OF THE ATLAS WITH THE AXIS. 269 



while the posterior common ligament, the ligamenta subflava, and the inter- 

 aiid supra-spinous ligaments, are stretched, as well as the posterior fibres of the 

 iutervertebral disk^. The interspaces between the laminae are widened, and 

 the inferior articular processes of the vertebra} above glide upwards, upon the 

 articular processes of the vertebra} below. Flexion is the most extensive of all 

 the movements of the spine. 



In Extension, or movement of the spine backwards, an exactly opposite dis- 

 position of the parts takes place. This movement is not extensive, being 

 limited by the anterior common ligament, and by the approximation of the 

 spinous processes. 



Flexion and extension are most free in the lower part of the lumbar, and in 

 the cervical regions; extension in the latter region being greater than flexion, 

 the reverse of which is the case in the lumbar region. These movements are 

 least free in the middle and upper part of the back. 



In Lateral Movement, the sides of the intervertebral disks are compressed, the 

 extent of motion being limited by the resistance offered by the surrounding liga- 

 ments, and by the approximation of the transverse processes. This movement 

 may take place in any part of the spine, but is most free in the neck and loins. 



Circumduction is very limited, and is produced merely by a succession of the 

 preceding movements. 



Eotation is produced by the twisting of the intervertebral substances ; this, 

 although only slight between any two vertebra}, produces a great extent of 

 movement, when it takes place in the whole length of the spine, the front of the 

 column being turned to one or the other side. This movement takes place 

 only to a slight extent in the neck, but is more free in the lower part of the 

 dorsal and lumbar regions. 



It is thus seen, that the cervical region enjoys the greatest extent of each 

 \ variety of movement, flexion and extension especially being very free. In the 

 dorsal region, especially at its upper part, the movements are most limited ; 

 < flexion, extension, and lateral motion taking place only to a slight extent. 



II. AKTICULATION OF THE ATLAS WITH THE AXIS. 



The articulation of the anterior arch of the atlas with the odontoid process 

 forms a lateral ginglymoid joint, whilst that between the articulating processes 

 of the two bones forms a double arthrodia. The ligaments which connect these 

 bones are, the 



Two Anterior Atlo-axoid. Transverse. 



Posterior Atlo-axoid. Two Capsular. 



Of the two Anterior Atlo-axoid Ligaments (Fig. 184), the more superficial is 

 a rounded cord, situated in the middle line; it is attached, above, to the 

 tubercle on the anterior arch of the atlas ; below, to the base of the odontoid 

 process and body of the axis. The deeper ligament is a membranous layer, 

 attached, above, to the lower border ^pf the anterior arch of the atlas ; below, 

 to the base of the odontoid process, and body of the axis. These ligaments 

 are in relation, in front, with the Eecti Antici Majores. 



The Posterior Atlo-axoid Ligament (Fig. 185) is a broad and thin membranous 



layer, attached, above, to the lower border of the posterior arch of the atlas ; 



below, to the upper edge of the lamina} of the axis. This ligament supplies 



, the place of the ligamenta subflava, and is, in relation, behind, with the Inferior 



i Oblique muscles. 



The Transverse Ligament 1 (Figs. 186, 187) is a thick and strong ligamentous 

 band, which arches across the ring of the atlas, and serves to retain the odon- 



1 It has been found necessary to describe the transverse ligament with those of the atlas and 

 axis ; but the student must remember that it is really a portion of the mechanism by which the 

 movements of the head on the spine are regulated ; so that the connections between the atlas 

 and axis ought always to be studied together with those between the latter bones and the skull. 



