310 



THE ARTICULATIONS OE JOINTS. 



Lig. Obturatorium Atlantoepistrophica Posterior. The posterior covering 

 atlanto-epistropheal ligament (O.T. posterior atlo-axoid ligament) (Fig. 296) occupies 

 the position which is elsewhere taken by the ligamenta flava. It extends from 

 the posterior arch of the atlas to the upper border of the vertebral arch of the 

 epistropheus. 



Lig. Trans versum Atlantis. The transverse ligament of the atlas (Figs. 296 

 and 297) is a strong band, placed transversely, which arches backwards behind 

 the neck of the dens of the epistropheus. By its extremities it is attached to 

 the tubercle on the medial aspect of each lateral mass of the atlas. A thin plate 

 of fibro-cartilage is developed in its central part. 



A stratum synoviale (synovial membrane) lines each of the three articular capsules, 

 and in addition a synovial sac is developed between the dens and the lig. transversum 

 atlantis. This is more extensive than the synovial cavity between the dens and the 

 atlas. 



ARTICULATIO ATLANTO-OCCIPITALIS. 



There are two articulations between the atlas and the occipital bone. Each 

 is a diarthrosis in which movement takes place in relation to two axes, viz., the 



Membrana tectoria 



Crus superius 



Occipital bone 



Lateral mass of atlas 



Atlanto-epistropheal joint 



Body of epistropheus 



-Ligamentum apicis dentis 



Ligamentum alare 



Crus superius 



Ligamentum crucia- 

 tum atlantis 



Accessory atlanto- 

 epistropheal ligament 



Crus inferius 



Membrana tectoria 



FIG. 297. DISSECTION FROM BEHIND OF THE LIGAMENTS CONNECTING THE OCCIPITAL BONE, THE ATLAS, 

 AND THE EPISTROPHEUS WITH EACH OTHER. 



transverse and the antero-posterior. The condyle of the occipital bone is bi- 

 convex, and fits into the bi-concave superior articular surface of the atlas, while the 

 long axes of the two joints are directed horizontally forwards and medially. 



Ligamenta. Each articulation is provided with a capsula articularis which 

 thin but complete. It is attached to the rough non-articular surfaces surrounding 

 the articular areas on the atlas and occipital bone. 



The following supplementary ligaments are the chief structures which bind the 

 atlas to the occipital bone : 



The membrana atlanto-occipitalis anterior (anterior occipito-atloid membrane' 

 (Fig. 296) is a strong although thin membrane, attached inferiorly to the anterioi 

 arch of the atlas, and superiorly to the anterior half of the circumference of tl 

 foramen magnum. Laterally it is in continuity with the articular capsules, whi] 

 in the median plane, where it extends from the anterior tubercle of the atlas to tl 

 basilar parfc of the occipital bone, it presents a specially well-defined thickened bai 

 which might be regarded as a separate accessory ligament or as the beginning 

 of the anterior longitudinal ligament of the vertebrae. 



The membrana atlanto-occipitalis posterior (posterior occipito-atloid membn 

 (Fig. 296) is another distinct but still thin membrane which is attached superiorly 

 to the posterior half of the circumference of the foramen magnum, and inferiorly t( 

 the upper border of the posterior arch of the atlas. Laterally it also is continuou: 

 with the articular capsules. On each side of the median plane its inferior borde: 



