Axis Vertebra: Introduction
Directly inferior to the atlas bone (C1) is the second cervical vertebra (C2), which is also referred to as the axis bone (axis, vertebra cervicalis II).
One of the most prominent features of the axis bone is a superior, tooth-like projection called the odontoid process (or dens).
The odontoid process articulates with the anterior arch of the atlas bone, where they form a pivot joint. When the head rotates side-to-side, the atlas bone rotates around the peg-like odontoid process. This is the reason why the second cervical vertebra is also called the axis bone.
a. Body or centrum (Corpus vertebrae) is a cylindrical mass on anterior side of the vertebra. It articulates with body of the vertebra below (C3).
b. Odontoid process or dens (Dens axis) is a prominent rounded superior projection from the body. This tooth-like elevation serves a stable point around which the atlas bone rotates (Gr., odous, tooth).
c. Superior articular facet (Facies articularis superior) is a smooth, sightly convex surface that articulates with the inferior articular facet on the altas bone above (C1).
d. Transverse process (Processus transversus) is a small extension that lies inferior to the superior articular facet and projects laterally. It is an attachment site for muscles that move the head and neck.
e. Transverse foramen (Foramen transversarium) is a hole in the transverse process that serves as a passageway for the vertebral artery and vein.
f. Vertebral foramen (Foramen vertebrale) is a large opening in the center of the axis bone that forms a passageway for the spinal cord.
g. Vertebral arch or neural arch (Arcus vertebrae) is the bony archway that encloses the posterior vertebral foramen and protects the spinal cord from damage.
h. Lamina of the vertebral arch (Lamina archus vertebrae) is one of two plates that forms the dorsal portion of the vertebral arch.
i. Spinous process (Processus spinosus) is a narrow posterior projection from the junction of the two laminae. It is an attachment point for ligaments and muscles that move the head and neck.
j. Inferior articular process (Processus articularis inferior) is a downward projection from the vertebral arch. Its flattened inferior surface or facet makes contact with the superior articular facet on C3.