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arches across the ring of the atlas, and retains the odontoid process in contact with the anterior arch. It is concave in front, convex behind, broader and thicker in the middle than at the ends, and firmly attached on either side to a small tubercle on the medial surface of the lateral mass of the atlas. As it crosses the odontoid process, a small fasciculus (crus superius) is prolonged upward, and another (crus inferius) downward, from the superficial or posterior fibers of the ligament. The former is attached to the basilar part of the occipital bone, in close relation with the membrana tectoria; the latter is fixed to the posterior surface of the body of the axis; hence, the whole ligament is named the cruciate ligament of the atlas. The transverse ligament divides the ring of the atlas into two unequal parts: of these, the posterior and larger serves for the transmission of the medulla spinalis and its membranes and the accessory nerves; the anterior and smaller contains the odontoid process. The neck of the odontoid process is constricted where it is embraced posteriorly by the transverse ligament, so that this ligament suffices to retain the odontoid process in position after all the other ligaments have been divided.

Synovial Membranes.—There is a synovial membrane for each of the four joints; the joint cavity between the odontoid process and the transverse ligament is often continuous with those of the atlantoöccipital articulations.

Movements.—The opposed articular surfaces of the atlas and axis are not reciprocally curved; both surfaces are convex in their long axes. When, therefore, the upper facet glides forward on the lower it also descends; the fibers of the articular capsule are relaxed in a vertical direction, and will then permit of movement in an antero-posterior direction. By this means a shorter capsule suffices and the strength of the joint is materially increased. 1
  This joint allows the rotation of the atlas (and, with it, the skull) upon the axis, the extent of rotation being limited by the alar ligaments.
  The principal muscles by which these movements are produced are the Sternocleidomastoideus and Semispinalis capitis of one side, acting with the Longus capitis, Splenius, Longissimus capitis, Rectus capitis posterior major, and Obliqui capitis superior and inferior of the other side.
5c. Articulations of the Vertebral Column with the Cranium
  The ligaments connecting the vertebral column with the cranium may be divided into two sets: those uniting the atlas with the occipital bone, and those connecting the axis with the occipital bone.

Articulation of the Atlas with the Occipital Bone (articulatio atlantoöccipitalis).—The articulation between the atlas and the occipital bone consists of a pair of condyloid joints. The ligaments connecting the bones are:
Two Articular Capsules.
The Posterior Atlantoöccipital membrane.
The Anterior Atlantoöccipital membrane.
Two Lateral Atlantoöccipital.

The Articular Capsules (capsulœ articulares; capsular ligaments).—The articular capsules surround the condyles of the occipital bone, and connect them with the articular processes of the atlas: they are thin and loose.

The Anterior Atlantoöccipital Membrane (membrana atlantoöccipitalis anterior; anterior atlantoöccipital ligament) (Fig. 304).—The anterior atlantoöccipitalis membrane is broad and composed of densely woven fibers, which pass between the anterior margin of the foramen magnum above, and the upper border of the anterior arch of the atlas below; laterally, it is continuous with the articular capsules; in front, it is strengthened in the middle line by a strong, rounded
Note 1.  Corner (“The Physiology of the Atlanto-axial Joints,” Journal of Anatomy and Physiology, vol. xli) states that the movements which take place at these articulations are of a complex nature. The first part of the movement is an eccentric or asymmetrical one; the atlanto-axial joint of the side to which the head is moved is fixed, or practically fixed, by the muscles of the neck, and forms the center of the movement, while the opposite atlantal facet is carried downward and forward on the corresponding axial facet. The second part of the movement is centric and symmetrical, the odontoid process forming the axis of the movement [back]


