Fig. 11.—Sixth Cervical Vertebra, Cranial End.
Fig. 12.—Atlas, Ventral View.
Fig. 11.—a, spinous process; b, cranial articular facet; c, lamina; d, radix or pedicle; e, transverse process proper; f, f′, processus costarius; g, foramen transversarium; h, centrum; i, vertebral canal.
Fig. 12.—a, ventral arch; b, tuberculum anterius; c, lateral masses; d, transverse processes; e, cranial articular facets; f, groove connecting the foramen transversarium with the atlantal foramen; g, atlantal foramen; h, caudal articular facets.
The Atlas ([Fig. 10], 1; [Fig. 12]).—The first cervical vertebra or atlas has somewhat the form of a seal ring. The centrum is absent; it has united with the second vertebra to form the odontoid process or dens. Its place is taken in the atlas by a narrow flat arch of bone, narrower at the ends than in the middle, the ventral arch ([Fig. 12], a) of the atlas. This connects the lateral, thicker portions of the ring ventrally and bears on its caudal margin a blunt tubercle (tuberculum anterius, [Fig. 12], b). Laterally the ring is thickened, forming thus the lateral masses (c) which are continued into the broad thin transverse processes ([Fig. 10], g; [Fig. 12], d). Each lateral mass bears at its cranial end on its medial surface a concave, pear-shaped facet, cranial (or superior) articular facet, ([Fig. 12], e) for articulation with the condyles of the skull. These facets look craniomediad. Dorsad of each is a foramen, the atlantal foramen ([Fig. 10], i; [Fig. 12], g), which pierces the dorsal arch at its junction with the lateral mass. Caudal to the facet, on the medial face of each lateral mass, within the vertebral canal, is a tubercle. To the two tubercles are attached the transverse ligament ([Fig. 14], b) which holds in place the odontoid process (dens) of the axis.
That part of the lateral mass which bears the articular facet projects craniad of the dorsal arch and is separated by a deep triangular notch from the transverse process. Along the bottom of this notch runs a groove ([Fig. 12], f), convex craniad, which connects the cranial end of the foramen transversarium and the atlantal foramen. The vertebral artery passes along it. The foramen transversarium is circular. It is bounded laterally by the lateral masses, and dorsally by the dorsal arch.
The dorsal arch ([Fig. 10], h) is two to three times as broad as the ventral, has a thick convex cranial border with a median notch, and a thin concave caudal border.
Fig. 13.—Axis or Epistropheus, Side View.