Overview
The atlas (atlas) is the first cervical vertebra. It has no body and no spinous process: it is a bony ring formed by a dorsal and a ventral arch, with two large lateral wings. It articulates cranially with the occipital condyles (atlanto-occipital joint, which allows the nodding movement) and caudally with the axis (atlantoaxial joint, which allows rotation of the head).
Key features
- Dorsal arch (arcus dorsalis): with a dorsal tubercle in place of a spinous process. The nuchal ligament runs over it, separated from the bone by the cranial nuchal bursa.
- Ventral arch (arcus ventralis): with a ventral tubercle for the longus colli; its dorsal surface forms the fovea for the dens of the axis.
- Wings (alae atlantis): broad and flat, easily palpated caudal to the base of the ear. Ventrally, the guttural pouch reaches the level of the wing.
- Foramina: the horse has three on each side: the lateral vertebral foramen, which transmits the first cervical nerve; the alar foramen, at the cranial border of the wing; and the transverse foramen, in the caudal part of the wing. The dog has an alar notch instead of an alar foramen, and cattle lack the transverse foramen.
- Cranial articular foveae: deep and cup-shaped, for the occipital condyles.
Clinical relevance
- Occipitoatlantoaxial malformation: a congenital anomaly, described mainly in Arabian horses, in which the atlas fuses with the occipital bone or takes on an occipital shape; it causes ataxia or an abnormal neck posture in foals.
- Atlanto-occipital puncture: under general anaesthesia or deep sedation, cerebrospinal fluid is collected from the cisterna magna, between the occipital bone and the dorsal arch of the atlas.
- Nuchal bursitis ("poll evil"): inflammation or infection of the cranial nuchal bursa over the dorsal arch of the atlas, with swelling and pain at the poll.