The axis the second cervical vertebra
The second vertebra, vertebra dentata, or axis, (so called from forming the pivot on which the head rotates,) is somewhat triangular in its form.
The body, presents anteriorly a vertical ridge, bounded on each side by a depression for the attachment of the longus colli muscle; superiorly it is surmounted by a process (odontoid, p. dentatus ; whence is derived the name vertebra dentata) presenting two smooth surfaces, one for its articulation with the atlas, the other with the transverse ligament, which retains it in its situation; being constricted inferiorly, and somewhat enlarged towards the summit, these parts of the process are called respectively its neck and head. The superior articulating processes are of considerable size, and nearly horizontal; they are close to the body, so as to communicate to it the weight of the head, transmitted to them by the articular processes of the atlas ; the inferior pair are oblique, and of the same size as in the vertebras beneath them. The transverse processes are neither grooved nor bifurcated, and the foramen at their root is inclined obliquely outwards. The spinous process is very large, and gives attachment to several muscles; it is deeply grooved on its inferior surface; the plates which support it are of proportionate size.