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The axis vertebra is a bone of the cervical spine in the neck. Second from the top, it is situated beneath the atlas bone, which supports the weight of the head. Other names for the axis include C2, for its position; epistropheus, its scientific name; and vertebra dentata, for an unusual bony protrusion that projects upward from the vertebral body like a tooth and is therefore known as the dens.
This bone possesses many of the same physical characteristics of the other cervical vertebrae, including a body shaped like a hollow ring in order for the contents of the spinal column to pass through. Its spinous processes, the backward-projecting bony protrusions on the posterior side of the bone that are shaped like a whale’s tale, angle slightly downward and serve as a point of attachment for the spinalis and semispinalis cervicis muscles in the upper back. Both play a role in flexing the head laterally, or bending it to one side, and in extending the cervical portion of the spinal column.
Like the other cervical vertebrae, the axis vertebra also features a pair of transverse processes, minor prominences on either side of the vertebra that act as attachment sites for a number of muscles. These include the scalene, splenius, and longissimus muscles in the neck. Because these muscles stretch from the axis to attach to other points along the spine, they make spinal movement possible, particularly rotation and lateral flexion of the head and neck.
One distinguishing feature of the axis vertebra is the dens, also known as the odontoid process. So known for its resemblance to a tooth, the odontoid process is what allows the atlas to rotate around the axis, thus causing the head to turn. This bony protrusion juts vertically from the anterior side of the body of the axis and articulates with the inner surface of the anterior arch of the atlas, where it is held in place across its posterior side by the transverse atlantal ligament. Thanks to this and several other ligaments holding the dens in place, the articulation, a pivot joint, is well fortified and allows for a limited rotation of the head on the neck.
Another anatomical structure on the axis vertebra that is characteristic of its unique function is the pair of superior articulating surfaces, one on either side of the anterior segment of the body of the vertebra. Perched atop the vertebral body above the transverse processes, the superior articulating surfaces are somewhat bean-shaped and slightly convex. They are shaped this way because they articulate with the concave inferior surfaces of the lateral masses of the atlas vertebra above to form the lateral atlanto-axial joints, one to either side. This articulation between the axis vertebra and the atlas is a synovial gliding joint, meaning that these two nearly flat surfaces can slide past each other to a very small degree.