2 33 Anatomy of IOM a Facet for dens and axis Anterior arch Lateral mass with facet for articulation with occipital condyle Transverse foramen Transverse process Vertebral foramen Groove for vertebral artery Posterior arch b Odontoid process Facet for atlas Transverse foramen Spine Fig. 2.33 (a) The C1 vertebra, atlas. (b) The C2 vertebra, axis The C2 vertebra is named axis for the fact that it provides an axis of rotation for C1. Its most prominent feature is the bony projection known as the dens. The dens is also called the odontoid process, since it resembles a tooth (Fig. 2.33b). The union between C1 and the occiput is the atlanto-occipital joint which is responsible for the nodding movement of the head. The C1âC2 joint is also known as the atlanto-axial joint and provides for rotation of the head on the neck. The C7 vertebra has a prominent spinous process and therefore is named vertebra prominens. Occasionally C7 has an abnormal pair of ribs associated with it that can cause compression of blood vessels as well as the nerves of the brachial plexus. This condition, known as thoracic outlet syndrome, can cause pain, numbness, and tingling in the upper extremity. Thoracic Vertebrae The thoracic segment of the vertebral column is the least flexible due to the attachment of the ribs. The ribs attach to facets located both on the vertebral body and the transverse processes. These facets are called costal facets. The bodies of the thoracic vertebrae are larger than the cervical but smaller than the lumbar (Fig. 2.34). The size increases with progression from T1 to T12. The vertebral canal contains the spinal cord, and the intervertebral foramina remain the exit points for the spinal nerves.