Now we've looked at the part of the skull that we’re concerned with in this section. We'll move on now to look at the bones below it. First we'll look at the special features of the first two cervical vertebrae - the atlas and the axis. Then we'll look at the continuity of the cervical spine with the bones of the upper part of the trunk.
Here’s the atlas, here’s the axis. These two vertebrae are adapted to allow movement of the head. Forward flexion and extension of the head take place up here at the atlanto-occipital joints. Lateral flexion of the head takes place at these joints too. Rotation of the head, together with the atlas, happens here, at the joints between the atlas and the axis, the atlanto-axial joints
Because of their special functions, the atlas and the axis differ in several ways from typical cervical vertebrae. As we’ve seen in Volume 3, a typical cervical vertebra has a body in front and a neural arch behind, enclosing the vertebral foramen.
It has a spinous process behind with two tuberosities, and a transverse process on each side, also with two tuberosities. On each side there are two articular surfaces, one above, and one below, which form the intervertebral joints. The articular surfaces slope upward and forward. They’re connected by this mass of bone, the articular pillar.
Each vertebra is joined to its neighbors by an interbertebral disk in front, and by two intervertebral joints behind, one on each side. Now let’s look at the ways in which the atlas and the axis are different.
The atlas vertebra doesn’t have a body. In front it just has this narrow anterior arch which matches the posterior arch. The two arches of the atlas, together with these two lateral masses, enclose an unusually large vertebral foramen. This part is occupied by the spinal cord, this part by the odontoid process of the axis, which we’ll meet in a moment.
The upper articular surfaces of the atlas are shaped like parts of the inside of a cup, to match the shape of the occiptal condyles. The lower articular surfaces are shaped like parts of the inside of a cone.
Now let’s look at the axis vertebra. The body of the axis is prolonged by this important projection, the odontoid process. In terms of development the odontoid process represents the missing body of the atlas. In terms of function it’s the pivot around which the head, together with the atlas, rotates.
The upper articular surfaces of the axis are placed well in front of the lower ones. The upper surfaces are in a straight line with the odontoid process. As rotation occurs between these surfaces and those of the atlas, the odontoid process stays in the middle.
The odontoid process is surrounded in front and on each side by bone. It’s held in place behind by a strong ligament, the transverse ligament of the atlas. The odontoid process is also held in place from above by two strong ligaments, the alar ligaments, which are attached here and here. We’ll see these ligaments shortly.
The odontoid process has two small articular surfaces, one behind for the transverse ligament, and one in front for the anterior arch of the atlas.
To see how these structures relate to the base of the skull, we’ll take an inside look from behind at a specimen in which the neural arches, and the back of the occipital bone have been removed.
Here’s the foramen magnum, here’s the inside of the basal part of the occipital bone. Here’s the atlas, here’s the axis, here’s the odontoid process. Here are the atlanto-occipital joints, and the atlanto-axial joints.