296 SYNDESMOLOGY 



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 Atlantobccipital Membrane (membrana atlantooccipitalis posterior; 

 posterior atlantooccipital ligament} (Fig. 305) . The posterior atlantooccipital mem- 

 brane, 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 r , 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 (6) 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 su- 

 perior, 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. Two Alar. The Apical Odontoid. 



The Membrana Tectoria (occipitoaxial ligament) (Figs. 307, 308). The mem- 

 brana 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 prolon- 

 gation upw r ard 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 w T ith the cranial dura mater. Its anterior sur- 

 face is in relation with the transverse ligament of the atlas, and its posterior 

 surface with the dura mater. 



The Alar Ligaments (ligamenta alarm; 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 occipi- 

 tal 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 odon- 

 toid process to the anterior margin of the foramen magnum, being intimately 

 blended with the deep portion of the anterior atlantooccipital 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. 



In addition to the ligaments which unite the atlas and axis to the skull, 

 the ligamentum nuchae (page 290) must be regarded as one of the ligaments 

 connecting the vertebral column with the cranium. 



