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:
The Articular Capsules (capsul 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 cord, which connects the basilar part of the occipital bone to the tubercle on the anterior arch of the atlas. This membrane is in relation in front with the Recti capitis anteriores, behind with the alar ligaments.
The Posterior Atlantoöccipital Membrane (membrana atlantoöccipitalis posterior; posterior atlantoöccipital ligament) (Fig. 305).The posterior atlantoöccipital membrane, broad but thin, is connected above, to the posterior margin of the foramen magnum; below, to the upper border of the posterior arch of the atlas. On either side this membrane is defective below, over the groove for the vertebral artery, and forms with this groove an opening for the entrance of the artery and the exit of the suboccipital nerve. The free border of the membrane, arching over the artery and nerve, is sometimes ossified. The membrane is in relation, behind, with the Recti capitis posteriores minores and Obliqui capitis superiores; in front, with the dura mater of the vertebral canal, to which it is intimately adherent.
The Lateral Ligaments.The lateral ligaments are thickened portions of the articular capsules, reinforced by bundles of fibrous tissue, and are directed obliquely upward and medialward; they are attached above to the jugular processes of the occipital bone, and below, to the bases of the transverse processes of the atlas.
Synovial Membranes.There are two synovial membranes: one lining each of the articular capsules. The joints frequently communicate with that between the posterior surface of the odontoid process and the transverse ligament of the atlas.
Movements.The movements permitted in this joint are (a) flexion and extension, which give rise to the ordinary forward and backward nodding of the head, and (b) slight lateral motion to one or other side. Flexion is produced mainly by the action of the Longi capitis and Recti capitis anteriores; extension by the Recti capitis posteriores major and minor, the Obliquus superior, the Semispinalis capitis, Splenius capitis, Sternocleidomastoideus, and upper fibers of the Trapezius. The Recti laterales are concerned in the lateral movement, assisted by the Trapezius, Splenius capitis, Semispinalis capitis, and the Sternocleidomastoideus of the same side, all acting together.
Ligaments Connecting the Axis with the Occipital Bone.
The Membrana Tectoria.
The Apical Odontoid.
The Membrana Tectoria (occipitoaxial ligament) (Figs. 307, 308).The membrana tectoria is situated within the vertebral canal. It is a broad, strong bands which covers the odontoid process and its ligaments, and appears to be a prolongation upward of the posterior longitudinal ligament of the vertebral column. It is fixed, below, to the posterior surface of the body of the axis, and, expanding as it ascends, is attached to the basilar groove of the occipital bone, in front of the foramen magnum, where it blends with the cranial dura mater. Its anterior surface is in relation with the transverse ligament of the atlas, and its posterior surface with the dura mater.
The Alar Ligaments (ligamenta alaria; odontoid ligaments) (Fig. 307).The alar ligaments are strong, rounded cords, which arise one on either side of the upper part of the odontoid process, and, passing obliquely upward and lateralward, are inserted into the rough depressions on the medial sides of the condyles of the occipital bone. In the triangular interval between these ligaments is another fibrous cord, the apical odontoid ligament(Fig. 308), which extends from the tip of the odontoid process to the anterior margin of the foramen magnum, being intimately blended with the deep portion of the anterior atlantoöccipital membrane and superior crus of the transverse ligament of the atlas. It is regarded as a rudimentary intervertebral fibrocartilage, and in it traces of the notochord may persist. The alar ligaments limit rotation of the cranium and therefore receive the name of check ligaments.