AKTICULATION OF ATLAS WITH AXIS. 



309 



lig. sacrococcygeum laterale supports the joint on each side, whilst strong bands pass 

 between the cornua of the two bones and constitute the interarticular ligaments. 



Intercoccygeal Joints. So long as they remain separate, the different pieces 

 of the coccyx are joined by intervertebral fibre-cartilages and by anterior and 

 posterior ligaments. 



Movements of the Vertebral Column. Although the amount of movement permissible 

 between any two vertebrae is extremely limited, yet the total range of movement capable of 

 being attained by the entire vertebral column is very considerable. 



Flexion may occur both forwards and backwards at the articulations of vertebral bodies, but 

 more freely in the lumbar and cervical regions than in the thoracic region, where the limited 

 amount of intervertebral fibre-cartilage and the imbrication of the laminae and spines restrict 

 the movement. Backward flexion is most pronounced in the cervical region, and forward flexion 

 in the lumbar region. Between the articular surfaces of the articulations between vertebral 

 arches a variety of movements are permitted, dependent upon the directions of these surfaces. 

 Thus lateral flexion is permitted in the lumbar, but not in the cervical or dorsal regions. 

 Again, in the lumbar region rotation does not occur, owing to the shape of the articular 

 processes, while it is possible in the thoracic region. In the cervical region the shape and position 

 of the articular surfaces prevent the occurrence both of lateral flexion and of rotation as isolated 

 movements, but a combination of these two movements may take place, whereby rotatory move- 

 ment in an oblique median axis results. Finally, in the lumbar region, by combining the four 

 forms of flexion, viz., forward, backward, and lateral, a certain amount of circumduction is possible. 



ARTICULATIO ATLANTOEPISTROPHICA. 



Between the atlas and epistropheus vertebrae three diarthroses occur. Two of 

 them are situated laterally, in relation to the articular processes, and are called 



Membrana tectoria 



Ba.silar part of occipital bone 



srior atlanto-occipltal ligament 

 Ligamentum apicis dentis 



Synovial cavity 



Dens 

 Anterior arch of atlas 



Transverse ligament of atlas 



iferior crus of cruciate ligament 

 Rudimentary intervertebral 

 tibro-cartilage 



Body of epistropheus 



Superior crus of cruciate 

 ligament of the atlas 



Synovial cavity 



Posterior atlan to-occipital 

 membranes 



Occipital bone 



Posterior longitudinal ligament 

 Posterior arch of atlas 



Root of spine of epistropheus 



IG. 296. MEDIAN SECTION THROUGH THE ATLANTO-OCCIPITAL AND ATLANTO-KPISTROPHEAL JOINTS. 



rodial diarthroses, because of the flattened nature of the articulating surfaces. 

 I The third articulation is median in position. It is found between the smooth 

 anterior surface of the dens of the epistropheus and the articular facet on the 

 posterior aspect of the anterior arch of the atlas. This joint is a rotatory diarthrosis. 



Ligamenta. Each of the joints is furnished with a capsula articularis, whereby 

 the articular cavity is circumscribed. In the case of the lateral articulations, each 

 articular capsule presents a distinct band, named the accessory ligament, which is 

 situated within the vertebral canal (Fig. 297),and passes downwards and medially from 

 the lateral mass of the atlas to the superior aspect of the body of the epistropheus. 



The following additional ligaments constitute the leading bonds of union : - 



Lig. Obturatorium Atlantoepistrophica Anterior. The anterior covering 

 atlanto-epistropheal ligament (O.T. anterior atlo-axoid ligament) (Fig. 296) is a mem- 

 branous structure which is thin laterally, but strong in the median plane, where it is 

 thickened by a prolongation of the lig. longitudinale anterius. It extends from 

 the anterior arch of the atlas to the front of the body of the epistropheus. 



