The atlas A(C1) is the most superior (first) cervical vertebra of the spine. It is named for the Atlas of mythology, because it supports the globe of the head. The atlas is the topmost vertebra, and – along 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. (adapted from wikipedia)
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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. The Atlas’ 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.