CEEVICAL VERTEBE^. 



91 



bone fused with the arch where the roots of the vertebral arches and the laminae 

 join. These cylinders are sliced away obliquely above and below, so that the superior 

 articular facets, more or less circular in form, are directed upwards and backwards, 

 whilst the corresponding inferior surfaces are turned downwards and forwards. 



The Atlas or First Cervical Vertebra. This bone may be readily recognised 

 by the absence of the body and spinous process. It consists of two lateral masses, 

 which support the articular and transverse processes. The lateral masses are them- 

 selves united by two curved bars of bone, the anterior and posterior arches, of which 

 the former is the stouter and shorter. Each lateral mass is irregularly six-sided, 

 and so placed that it lies closer to its fellow of the opposite side in front than 

 behind. Its upper surface is excavated to form an elongated oval facet called the 

 superior articular fovea, which is concave from before backwards, and inclined 

 obliquely medially ; not infrequently this articular surface displays indications of 

 division into two parts. The superior articular fovese are for the reception of the 

 condyles of the occipital bone. 



The inferior articular fovese or facets are placed on the inferior surfaces of 

 the lateral masses. Of circular form, they display a slight side-to-side con- 

 cavity, though flat in the 

 antero- posterior direction. 

 Their disposition is such 

 that their surfaces incline 

 downwards and slightly 

 medially. They rest on the 

 superior articular processes 

 of the second cervical 2 

 vertebra or epistropheus. 

 Springing from the an- 

 terior and medial aspects 

 of the lateral masses, and 

 uniting them in front, is a 

 curved bar of bone, the 

 arcus anterior (anterior arch); 

 compressed on each side, 

 and thickened centrally 

 so as to form on its an- 

 terior aspect the rounded 

 tuberculum anterius (an- 

 terior tubercle). In corre- 

 spondence with this, on the posterior surface of this arch is a circular facet 

 (fovea dentis) for articulation with the dens of the epistropheus. 



The medial surface of the lateral mass is rough and irregular, displaying a 

 tubercle for the attachment of the transverse ligament of the atlas, which passes 

 across the space included between the two lateral masses and the anterior arch, 

 thus holding the dens of the epistropheus in position. Behind each tubercle there 

 is usually a deep pit, opening into the bottom of which are the canals for the 

 nutrient vessels. 



Laterally to the lateral mass, and principally from its upper half, the transverse 

 process arises by two roots which include between them the foramen trans- 

 versarium. The transverse process is long, obliquely compressed, and down-turned ; 

 the anterior and posterior tubercles 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, which groove is also continuous 

 laterally with the foramen transversarium, the posterior arch becomes thicker 

 medially, at which point it displays posteriorly a rough irregular projection the 

 tuberculum posterius (posterior tubercle), the feeble representative of the spinous 

 process. A prominent little tubercle, arising from the posterior extremity of the 

 superior articular process, overhangs the groove above mentioned, and not in- 



1. Posterior arch. 



2. Transverse process. 



3. Tubercle for transverse 



ligament. 



4. Anterior arch. 



5. Anterior tubercle. 



10 

 FIG. 108. THE ATLAS FROM ABOVE. 



6. Surface for articulation with dens. 



7. Superior articular surface. 



8. Foramen for vertebral artery. 



9. Groove for vertebral artery. 

 10. Posterior tubercle. 



