AXIS
The axis is characterized by its length and its enormous spinous process, which overhangs both the dorsal arch of the atlas and the laminae of the third vertebra and carries the caudal articular processes.
The cranial extent of the spinous process matches that of the dens, which rests on the dorsal surface of the ventral arch of the atlas (see Figure 2-7 and Figure 2-8). The dens, the displaced body of the atlas, is the pivot around which the atlas and thus the head rotates. The atlantoaxial joint is enclosed by a single joint capsule. The two bones are held in apposition by a thin median ligament (ligamentum apicis dentis), which connects the tip of the dens with the ventral border of the foramen magnum (see Figure 2-13), and paired (alar) ligaments, which pass obliquely from the dens to the ventrolateral borders of the foramen. The dens is further secured by a transverse ligament connecting the inner walls of the ventral arch of the atlas across its dorsal surface. This transverse ligament allows rotations but prevents impingement of the dens on the spinal cord. The dens plays an| Table 12-1 Development and Maturation of the Canine* Vertebral Column | |
| Ossification Centers Present of Birth (after Birth) | Approximate Age at Growth Plate Closure Observed on Radiographs |
| Vertebrae, except C1 and C2 | |
| Cranial epiphysis (2-8 wk) | 7-14 mo↑ |
| Body | |
| Caudal epiphysis (2-8 wk) | 7-14 mo↑ |
| Two sides of arch | |
| Atlas | |
| Ventral arch | |
| Two sides of dorsal arch | 4 moJ |
| Axis | |
| Apex of dens (3-4 mo) | 3-4 moJ |
| Dens and cran. articular | 7-9 moJ |
| surface | |
| Intercentrum (3 wk) | 4 moJ |
| Body | |
| Caudal epiphysis (3 wk) | 7-9 moJ |
| Two sides of arch | 3 moJ |
*Similar information for the cat appears to be lacking.
JBased on Hare WCD: Zur Ossifikation und Vereinigung der Wirbelepiphysen beim Hund, Wien Tierarztl Monatsschr 48:210-215, 1961.JBased on Hare WCD: Radiographic anatomy of the cervical region of the canine vertebral column, JAVMA 139:209-220, 1961.
From de Lahunta, A., and R.E. Habel, 1986.
important role in stability of the atlantoaxial joint, and in cases of aberrant development of the growth plate (early fusion, partial fusion, or nonfusion) there is instability; this is most often seen in miniature breeds (Chihuahua, Toy Pomeranian, Pekingese, and Toy Poodle).
The atlantoaxial membrane closes the interarcuate space; its median part is thickened by elastic fibers that connect the cranial tip of the spine of the axis with the tubercle on the dorsal arch of the atlas (dorsal atlantoaxial ligament) (Figure 12-4/5).