The first cervical vertebra (Atlas; C1) differs considerably from other cervical vertebrae to allow free movement of the head:
The atlas posses no body and is composed by two lateral masses joined by dorsal and ventral arches, constituting a bony ring for the beginning of the vertebral canal. The lateral masses present two cranial articular fovea for articulation with the paired occipital condyles and two caudal articular fovea for articulation with articular surfaces of axis.
The transverse processes are expanded and project laterally of each masses, forming shelf-like processes termed the wing of atlas. Each wing is hollowed to form the atlantic fossa on their ventral surface (especially deep in horses, flat in dogs). The base (cranial part) of each wing is perforated by the alar foramen (absent in carnivores, replaced by the alar notch) for passage of vertebral artery.
The transverse foramen (absent in ruminants) passes through the caudal part of the wing of the atlas, as a short canal.
The lateral vertebral foramen opens in the craniodorsal part of the vertebral arch.
The ventral arch represents of small part of the remnant body of the atlas (the larger part being incorporate din the dens and cranial articular surfaces of the axis).
On the dorsal part of the ventral arch of the atlas is the articular fovea for dens of axis.
There is cranioventral branch of the transverse process of the ventral arch termed the ventral tubercle.
The dorsal arch presents a dorsal tubercle, vestige of a spinous process.
Text by Antoine Micheau, MD - Copyright IMAIOS Veterinary Anatomy of Domestic Mammals: Textbook and Colour Atlas, Sixth Edition - Horst Erich König, Hans-Georg Liebich - Schattauer - ISBN-13: 978-3794528332 Illustrated Veterinary Anatomical Nomenclature - 3rd edittion - Gheorghe M. Constantinescu, Oskar Schaller - Enke