygapophyses) 

 having their smooth articular surfaces directed 

 upwards ; the posterior (post-zygapophyses) with 

 similar surfaces directed downwards. 



The short spinous process ; springing from the 

 dorsal aspect of the arch and directed back- 

 wards. 



y. The neural canal ; bounded, above and at the 

 sides by the arch, below by the centrum. 



b. Examine the remaining vertebra;. 



a. The first vertebra : its body, produced forwards 

 into a wedge-shaped process (odontoid process) 

 which lies between the occipital condyles of 

 the skull: its arch, usually incompletely ossified 

 in the region of the spinous process, which is 

 rudimentary; posterior zygapophyses alone pre- 

 sent : the large concave anterior facets (condylar 

 facets), partly on the arch and partly on the 

 centrum, for articulation with the skull. 



