This joint may be classed as a ginglymus. The =articular surfaces= of
this joint are: (1) On the atlas, two deep oval cavities; (2) the
corresponding condyles of the occipital bone.
The joint surfaces are oblique, coming very close to the median line
ventrally, but separated by a considerable interval dorsally.
Posteriorly, a triangular rough area cuts into the middle of each
articular surface on the atlas.
There are two roomy =joint capsules=, which sometimes communicate
ventrally, especially in old subjects.
The =superior atlanto-occipital membrane= extends from the dorsal arch
of the atlas to the upper margin of the foramen magnum. It is blended
with the capsules and contains many elastic fibers.
The =inferior atlanto-occipital membrane= extends from the ventral arch
of the atlas to the lower margin of the foramen magnum. It is narrower
and thinner than the superior membrane, and also fuses with the joint
capsules.
The =lateral ligaments= are two short bands which are partially blended
with the capsules. Each is attached to the border of the wing of the
atlas near the intervertebral foramen, and to the outer surface of the
paramastoid or styloid process of the occipital bone.
=Movements.=—These are chiefly flexion and extension. A small amount of
lateral oblique movement is also possible.