<P> The atlas is the topmost vertebra and with the axis forms the joint connecting the skull and spine . The atlas and axis are specialized to allow a greater range of motion than normal vertebrae . They are responsible for the nodding and rotation movements of the head . </P> <P> The atlanto - occipital joint allows the head to nod up and down on the vertebral column . The dens acts as a pivot that allows the atlas and attached head to rotate on the axis, side to side . </P> <P> The atlas's chief peculiarity is that it has no body . It is ring - like and consists of an anterior and a posterior arch and two lateral masses . </P> <P> The atlas and axis are important neurologically because the brain stem extends down to the axis . </P>

What is the most posterior structure of the atlas c1 vertebra