ARTICULATIONS OF THE VERTEBRAL COLUMN WITH THE CRANIUM 295 



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 (cms 

 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 atlantooccipital 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 direc- 

 tion, 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 u, 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. 





III. 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 atlantooccipitalis). 

 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 Atlantooccipital 



The Anterior Atlantooccipital membrane. 



membrane. Two Lateral Atlantooccipital. 



The Articular Capsules (capsulce articular es; 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 Atlantooccipital Membrane (membrana atlantooccipitalis anterior; 

 anterior atlantooccipital ligament) (Fig. 304). The anterior atlantooccipital mem- 

 brane 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 



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 



