Atlas
The ventral arch of the atlas is considerably narrower (craniocaudally) than the dorsal arch. The lateral vertebral foramen for the first cervical nerve is close to the cranial border of the dorsal arch; a notch in the cranial border of the wing replaces the alar foramen of other species and transmits the ventral branch of the same nerve.
The wings, extended transverse processes, slope caudally and overlap the atlantoaxial junction. The base of the wing is perforated by the transverse foramen (see Fig. 2.7A).The dorsal and ventral arches of the atlas participate in the deep cranial articular foveae, which receive the occipital condyles. The single joint cavity has a U shape with its dorsal parts widely spaced and only narrowly joined ventrally. The atlanto-occipital membrane extends from the dorsal border of the foramen magnum to the dorsal arch of the atlas, and by attaching laterally to the joint capsules, it closes the atlanto-occipital aperture. This membrane is punctured in the collection of cerebrospinal fluid and in the injection of radiopaque contrast agent into the subarachnoid space (Figs. 12.3 and 12.6).