76 OSTEOLOGY. 
articular surface displays indications of division into two parts. These facets are 
for the reception of the condyles of the occipital bone. 
The inferior articular facets are placed on the under surfaces of the lateral 
masses. Of circular form, 
they display a slight side- 
to-side concavity, though 
flat in the antero-posterior 
direction. ‘Their position 
is such that their surfaces 
incline downwards and 
slightly wards. They rest 
on the superior articular 
processes of the second cer- 
vical vertebra. Springing 
from the anterior and inner 
aspects of the lateral masses, 
and uniting them in front, 
is a curved bar of. bone, the 
Fia. 57.—TuHr ATLAS FROM ABOVE. anterior arch (arcus an- 
1. Posterior arch. 6. Surface for articulation with odontoid terior) : compressed on 
2. Transverse process, _ process. miter as Perr rhe 3 Hide 
3. Tubercle for transverse 7. Superior articular process. elther side, this 1s thick- 
ligament. 8. Foramen for vertebral artery. ened centrally so as to form 
4, Anterior arch. oF Groove for vertebral artery. on its anterior aspect the 
5. Anterior tubercle. 10. Posterior tubercle. 
rounded anterior tubercle 
(tuberculum anterius). In correspondence with this on the posterior surface of 
this arch is a circular facet (fovea dentis) for articulation with the odontoid 
process of the second cervical vertebra (axis). 
The inner surface of the lateral mass is rough and irregular, displaying a little 
tubercle for the attachment of the transverse ligament which passes across the space 
included between the two lateral masses and the anterior arch, thus holding the 
odontoid process of the axis in position. Behind each tubercle there is usually a 
deep pit, opening into the bottom of which are the canals for the nutrient vessels. 
External to the lateral mass, and principally from its upper half, the transverse 
process arises by two roots which include between them the. vertebrarterial 
foramen. The transverse process is long, obliquely compressed, and down-turned ; 
the anterior and posterior tubercles are no longer distinguishable, as they have fused 
to form one mass. 
The posterior arch arises in part from the posterior surface of the lateral mass, 
and in part from the posterior root of the transverse process. Compressed from 
above downwards anteriorly, where it bounds a groove which curves around the 
posterior aspect of the superior articular process and is also continuous externally with 
the vertebrarterial foramen, the posterior arch becomes thicker mesially, where it 
displays posterior a a rough irregular projection—the posterior tubercle (tuberculum 
posterius), the feeble representative of the spinous process. A prominent little 
tubercle, arising from the posterior extremity of the superior articular process, over- 
hangs the groove above mentioned, and not infrequently becomes developed so as 
to form a bridge of bone across it, converting the groove into a canal through which 
the vertebral artery and the first cervical or suboccipital nerve pass—a condition 
normally met with in many animals. It is noteworthy that the grooves along 
which the two highest spinal nerves pass lie behind the articular processes, in 
place of in front, as in other parts of the column. 
The ring formed by the lateral masses and the anterior and posterior arches is 
of irregular outline. The anterior part, cut off from the rest by the transverse 
ligament, serves for the lodgment of the odontoid process of the axis; the larger 
part behind corresponds to the upper part of the neural or spinal canal. 
Variations.—The vertebrarterial foramen is often deficient in front. Imperfect ossification 
occasionally leads to the anterior and posterior arches being incomplete. The superior articular 
surfaces are occasionally partially or completely divided into anterior and posterior portions. 
In some instances the extremity of the transverse process has two tubercles. The transverse 
