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Cervical vertebra 2 - axis
Other Terms: Axis (CII), C2 vertebra, Second cervical vertebra, Axis [C II]
This vertebra differs from other cervical vertebrae in its possession of a dens or odontoid process. This tooth-like process projects superiorly from the vertebral body forming a pivot around which the ring-like atlas rotates. It is the bulkiest of the cervical vertebrae. Unlike other cervical vertebrae, its superior and inferior articular facets do not sit in line with one another, the superior facet sitting anterior to the inferior facet.
The word cervical arises from the Latin term cervix meaning neck. This is an ancient term that has passed through the centuries literally unchanged. The word vertebra is an old Latin term that meant a joint or something to be turned. It arises from the Latin verto meaning to turn. In A.D. 30 Celsus used the word to designate any joint. It was only in later years that the bone arrived at its present meaning. The term axis arises from the Greek word axon or axel, which in turn comes from the Sanscrit word aksha meaning wheel or that around which something turns. James Pollux, a 2nd Century Greek author, named the second cervical vertebra, the axis, concerning its functional relationship to the first cervical vertebra. Historically, this vertebra has also been called the epistropheus, literally meaning to pivot upon, a reference to its pivot-like function with the atlas.
The axis articulates with two other vertebrae: the atlas and the third cervical vertebra. The slightly convex superior articular facets articulate with the shallow inferior articular facets of the atlas. The articular surface on the anterior face of the dens articulates with the fovea of the fovea dentis of the anterior arch of the atlas. The round inferior articular facets, facing inferoventrally articulate with the opposite facing superior articular facets of the subjacent vertebra and the inferior surface of the body articulates with the superior surface of the third vertebra's body.
The axis ossifies from seven separate centers, five primary and two secondary. Primary centers, similar to all vertebrae, form the body and each half of the vertebral arch, the arch centers beginning at the junction of the lamina, the pedicle, and the transverse process. The body's center appears in the fourth fetal month, while those of the arches appear during the seventh week. Two centrolateral centers arise above the body during the sixth fetal month. These centers progress anteriorly, fusing by birth. They form the main portion of the dens, the apex, however forms from a secondary center that appears in the second year and fuses to the aforementioned primary centers during the twelfth year. (For an excellent review of the origins of the dens see: Jenkins, F. A. 1969. The evolution and development of the dens of the mammalian axis. Anat. Rec. 164: 173-184.) After birth, a second secondary center arises to form an inferior epiphyseal plate that joins the body around the seventeenth year.
Axis [C II]