90 THE BACK. 



brae join to form the intervertebral foramen for the out- 

 ward passage of nerves and the inward passage of blood- 

 vessels. 



The distinguishing mark of the cervical vertebra is the 

 foramen in each transverse process, through which the 

 vertebral arteries run to the skull. They are also smaller 

 than the dorsal and lumbar vertebrae. The dorsal vertebra 

 are distinguished by having on the transverse processes 

 and on the body smooth articular surfaces called facets 

 and demi-f acets for articulation with the ribs. The lumbar 



FIG. 33. The sacrum, from before. (Drawn by D. Gunn.) 



vertebrce are the largest and heaviest and have the thickest 

 spine. By the time the sacral region is reached, however, 

 the vertebrae have only a rudimentary spinous process. 

 Moreover, in adult age the sacral bones grow together 

 and form one triangular bone, the sacrum, which has a 

 broad base called the promontory of the sacrum and a blunt 

 apex. It is concave in front and convex behind and has an 

 articulating surface for joining the pelvic bones. In the 

 case of the coccyx also the four original bones, all rudi- 

 mentary in character and supposed to be the survival of a 

 tail, grow together to form one bone. Together the sacrum 

 and coccyx form the posterior wall of the true pelvis. 



