BETWEEN ATLAS AND AXIS 



18 < 



chock and central occipito-odontoid ligaments, and arises also along the sides of the 

 articular facet as far as the neck of the process; the fibres are thick, and blend with 

 the capsules of the lateral joint. At the atlas they are attached to the non-articular 

 part of the anterior arch in front of the tubercles for the transverse ligament, blend- 

 ing, aliove and l^elow the borders of the bone, with the anterior occipito-atlantal 

 and atlanto-axoidean ligaments, as well as with the inner portion of the capsular 

 ligaments. It holds the axis to the anterior arch of the atlas after all the other 

 ligaments have been divided. 



The synovial membranes (figs. 195 and 196) are two in number: — one for the 

 joint between the odontoid process and atlas; and another (transverso-odontoid) 

 for that between the transverse ligament and the odontoid. This last often com- 

 municates with the occipito-atlantal articulations; it is closed in by membranous 

 tissue between the borders of the transverse ligament and the margin of the facet on 

 the odontoid, and is separated from the front sac by the atlanto-odontoid capsular 

 ligament. 



The arterial supply is from the vertebral artery, and the nerve-supply from 

 the loop between the first and second cervical nerves. 



Movements. — The chief and characteristic movement at these joints is the 

 rotation, in a nearly horizontal plane, of the collar formed by the atlas and trans- 



FiG. 196. — HoRizoxTAL Sectiox through the Lateral Masses of the Atlas axd the 



Top of the Odoxtoid Process. 



Atlanto-odontoid 

 synovial sac 



Transverso-odon- 

 toid synovial sae 



Atlanto-odontoid 

 capsular ligament 



Transverse 

 ligament 



Posterior common 

 and cervieo-basi- 

 lar ligaments 



Dura mater 



verse ligament, round the odontoid process as a pivot, which is extensive enough 

 to allow of an all-round vicAV without twisting the trunk. Partly on account of 

 its ligamentous attachments, and partly on account of the shape of the articular 

 surfaces, the cranium must be carried with the atlas in these movements. The 

 rotation is checked by the ligaments passing from the axis to the occiput (check 

 ligaments), and also by the atlanto-axoidean. Owing to the fact that the facets 

 of both atlas and axis, which enter into the formation of the lateral atlanto- 

 axoidean articulations, are convex from before backwards, and have the articular 

 cartilage thicker in the centre than at the circumference, the motion is not quite 

 horizontal but slightly curvilinear. In the erect position, with the face looking 

 directly forwards, the most convex portions of the ai*ticular surfaces are alone in 

 contact, there being a considerable interval between the edges; during rotation, 

 therefore, the prominent portions of the condyles of the atlas descend upon those 

 of the axis, diminishing the space l^etween the bones, slackening the ligaments, 

 and thus increasing the amount of rotation, without sacrificing the security of the 

 joint in the central position. 



Besides rotation, forward and backward movements and some lateral flexion are 

 permitted between the atlas and axis, even to a greater extent than in most of the 

 other verteViral joints. 



