Till: SKKLKTON 



39 



FlG. 12. The alias and a.\i>. 



A, the atlas from above; a, areas for articulation with skull; /, 



odontoid peg of axis, with ligament beliind it. 



B, side view of axis vertebra, right side ; a, areas for articula- 



tion with atlas ; /, odontoid process or peg. 



ring of the atlas. This pnx es>, \vhi< h is called the odontoid 

 process, lias a smooth part where it meets the inner side 

 of the front part of the ring of the atlas, and so has the ring 

 itself, so that the atlas can mo\c round for a considerable way 

 on the axis, with the odontoid process as a pivot. The 

 odontoid pro- 

 cess is kept 

 in place by 

 'a ligament 

 lying just be- 

 hind it, across 

 the ring of 

 tin- atlas, and 



this separ- B 



ates the pro- 

 cess from the 

 spinal cord, 

 which passes 

 through the 



ring. The lower surface of the atlas has also two surfaces 

 by which it forms joints with the axis, as do the other ver- 

 tebrae to one another ; it is the looseness of these joints 

 which allows the atlas to turn on the axis as far as it does. 

 The upper surface of the atlas has two smooth surfaces on 

 which the skull rests. These are joints which allow a backward 

 and forward movement of the skull ; when the head is moved 

 backwards and forwards as in nodding, it moves on the atlas, 

 which is kept fixed. When the head is moved round, the atlas 

 is moved with it, the latter turning on the axis with the odontoid 

 process of the axis as the pivot. There arc two ligaments, the 

 i heck ligaments, passing from the top of the odontoid process 

 to the skull, which limit the amount of rotation. When the 

 head is tilted to one side, the cervical region of the spinal 

 column is bent, the cervical vertebra: allowing a considerable 

 amount of play from side to side. 



The sacrum is a wedge-shaped bone with the broad end 

 above and the narrow end below. The upper end articulates 

 with the last lumbar vertebra by a joint on each side, and is 

 connected to it in the centre by an intervertebral disc as well 

 .i-j by ligaments. The lower end carries the coccyx. The 



