44 CONSCIOUS NERVOUS OPERATIONS 



vertebra (Fig. 32, A) supports the skull, being articulated 

 by two shallow hollows (a, J) with corresponding pro- 

 jections on the occipital bone above. This permits the 

 head to rock back and forth. The atlas has a very small 

 body or centrum, and a large neural ring subdivided by 

 the transverse ligament. Into the front portion of the 

 ring projects the odontoid process, a thick bony peg arising 

 from the axis or second cervical vertebra (Fig. 32, jB). 

 Around the odontoid process the atlas rotates, carrying 

 the head with it from side to side. 



Fig. 



A atlas. B atlas and axis, a, b articulations with occipital bone. 

 c transverse ligament, o odontoid process. 



43. By means of the variations in the form of the ver- 

 tebrae, and by the four curvatures seen in the spinal 

 column, a considerable range of movement is provided for. 

 The vertebrae furnish strong support for the great mus- 

 cles of the trunk and a safe channel for the spinal cord, 

 while a firm but flexible and elastic column is secured for 

 the support of the whole frame. 



44. The Ribs. Attached by their heads to the tho- 

 racic vertebrae are the twelve pairs of slender curved bones 

 called ribs (Fig. 33). The upper seven pairs are attached 

 in front by costal cartilages to the sternum, or breastbone. 

 The next three pairs have their costal cartilages joined 

 each to the cartilage of the preceding rib ; while the last 



