The defining feature of the axis is its strong bony protrusion known as the dens, which rises from the superior aspect of the bone.

It presents a median longitudinal ridge in front, separating two lateral depressions for the attachment of the longus colli muscles.

The condition where the dens is separated from the body of the axis is called os odontoideum and may cause nerve and circulation compression syndrome.

On the back of the neck, and frequently extending on to its lateral surfaces, is a shallow groove for the transverse atlantal ligament which retains the process in position.

The superior articular surfaces are round, slightly convex, directed upward and laterally, and are supported on the body, pedicles, and transverse processes.

The dens, or odontoid process, consists originally of a continuation upward of the cartilaginous mass, in which the lower part of the body is formed.

During about the sixth month of fetal life, two centres make their appearance in the base of this process: they are placed laterally, and join before birth to form a conical bilobed mass deeply cleft above; the interval between the sides of the cleft and the summit of the process is formed by a wedge-shaped piece of cartilage.

The apex of the dens has a separate centre that appears in the second and joins about the twelfth year; this is the upper epiphyseal plate of the atlas.

Fractures of the dens are classified into three categories according to the Anderson Alonso system: This article incorporates text in the public domain from page 99 of the 20th edition of Gray's Anatomy (1918)