LIGAMENTS UXITIXG OCCIPUT AXD AXIS 



189 



a slender l)an(l oi tiljres ascending from the siininiit of tlie odontoid process to the 

 under surface of the occipital bone, close to the foramen magnum. It is best seen 

 from the front, after remoWng the anterior occipito-atlantal ligament, or from 

 behind l\v drawing aside the crucial ligament. 



The suspensory ligament is tightened by extension and relaxed by flexion or 

 nodding; the lateral odontoids not only limit the rotatory movements of the head 

 and atlas upon the axis, but by binding the occiput to the pivot, round which 

 rotation occurs, they steady the head and prevent its undue lateral inclination upon 

 the spine. (See Transverse Ligament, p. 186.) 



By experiments, it has been proved that the head, when placed so that tlie 

 orbits look a little upwards, is poised upon the occipital condyles in a line drawn a 

 little in front of their middle; the amount of elevation varies slightly in different 

 cases,' Init the balance is always to be obtained in the human body — it is one of the 

 characteristics of the human figure. It sers^es to maintain the head erect without 

 undue muscular effort, or a strong ligamentum nucha? and prominent dorsal spines 



Fig. 198. — Vertical Tkaxsveese Section of the Spixal Columx and the Occipital 



BoxE TO SHOW Ligaments. 



(Tlie oervico-basilar (1), though shown as a distinct stratum, is really the deeper part of 



the posterior common ligament (2).) 



Vertical portion of crucial ^. 



ligament 

 Central odontoid ligament 



Lateral odontoid ligaments 



Transverse portion of 

 crucial ligament 

 Accessory band of atlanto 

 azoidean capsules 

 Atlanto-axoidean joint 



Oecipito-cervieal or eervieo-basilar 

 ligament 



Posterior common ligament 



such as are seen in the lower animals. Disturb this balance, and let the muscles 

 cease to act, the head will either drop forwards or backwards according as the 

 centre of the gravity is in front or behind the balance line. The ligaments Avhich 

 pass over the odontoid process to the occiput are not quite tight when the head is 

 erect, and only become so when the head is flexed; if this were not so, no flexion 

 would be allowed; thus, muscular action, and not ligamentous tension, is employed 

 to steady the head in the erect position. It is through the combination of the joints 

 of the atlas and axis, and occiput and atlas (consisting of two pairs of joints placed 

 svmmetrically on either side of the median line, while through the median line 

 there passes a pivot, also with a pair of joints) that the head enjoys such freedom 

 and celerity of action, remarkable strength, and almost absolute security against 

 violence, which could only be obtained by a ball-and-socket joint; but the ordinary 

 ball-and-socket joints are too prone to dislocations l)y even moderate twists to Ijc 

 reliable enough when the life of the individual depends on the perfection of the 

 articulation : hence the importance of this combination of joints. 



