THE HUMAN BODY 



Where the arch joins the centrum it is narrowed to a 

 stalk or pedicle, li, Fig. 12. When the vertebrae are placed 

 together in their natural relative positions, apertures (Fi), 

 leading into the neural canal, are left between their nar- 

 rower portions ; through these apertures (called the inter- 

 vertebral foramina) nerves pass out from the spinal cord. 



The atlas and axis. The first and second cervical ver- 

 tebras differ considerably from the others. The first, 

 called the atlas (Fig. 13), carries the head ; it has a very 



Aa Fas 



D 



Fas 



Fai 



FIG. 13. FIG. 14. 



FIG. 13. The atlas. FIG. 14. The axis. Aa, body of atlas?. Z>, odontoid pro- 

 cess of axis ; Fas. facet on upper side of atlas with which the skull articulates ; 

 and in Fig. 13, anterior articular surface of axis ; L, transverse ligament ; Frt, 

 vertebral foramen. 



small body and a very large neural ring. A ligament, L, 

 divides the ring into a ventral and a dorsal portion ; the 

 spinal cord passes through the latter and a bony peg, D, 

 lies in the former. The peg is the odontoid or tooth-like, 

 process. This reaches up from the second cervical or axis 

 vertebra (Fig. 14) and forms a pivot around which the atlas, 



How are the mtervertcbral foramina formed? What is their pur- 

 pose? 



What is the first cervical vertebra called? Describe its general 

 form. How is its neural ring divided? What lies in each division? 

 What is the second cervical vertebra named? What is the odontoid 

 process? Around what pivot does the head rotate when the face is 

 XUrnod on either side? 



