220 



THE ARTICULATIONS 



in advance of the elevated side. In this motion, which takes place on the antero-posterior axis, 

 one condyle advances slightly and approaches the middle Une, while the other recedes. Thi^is 

 of the nature of rotation, though there is no true rotation round a vertical axis possible between 



the occiput and atlas. 



r These lateral movements are checked by the alar ligaments and the lateral part of the 

 ! capsules; extension is checked by the anterior atlanto-occipital and anterior oblique ligaments, 

 and flexion by the posterior part of the capsule and the tectorial membrane. ^*' 



^Muscles acting upon the occipito-atlantal joint. — ^Flexion whereby the chin is approxP 

 mated toward the sternum is produced by the weight of the anterior part of the head and by ' 

 all muscles which are attached to the hyoid bone or to the bones of the skull in front of a trans- 

 verse axis between the two condyles. These muscles take their fixed point below either from 

 the vertebral columUj the sternum, or the bones of the shoulder girdle. Before those connected 

 with the mandible can act that bone must be fixed by the muscles of mastication which, there- 

 fore, also take part in the movements. It must be noted that the sterno-mastoid muscles are 

 powerful flexors, although a part of their insertion is behind the transverse axis between the 

 two condyles. 



Extension is due to the action of muscles or portions of muscles inserted into the skull 

 behind the transverse axis above mentioned, and connected below either with the vertebral 

 column, shoulder girdle, or sternum. 



Lateral movement is produced by the anterior and posterior groups of muscles on the same 

 ide acting simultaneously and aided by the rectus capitis lateralis of that side. 



Fig. 260. — Median Sagittal, Section of Vertebral Column showing Ligaments. 



Ascending 

 portion of 

 Left alar crucial 



Transverse ligament 



Inner part of capsular ligament of 

 atlanto-occipital joint 



Posterior atlanto-occipital 

 ligament 



Descending portion of crucial 



lieament 

 Posterior atlanto-epistropbic 



ligament 



ligament 



Interspinous ligament 



Ligamentum flaTum 



Apical dental ligament 



Anterior atlanto-occipital 

 ligament 



Atlanto-dental synovial 

 sac 



Anterior atlanto-epistrophic 

 ligament 



(6) The Articulations between the Atlas and Epistropheus (Axis). 



1. The Lateral Atlanto-epistrophic Joints. 



2. The Central Atlanto-epistrophic Joint or 



The Atlanto-dental. 



Class. — Diarthrosis. 

 Subdivision. — Arthrodia. 

 Class. — Diarthrosis. 

 \ 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 epistropheus (axis); the 

 central joint is formed hy the dens (odontoid jirocess) articulating in front with 

 the atlas, and behind with the transverse ligament. 



