The atlo-axoid joint is formed by the projection of the odontoid process of the axis into the ring of the atlas, where it is retained by the odontoid ligament (fig. 351). Other ligaments, the superior and inferior atlo-axoid and the capsular, also enter into the structure of the articulation.

Fig. 350. - Occipito-Atloid Articulation.

1, Occipital Bone. 2, Atlas (the upper wall removed to show the odontoid ligament). 3, Axis or Dentata. 4, Third cervical vertebra. 5, Capsular ligament. 6, Odontoid ligament. 7, Fibrous capsule (partly removed). 8, Interspinous ligament. 9, Fibrous capsule, uniting the articular processes of the vertebras.

Fig. 351. - The Occipito-Atloid and Atlo-Axoid Articulations A A, Posterior portion of skull. B B, Atlas. c c, Axis or Dentata.

I. - Vertical section through the occiput, atlas, and axis. (The occiput is drawn apart from the atlas in order to show the articular cavity of the latter bone.) 1, The occipital hone. 2, Its basilar process. 3, Occipital condyle. 4, Superior arch of the atlas. 5, Its inferior tubercle. 6, Articular cavity (shown viewed in front at 3, Fig. 2, Plate XXXVIII). 7, Spinal canal. 8, Odontoid process of the axis. ft, Its superior spinous process. 10, Its inferior spinous process. 11, Spinal canal.

II. - Vertical section through the same bones in their natural position, showing the ligaments. 12, Superior occipito-atloid ligament. 13, Inferior occipito-atloid ligament. 14, Superior atlo-axoid ligament. 15, Inferior atlo-axoid ligament. 16, Odontoid ligament. 17, Spinal canal, with the dura mater in position. (The spinal cord has been removed.)