The vertebral column, or spinal column, is made up of a total of 33 vertebrae, which are subdivided into five regions: cervical, thoracic, lumbar, sacral, and coccyx. At the cervical region the spinal column is further classified into an upper and lower cervical region. The axis is the second of seven bones in the cervical spine.

The axis, also known as the C2 bone, creates a pivot that allows the C1, or atlas, to rotate. This action gives the head and neck a greater range of motion from side to side. The pivoting motion occurs on part of the bone called the dens, a tooth-like upright section on the bone. The dens in the axis is larger than any other vertebral bone.

Another special characteristic of the axis and all other cervical vertebrae is the absence of a body, or large flat portion of the bone. Because these two bones lack this body, the neck has the widest range of motion of all the sections of the spinal column.

Injury to the atlas or axis, such as fracture from a high-impact collision or sudden, violent jerking of the head, may cause paralysis and it some cases can even be fatal. The atlas and axis are the two most commonly injured cervical vertebrae bones.