BETWEEN ATLAS AND AXIS 185 



outer part of the capsules; extension is checked by the anterior occipito-atlantal 

 and anterior oblique ligaments, and flexion by the posterior part of the capsule and 

 cervico-basilar ligaments. 



(/>) The Articulations between the Atlas and Axis 



1. The Lateral Atlaxto-axoidean Joints. I Class.— Diarthros Is ^ 



( Subdivision. — Arthroaio. 



2. The Central Atlanto-axoidean Joint, or f Class. — Diarthrosis. 



The Atlanto-odoxtoid. | Subdivision. — Trochoides. 



The bones that enter into the formation of the lateral joints are the inferior 

 articular processes of the atlas and the superior of the axis; the central joint is 

 formed by the odontoid process articulating in front Avith the atlas, and behind with 

 the transverse ligament. 



The ligaments which unite the axis and atlas are: — 



1. The anterior atlanto-axoidean. 3. Two capsular (for lateral joints). 



2. The posterior atlanto-axoidean. 4. The transverse ligament. 



5. The atlanto-odontoid capsular ligament. 



The anterior atlanto-axoidean ligament (figs. 194 and 195) is a narrow but 

 strong menil^rane filling up the interval Ijetween the lateral joints. It is attached, 

 above to the front surface and lower border of the anterior arch of the atlas, and 

 below to the transverse ridge on the front of the body of the axis. Its fibres are 

 vertical, and are thickened in the median line by a dense band which is a con- 

 tinuation upwards of the anterior common ligament of the spine. 



This band is fixed above to the anterior tubercle of the atlas, where it becomes 

 continuous with the central part of the anterior occipito-atlantal ligament (fig. 

 194); it is sometimes separated by an interval from the deeper ligament, and is often 

 described as the superficial atlanto-axoidean ligament. It is in relation with the 

 longus colli muscle. 



The posterior atlanto-axoidean ligament (fig. 195) is a deeper, but thinner 

 and looser membrane than the anterior. It extends from the posterior root of the 

 transverse process of one side to that of the other, projecting outwards beyond the 

 posterior part of the capsules which are connected with it. It is attached above to 

 the posterior surface and lower edge of the i)Osterior arch of the atlas, and below to 

 the superior edge of the laminse of the axis on their dorsal aspect. It is denser and 

 stronger in the median line, and has a layer of elastic tissue on its anterior surface 

 like the ligamenta suljflava, to which it corresponds in })Osition. It is connected in 

 front with the dura mater; behind, it is in relation M'ith the inferior oblique muscles, 

 and is perforated at each side by the second cerviccd nerve. 



1. The Lateral Atlanto-axoidean Joints are provided with short, liga- 

 mentous fibres, forming capsular ligaments (fig. 194), which completely surround 

 the lateral articular facets. Outside the canal, they are attached some little dis- 

 tance from the articular margins, extending along the roots of the transverse pro- 

 cesses of the axis nearly as far as the tips, but between the roots they skirt the 

 inner edge of the costo-vertebral foramina. They are strengthened in front and 

 behind by the atlanto-axoidean ligaments. Internally each capsule is thinner, and 

 attached close to the articular margins, being strengthened behind by a stroDS band 

 of slightly ol:)lique fibres passing upwards along the outer edge of the cervico-basilar 

 ligament from the body of the axis to the lateral mass of the atlas behind the trans- 

 verse ligament; some of these fibres pass on, thickening and blending witli the 

 occipito-atlantal capsule, to be inserted into the margin of the foramen magnum. 

 This band is sometimes called the accessory band (fig. 198). 



There is a synovial sac for each joint. 



2. The Central Atlanto-axoidean Joint, although usually described as one, 

 is composed of two articulations, which are quite separate from one another: an 

 anterior between the odontoid ]irocess and the arch of the atlas, and a j)Osteriov 

 between the odontoid })rocess and tlie transverse ligament. 



