2-20. ATYPICAL VERTEBRAE
Atypical vertebrae (figure 2-20) are those vertebrae whose structure is highly
modified by function and position. They consist of the first cervical vertebra (C-1) or
atlas, the second cervical vertebra (C-2) or axis, the sacrum, and the coccyx.
a. The Atlas.
(1) The first cervical vertebra (C-1) is named the atlas because it supports
the head (figure 2-20A). It is characterized by the absence of both body and spinous
process and consists of an anterior and posterior arch, two lateral masses, and a
vertebral foramen. The anterior surface of the anterior arch presents a slight projection,
the anterior tubercle. The posterior surface is marked by a dental facet for articulation
with the dens or odontoid process of the axis. On the superior surface are two grooves
for the vertebral arteries.
(2) The morphology of the atlas affords freedom of movement of the skull.
The body of the atlas is transferred to the axis (second cervical vertebra) where it
becomes the dens (odontoid process), which articulates with the dental facet (facet for
odontoid) of the anterior arch of the atlas, thus making possible the rotary movements of
the skull. The superior articular surfaces of the atlas are concave for reception of the
condyles of the occipital bones, permitting flexion, extension, and hyperextensions of
b. The Axis. The second cervical vertebra (C-2) is named the axis, or
epistropheus, because it forms the pivot upon which the atlas rotates when the head is
turned from side to side (figure 2-20B). The axis differs primarily from a typical vertebra
by the presence of a tooth like projection, called the dens or odontoid process, which
rises perpendicularly from the upper surface of the body. On its anterior surface, the
dens presents an oval facet for articulation with the dental facet on the anterior arch of
the atlas. On its posterior surface is a shallow groove that receives the transverse
ligament of this articulation. The relationship of the atlas and axis to the skull is shown
in figure 2-21A. Other spinal articulations are also illustrated in figure 2-21.