THE SKELETON 



223 



Bone Groups. On account of the close relation between 

 the bones of the same group, they cannot profitably be 

 studied as individual bones, but each 

 must be considered as a part of the 

 group to which it belongs. By first 

 making out the relation of a given bone 

 to its group, its value to the whole body 

 can be determined. The most important 

 of the groups of bones are as follows : 



i . The Spinal Column. This group 

 consists of twenty-four similarly shaped 

 bones, placed one above the other, 

 called the vertebra, and two bones 

 found below the vertebrae, known as 

 the sacrum and the coccyx (Fig. 98). 

 These twenty-six bones supply the 

 central axis of the body, support the 

 head and upper extremities, and in- 

 close and protect the spinal cord. 



The upper seven vertebrae form the 

 neck and are called the cervical ver- 

 tebrae. They are smaller and have 

 greater freedom of motion than the 

 others. The first and second cervical 

 vertebrae, known as the atlas and the 

 axis, are specially modified to form a 

 support for the head and provide for its 

 movements. The head rests upon the 

 atlas, forming with it a hinge joint (used 

 in nodding to indicate " yes "); and the 

 atlas turns upon an upward projection 



of the axis forming a pivot joint (used p IG 9 g_ The spinal 



in shaking the head to indicate " no "). column. 



V.1 



<Sacrui 



Coccyx. 



