The skull articulates with the spinal column by means of the condyles of the occipital bone and the articular facets on the lateral masses of the atlas. The condyles are convex in both lateral directions, and the facets concave, so the joint is freely movable round two axes.

Transverse - flexion and extension.

Antero-posterior - lateral flexion.

A combination of these movements causes rotation or "rolling."

A capsule, which is complete, surrounds both the joints.

A synovial membrane lines both the joint cavities.

Accessory Ligaments

These are very numerous and more important than the ligaments of the actual joints.

Odontoid ligaments, are three in number, one from the apex of the odontoid peg and one on either side; these latter are called check ligaments.

Occipito-Atlantoid ligaments, anterior and posterior; their membranous structure continuous with the capsules of the joints, thus forming a complete ligament between the foramen magnum and the anterior and posterior arches of the atlas.

Posterior Occipito-Axoid ligament, a continuation upwards of the posterior longitudinal ligament.

Ligamentum Cruciatum, a cross-shaped ligament, the horizontal part formed by the transverse ligament of the atlas, the vertical bars being attached to the occipital bone and axis respectively.

Muscles acting on the Joints of the Skull.

The muscles of the head and face are divided into four sets:

1. Muscles of the scalp.

2. Muscles of expression.

3. Muscles of mastication.

4. Muscles connecting the skull to the spinal column.