a, odontoid process or dens; b, cranial articular facets; c, spinous process; d, caudal articular facet; e, transverse process; f, foramen transversarium.
The caudal articular facets ([Fig. 12], h) are borne by the caudal ends of the lateral masses. They are slightly concave, triangular, and look caudomediad, so that their dorsal borders form with the caudal border of the dorsal arch nearly a semicircle. The transverse processes are flat and directed laterad. The attached margin of each is about two-thirds the length of the thinner free margin. The somewhat thicker caudal end of the transverse process projects further caudad than any other part of the vertebra and is separated by a slight notch from the caudal articular facet. From the bottom of this notch the foramen transversarium extends craniad and opens at the middle of the ventral face of the transverse process.
Epistropheus or Axis ([Fig. 10], 2; [Fig. 13]).—The second cervical vertebra (epistropheus or axis) is not so wide as the atlas but is much longer. Craniad the centrum is continued into a slender conical, toothlike projection, the dens or odontoid process ([Fig. 13], a) which represents the centrum of the atlas. The dens is smooth below for articulation with the ventral arch of the atlas. It is rougher above. Laterad of the dens the centrum bears a pair of large cranial articular facets (b) which look craniolaterad. These have each the form of a right-angled triangle with rounded angles, one side of the triangle being nearly horizontal. Each is separated from the articular face of the dens by a roughened groove. The spinous process (c) runs the length of the vertebral arch. It extends craniad of the vertebral arch nearly as far as the dens, as a flat rounded projection. Caudad of the vertebral arch it projects for a short distance as a stout triangular spine. The caudal articular facets (d) are borne on thickenings of the caudolateral portions of the arch; they face almost directly ventrad. The transverse process (e) is slender and triangular and directed nearly caudad. Its apex reaches no farther than the caudal or articular face of the centrum. Its base is traversed by the foramen transversarium (f).
Differential Characters of the Cervical Vertebræ.—It is possible to identify each of the cervical vertebræ:
The first by the absence of the centrum.
The second by the dens or odontoid process.
The third by the small spinous process and slightly marked tubercle of the transverse process, and by a median tubercle on the cranial border of the vertebral arch.
The fourth by the spinous process directed dorsad, and the short thick tubercle of the transverse process not trifid.
The fifth by the spinous process directed craniad, and the more slender spine-like tubercle of the transverse process not trifid.
The sixth by the trifid transverse process.