Overview
The axis is the second cervical vertebra and the longest of the neck. Its defining feature is the dens, a cranial projection of the body that sits inside the ring of the atlas and serves as the pivot for rotation of the head.
Key features
- Dens: cylindrical projection resting in the fovea of the dens of the atlas. It is held in place by the transverse ligament of the atlas, which passes over it, and by the apical and alar ligaments, which tie it to the occipital bone.
- Spinous process: a tall, elongated crest that extends cranially over the dorsal arch of the atlas and caudally over the third cervical vertebra. In other species it anchors the nuchal ligament; in the cat the nuchal ligament is poorly developed or absent.
- Cranial articular surfaces: on either side of the dens, for the caudal foveae of the atlas.
- Caudal articular processes: articulate with C3.
- Transverse processes: small and directed caudally, pierced at their base by the transverse foramen for the vertebral artery.
Clinical relevance
- Atlantoaxial injuries: fractures of the dens or the body of the axis and atlantoaxial subluxation are uncommon in the cat; they follow severe trauma and cause neck pain and deficits in all four limbs.
- Radiography: the length of the axis and the height of its spinous process are landmarks for identifying the remaining cervical vertebrae.