Vertebral Column 19 



perform nodding movements : so the skull and the Axis are on opposite sides in both 

 these forms of movement, and the occipito-odontoidal ligaments become the most 

 important bands associated with limitations of these movements. 



Flexion of the head is checked by tightening of these ligaments : this is seen in 

 Fig. 14, A, where it is shown that when the skull is brought down from the level AA 

 to BB the condyle rotates in the articular cup of the Atlas, and the upper attachment 

 of the check ligament is carried backwards and upwards (from a to b) on the condyle, 

 so tightening the band and checking the movement. 



The opposite movement is checked by the tightening of the anterior occipito- 

 atloid ligament (aoa) and the front fibres of the articular capsule. 



Rotation is also limited by the lateral occipito- odontoid ligaments. These 

 are shown in Fig. 13, and consist of a thick mass of fibres on each side, the posterior 

 and deeper ones showing a tendency to pass forward under the more superficial ones : 

 some fibres, not shown, also run across without any definite connection with the 



dlYS 



FIG. 14. Schemes to illustrate the action of the lateral occipito-odontoid ligaments in 

 (A) flexion and (B) rotation of the head. 



process. The area of insertion into the odontoid is on its postero-lateral aspect, so 

 that the deeper fibres reach the bone in front of the position of the centre of rotation, 

 while the " anterior " and superficial fibres, crossing these, reach the process behind 

 this centre. Rotation takes place between the Atlas and Axis, but for this movement 

 the Atlas and Occipital may be looked on as one mass, so that the ligaments check the 

 rotation as seen in Fig. 14, B. 



Here it is shown that when rotation occurs, as for instance from the position 

 A to B, that is to the right, the right posterior fibres p are carried back on the margin 

 of the foramen magnum and so stretched, and the left anterior fibres are rendered 

 taut by being carried forward, both of these sets of fibres being attached to the odontoid 

 in a relation to the centre of rotation opposite to that of the direction in which they are 

 carried by the occipital bone. 



A certain amount of " screw " also occurs. A glance at the surfaces shows that, as 

 the atlas turns, one of its surfaces must descend while the other ascends. The descent 

 leads to slackening of the ligament on that side, so giving more range. Any decrease 

 in range on the other side is compensated by slight tilting of the head to the side of the 

 ascent, a movement possible to some degree at the occipital joint. 



22 



