Which vertebra is known as the axis?

The second cervical vertebra is known as the axis. This vertebra is crucial for the rotation of the head, as it has a distinctive structure called the odontoid process, or dens. The axis allows the first cervical vertebra, known as the atlas, to pivot around it, enabling the head to turn from side to side. This unique design is essential for the range of motion in the cervical region of the spine.

In contrast, the first cervical vertebra, or atlas, supports the skull and allows for nodding movements, while the third and fourth cervical vertebrae do not possess the same pivoting capability as the axis. Hence, the axis is essential for understanding cervical mobility and the actions that involve head movement.