228 



THE ARTICULATIONS. 



each side to a small tubercle on the inner surface of the lateral mass of the atlas. 

 As it crosses the odontoid process, a small fasciculus is derived from its upper, and 

 another from its lower, border ; the former passing upward, to be inserted into the 

 basilar process of the occipital bone ; the latter, downward, to be attached to the 

 posterior surface of the body of the axis ; hence, the whole ligament has received 

 the name of cruciform. 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 cord and its membranes and the spinal accessory nerves ; 

 the anterior and smaller contains the odontoid process. Since the space between 

 the anterior arch of the atlas and the transverse ligament is smaller at the lower 



CAPSULAR LIGA- 

 MENT and 

 synovial mem- 

 brane. 



SULAR LIGAMENT 



i.nd synovial 

 membrane. 



FIG. 153. Occipito-atlantal and atlanto-axial ligaments. Front view. 



part than the upper (because the transverse ligament embraces firmly 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 are two thin and loose capsules, connecting the lateral 

 masses of the atlas with the superior articular surfaces of the axis, the fibres being 

 strengthened at the posterior and inner part of the articulation by an accessory 

 ligament, which is attached below to the body of the axis near the base of the 

 odontoid process. 



There are four 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 atlanto-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. 



Actions. This joint allows the rotation of the atlas (and, with it, of the cra- 

 nium) upon the axis, the extent of rotation being limited by the odontoid liga- 

 ments. 



The principal muscles by which this action is produced are the Sterno-mastoid 

 and Complexus of one side, acting with the Rectus capitis anticus major, Splenius, 

 Trachelo-mastoid, Rectus capitis posticus major, and Inferior oblique of the other 

 side. 



