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.
[Illustration: FIG. 13.--AXIS OR EPISTROPHEUS, SIDE VIEW.
_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.
The seventh by the long spinous process and the slender simple
transverse process, and by the usual absence of the foramen
transversarium.