THE CERVICAL VERTEBR2. 77 
process may, in rare cases, articulate with a projecting process (paroccipital) from the under 
surface of the jugular process of the occipital bone (see p. 109). An upward extension from the 
median part of the anterior arch, due probably to an ossification of the anterior occipito-atlantal 
ligament, may articulate with the anterior surface of the summit of the odontoid process of 
the axis. Allen has noticed the articulation of the superior border of the posterior arch with 
the posterior border of the foramen magnum. Cases of partial or complete fusion of the atlas 
with the occipital bone are not uncommon (see p. 109). 
Second Cervical Vertebra, Axis, or Epistropheus.—This is characterised by 
the presence of the tooth-lhke odontoid process (dens) which projects upwards from 
the superior surface of the body. Shghtly constricted where it joins the body, the 
odontoid process tapers to a blunt point superiorly, on the sides of which there are 
surfaces for the attachment of the odontoid or check ligaments. When the atlas 
and axis are articulated together this process hes behind the anterior arch of the 
atlas, and displays on its anterior surface an oval or circular facet which articu- 
lates with that on the posterior surface of the anterior arch of the atlas. On the 
Odontoid process Groove for transverse ligament 
Superior articular 
surface Odontoid process 
Articular 
surface for 
anterior arch 
of atlas 
Transverse 
process 
Groove for 
| -—_ transverse 
ligament hl 
Spine 
Superior 
articular 
process 
Body 
Foramen for 
vertebral 
artery \ Foramen Inferior articular 
Inferior articular er for verte- dol Ue preeese 
process g Xo bral artery Transverse process 
Spine 
Fia. 58.—AxIsS FROM BEHIND AND ABOVE. Fic. 59.—AXIS FROM THE LEFT SIDE. 
posterior aspect of the neck of the odontoid process there is a shallow groove which 
receives the transverse ligament which holds it in position. 
The anterior surface of the body has a raised triangular surface, which ends 
superiorly in a ridge passing upwards to the neck of the odontoid process. The 
pedicles are concealed above by the superior articular processes ; inferiorly, they are 
deeply grooved. The lamine—prismatic on section—are thick and strong, ending 
in a stout, broad, and bifid spinous process, the under surface of which 1s deeply 
grooved, whilst its sides meet superiorly in a ridge. Placed over the pedicles 
and the anterior root of the transverse processes are the superior articular surfaces. 
These are more or less circular in shape, slightly convex from before backwards, 
flat from side to side, and have a direction upwards and a little outwards. They 
are channelled inferiorly by the vertebrarterial foramina which turn outwards 
beneath them. The grooves by which the second cervical nerves leave the neural 
canal cross the laminze immediately behind the superior articular processes. The 
inferior articular processes agree in their form and position with those of the 
remaining members of the series, and are placed behind the inferior intervertebral 
notches. 
The transverse process is markedly down-turned, with a single pointed 
extremity. 
Variations.—In some instances the summit of the odontoid process articulates with a promi- 
nent tubercle on the anterior border of the foramen magnum (third occipital condyle, see p. 109). 
Bennet (Trans. Path. Soc. Dublin, vol. vii.) records a case in which the odontoid process was 
double, due to the persistence of the primitive condition in which it 1s developed from two 
centres. Occasionally the odontoid process fails to be united with the body of the axis, forming 
an os odontoideum comparable to that met with in the crocodilia (Giacomini, Romiti, and 
Turner). The vertebrarterial foramen is not infrequently incomplete, owing to the imperfect 
ossification of the posterior root of the transverse process. 
