Overview
The atlas is the first cervical vertebra. Unlike the others it has no vertebral body and no spinous process: it is a ring made of a dorsal arch and a ventral arch, joined laterally by two masses from which the wings (alae atlantis) project. It articulates cranially with the occipital condyles and caudally with the axis.
Key features
- Cranial articular foveae: two deep cavities that receive the occipital condyles. The atlanto-occipital joint allows flexion and extension of the head (the "yes" movement).
- Caudal articular foveae: flatter surfaces for the axis. The atlantoaxial joint allows rotation of the head (the "no" movement).
- Fovea of the dens (fovea dentis): on the dorsal surface of the ventral arch, where the dens of the axis rests.
- Dorsal and ventral tubercles: small midline prominences on each arch.
- Wings of the atlas: wide horizontal plates, easily palpable on each side of the neck just caudal to the skull.
- Foramina: the lateral vertebral foramen transmits the first cervical nerve and the transverse foramen, at the base of the wing, the vertebral artery.
Clinical relevance
- Cerebellomedullary cistern puncture: cerebrospinal fluid is collected between the occipital bone and the dorsal arch of the atlas with the head flexed; the wings and the external occipital protuberance are the landmarks.
- Atlantoaxial instability: much less common than in toy-breed dogs; in cats it is described after trauma and, rarely, with malformations of the dens of the axis.
- Fractures: uncommon; associated with road accidents and falls.