Overview
The axis (axis) is the second cervical vertebra and the longest of the equine spine. Its characteristic feature is the dens, a cranial projection of the body that sits in the ventral arch of the atlas. The atlas, and with it the head, rotates around it at the atlantoaxial joint.
Key features
- Dens: in the horse it is broad and spade-shaped, concave dorsally, rather than cylindrical as in the dog. It is attached to the atlas by a strong longitudinal ligament of the dens.
- Spinous process: a long, high crest that widens caudally; it gives attachment to the nuchal ligament and neck muscles.
- Body: long, with a marked ventral crest and a broad cranial head that continues into the dens.
- Transverse processes: small and directed caudally, pierced by the transverse foramen.
- Caudal articular processes: articulate with the third cervical vertebra.
Clinical relevance
- Fractures of the axis: mainly in foals after falls or blows to the head; they usually involve the dens or the physis between the dens and the body, and can compress the spinal cord.
- Cervical stenotic myelopathy (wobbler syndrome): it affects the cervical vertebrae caudal to the axis, but radiographic examination of the neck always includes it.