54 



THE HUMAN BODY 



early in life to the ventral surface of each transverse process; the 

 foramina (Fig. 22, Frt) thus formed give passage to an important 

 artery which ultimately passes into the cranial cavity to carry 

 blood to the brain. 



The Atlas and Axis. The first and second cervical vertebra 

 differ considerably from the rest. The first, or atlas (Fig. 23), 

 which carries the head, has a very small body, Aa, and a large 

 neural ring. This ring is subdivided by a cord, the transverse 

 ligament, L, into a dorsal moiety in which the spinal cord lies and 



Aa Fas 



Fas 



Frt 



D 



i 



FIG. 24. 



FIG. 23. The atlas. FIG. 24. The axis. Aa, body of atlas; D, odontoid 

 process; Fas, facet on front of altas with which the skull articulates; and in Fig. 24, 

 anterior articular surface of axis; L, transverse ligament; Frt, vertebral foramen; 

 Ap, neural arch; Tp, spinous process. 



a ventral into which the bony process D projects. This is the 

 odontoid process, and arises from the front of the axis or second 

 cervical vertebra (Fig. 24). Around this peg the atlas rotates 

 when the head is turned from side to side, carrying the skull (which 

 articulates with the large hollow surfaces Fas} with it. 



The odontoid process really represents a large piece of the body 

 of the atlas which in early life separates from its own vertebra and 

 becomes united to the axis. 



The Thoracic Vertebrae have larger bodies and longer processes 

 than do the cervical vertebrae. They are specially modified for 

 carrying the ribs. Each rib is attached at two points (Fig. 21). 

 The head of the rib fits into an articulation at the junction of two 

 vertebrae, a part of the articular surface being on the centrum of 

 one and a part on the other (Fig. 20, Fes and Fci) . The second 

 attachment is between a point on the neck of the rib and an artic- 

 ular surface at the end of the transverse process of the posterior 

 of the two vertebrae which the rib touches (Fig. 20, Ft). 



