ARTICULATION OF THE ATLAS WITH THE OCCIPITAL BONE 275 



process and the anterior arch of the atlas. The transverse ligament divides the 

 vertebral foramen of the atlas into two unequal compartments; of these, the pos- 

 terior and larger serves for the transmission of the cord and its membranes and 

 the spinal accessory nerves; the anterior and smaller contains the odontoid pro- 

 cess. Since the space between the anterior arch of the atlas and the transverse 

 ligament is smaller at the lower part than the upper (because the transverse 

 ligament embraces tightly the narrow neck of the odontoid process), this process 

 is retained in firm connection with the atlas after all the other ligaments have been 

 divided. 



The capsular ligaments (capsulae articulares) (Figs. 225 and 227) are two thin 

 and loose capsules, connecting the lateral masses of the atlas w r ith the superior 

 articular surfaces of the axis, the fibres being strengthened at the posterior and 

 inner part of each articulation by an accessory ligament, which is attached below 

 to the body of the axis near the base of the odontoid process. 



Synovial Membranes (Fig. 226). There are/owr synovial membranes in this articulation 

 one lining the inner surface of each of the capsular ligaments; one between the anterior surface 

 of the odontoid process and the anterior arch of the atlas, the atlan to-odontoid joint; and 

 one between the posterior surface of the odontoid process and the transverse ligament, the 

 syndesmo-odontoid joint. The latter often communicates with those between the condyles of 

 the occipital bone and the articular surfaces of the atlas. 



Movements. This joint allows the rotation of the atlas (and, with it, of the cranium) upon 

 the axis, the extent of rotation being limited by the odontoid ligaments. 



The principal muscles by which this action is produced are the Sternomastoid and Corn- 

 plexus of one side, acting with the Rectus capitis anticus major, Splenius, Trachelomastoid, 

 Rectus capitis posticus major, and Inferior oblique of the other side. 



ARTICULATIONS OF THE VERTEBRAL COLUMN WITH THE CRANIUM. 



The ligaments connecting the vertebral column with the cranium may be divided 

 into two sets those connecting the occipital bone with the atlas, and those con- 

 necting the occipital bone with the axis. 



III. Articulation of the Atlas with the Occipital Bone (Articulatio 



Atlanto-occipitalis) . 



This articulation is a double condyloid joint. Its ligaments are the 



Anterior Occipito-atlantal. Posterior Occipito-atlantal. 



Two Capsular. 



The anterior occipito-atlantal ligament (membrana atlantooccipitalis anterior) 

 (Figs. 224 and 228) is a broad membranous layer, composed of densely woven 

 fibres, which passes between the anterior margin of the foramen magnum above, 

 and the whole length of the upper border of the anterior arch of the atlas below. 

 Laterally, it is continuous with the capsular ligaments. In the middle line in 

 front it is strengthened by a strong, narro\v, rounded cord, which is attached, 

 above, to the basilar process of the occiput, and, below, to the tubercle on the 

 anterior arch of the atlas, and which is a continuation of the anterior common 

 ligament. This ligament is in relation, in front, with the Recti antici minores; 

 behind, with the odontoid ligaments. 



The posterior occipito-atlantal ligament (membrana atlantooccipitalis posterior, 

 posterior occipito-atloid ligament) (Figs. 225 and 228) is a very broad but thin 

 membranous lamina intimately blended with the dura. It is connected, above, 

 to the posterior margin of the foramen magnum; below, to the upper border 

 of the posterior arch of the atlas. This ligament is incomplete at each side, and 



