Overview
The axis (C2) is the longest vertebra of the dog's neck. With the atlas it forms the atlantoaxial joint, a pivot joint responsible for most of the rotation of the head. Unlike the atlas, it has a long body and a very large spinous process.
Key features
- Dens (dens axis): peg-like cranial projection of the body that fits into the vertebral foramen of the atlas, on its ventral floor. It is held in place by the transverse ligament of the atlas and by the apical and alar ligaments.
- Cranial articular surfaces: on each side of the dens; they articulate with the caudal articular foveae of the atlas.
- Spinous process (processus spinosus): a long, blade-like crest that overhangs the atlas cranially and ends caudally in a thickened part bearing the caudal articular processes. It gives attachment to the nuchal ligament.
- Transverse processes: small and directed caudally; their bases are perforated by the transverse foramen.
- Caudal articular processes: articulate with the third cervical vertebra.
Clinical relevance
Atlantoaxial instability (subluxation) is a congenital disease of toy breeds (Yorkshire terrier, Chihuahua, toy poodle): an absent, hypoplastic or separated dens, or missing ligaments, allows the axis to move dorsally into the vertebral canal and compress the spinal cord, causing neck pain and tetraparesis. Traumatic fractures of the dens produce the same instability.
In this model the axis is reconstructed by deforming the scanned seventh cervical vertebra; its outline is approximate.