788 



PHYLUM CHORDATA : CLASS MAMMALIA 



ring is divided transversely by a ligament ; through the 

 upper part the spinal cord passes, into the lower the odon- 

 toid process projects. The transverse processes are very 

 broad ; the articular surfaces for the two condyles of the 

 skull are large and deep. 



The second vertebra or axis has a broad flat centrum 

 produced in front in the odontoid process. The neural 

 spine forms a prominent crest, the transverse processes are 

 small, the anterior articular surfaces are large. 



Fig. 471. — Side view of the axis of a Mammal. 



O.P., The odontoid process which protrudes into the lower part 

 of the neural canal of the atlas; CV., the convex surface of 

 the centrum, on which the posterior concavities of the atlas 

 move ; N.SP., strongly developed neural spine ; T.P., small 

 transverse process ; TV., foramen for spinal nerve ; 1'., verte- 

 brarterial canal for the vertebral artery. 



A typical lumbar vertebra will show the centrum and its 

 epiphyses, the neural arch and neural spine, the transverse 

 processes, the anterior and posterior articular processes or 

 zygapophyses, the median ventral hypapophysis, the small 

 anapophyses from the neural arch below the posterior 

 zygapophyses, below the anapophyses the posterior inter- 

 vertebral notches — passages through which the spinal 

 nerves pass out, and anteriorly a similar pair of notches. 

 There are twelve or thirteen pairs of ribs which support the 

 wall of the thorax and aid in the mechanism of respiration. 



