12 



ATLAS AND AXIS. 



Fig. 



tenor arch has a tu- 

 bercle on its anterior 

 surface, for the attach- 

 ment of the longus colli 

 muscle ; and on its pos- 

 terior aspect is a smooth 

 surface, for the articula- 

 tion of the odontoid 

 process of the axis. 



The posterior arch is 

 longer and more slender 

 than the anterior, and 

 flattened from above 

 downwards ; at its mid- 

 dle is a rudimentary 



spinous process ; and upon its upper surface, near the articular pro- 

 cesses, a shallow groove "f at each side, which represents a superior 

 intervertebral notch, and supports the vertebral artery previously to 

 its passage through the dura mater, and the first cervical nerve. The 

 intervertebral notches are peculiar from being situated behind the ar- 

 ticular processes instead of before them, as in the other vertebrae. The 

 transverse processes are remarkably large and long, and pierced by the 

 foramen for the vertebral artery. The articular processes are situated 

 upon the most bulky and strongest part of the atlas. The superior 

 are oval and concave, and look inwards, so as to form a kind of cup 

 for the condyles of the occipital bone, and are adapted to the nodding 

 movements of the head ; the inferior are circular, and nearly horizon- 

 tal, to permit of the rotatory movements. Upon the inner face of the 

 lateral mass which supports the articular processes, is a small tubercle 

 at each side, to which the extremities of the transverse ligament are 

 attached, a ligament which divides the ring of the atlas into two un- 

 equal segments ; the smaller for receiving the odontoid process of the 

 axis, and the latter to give passage to the spinal cord and its mem- 

 branes. 



The Axis (vertebra dentata) is so named from having a process upon 

 which the head turns as on a pivot. The body is of large size, and 

 supports a strong process, the odontoid, which rises perpendicularly 

 from its upper surface. The odontoid process presents two articulating 

 surfaces ; one on its anterior face, to articulate with the anterior arch 

 of the atlas ; the other on its posterior face, for the transverse liga- 



* The upper surface of the atlas. 1. The anterior tubercle projecting from 

 the anterior arch. 2. The articular surface for the odontoid process upon the 

 posterior surface of the anterior arch. 3. The posterior arch, with its rudi- 

 mentary spinous process. 4. The intervertebral notch. 5. The transverse 

 process, o. The vertebral foramen. /" Superior articular surface. 8. The 

 tubercle for the attachment of the transverse ligament. The tubercle referred 

 to is just above the head of the figure ; the convexity below it is the margin of 

 the inferior articulating process. 



t This groove is sometimes converted into a foramen. 



