- The axis is the second cervical vertebra (C2).
- The axis articulates with:
- the atlas (C1) above, forming the atlanto-axial joint.
- C3 below, forming the C2-C3 joint complex via the paired facet joints and the disc joint.
- What would have been the body of the atlas (C1 above) fused onto the axis instead, creating the odontoid process, aka the dens.
- The dens creates a vertical axis of motion that the ring-shaped atlas can rotate around (left and right rotation in the transverse plane). The atlanto-axial joint between the atlas and axis is responsible for 45% of all cervicocranial rotation.
- The axis has the largest spinous process in the cervical spine.