222 



THE ARTICULATIONS 



an anterior between the dens and the arch of the atlas, and a posterior between 

 the dens and the transverse hgament. 



The transverse ligament (figs. 260, 261, and 263) is one of the most important 

 structures in the body, for on its integrity and that of the alar ligaments our 

 lives largel}^ depend. It is a thick and verj^ strong band, as dense and closely 

 woven as fibro-cartilage, about a quarter of an inch (6 mm.) deep at the sides, 

 and somewhat more in the middle line. Attached at each end to a tubercle on 

 the inner side of the lateral mass of the atlas, it crosses the ring of this bone in a 

 curved manner, so as to have the concavity forward; thus dividing the ring into 

 a smaller anterior portion for the dens and a larger posterior part for the spinal 

 cord and its membranes, and the spinal accessory nerves. 



It is flattened from before backward, being smooth in front, and covered by synovial mem- 

 brane to allow it to glide freely over the posterior facet of the dens. Where it is attached to the 

 atlas it is smooth and well rounded off to provide an easy floor of communication between the 

 transverso-dental and occipito-atlantal joints. 



To its posterior surface is added, in the middle line, a strong fasciculus of 

 vertical fibres, passing upward from the root of the dens to the basilar border of 

 the foramen magnum on its cranial aspect. Some of these fibres are derived 

 from the transverse ligament. These vertical fibres give the transverse liga- 

 ment a cruciform appearance; hence the name, the crucial ligament (figs. 260 

 and 263) applied to the whole. 



The atlanto -dental articular capsule (fig. 261) is a tough, loose membrane, 

 completely surrounding the apposed articular surfaces of the atlas and dens. 



At the dens it blends above with the front of the alar and central occipito-odontoid liga- 

 ments, and arises also along the sides of the articular facet as far as the neck of the dens; 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 liga- 

 ment, blending, above and below the borders of the bone, with the anterior atlanto-occipital 

 and atlanto-epistrophic ligaments, as well as with the medial portion of the articular capsules. 

 It holds the dens to the anterior arch of the atlas after all the other ligaments have been divided. 



The synovial membranes (figs. 260 and 261) are two in number: — one for the 

 joint between the dens and atlas; and another (transverso-dental) for that 

 between the transverse ligament and the dens. This last often communicates 

 with the atlanto-occipital articulations; it is closed in by membranous tissue 

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

 the dens, and is separated from the front sac by the atlanto-dental articular 

 capsule. 



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

 the first and second cervical nerves. 



j 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 transverse ligament, round the 

 /dens as a pivot, which is extensive enough to allow of an all-round view without twisting the 

 /trunk. Partly on account of its ligamentous attachments, and partly on account of the shape 

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

 I rotation is checked by the ligaments passing from the dens to the occiput (alar ligaments), and 

 j also by the atlanto-epistrophic. Owing to the fact that the facets of both atlas and epistropheus, , 

 j which enter into the formation of the lateral atlanto-epistrophic articulations, are convex from i 

 J before backward, and have the articular cartilage thicker in the centre than at the circumfer- 

 / ence, the motion is not quite horizontal but slightly curvilinear. In the erect position, with the 

 ' face looking directly forward, the most convex portions of the articular 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 epistropheus, dimin- 

 ishing the space Ijetweon the l)ones, slackening the ligaments, and thus increasing the amount of 

 rotation, without sacrificing the sccui'ity of the joint in the central position. 



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

 between the atlas and epistropheus, even to a greater extent than in most of the other vertebral 

 I joints. 



The muscles acting upon the atlanto-epistrophic joints. — The muscles capable of producing 

 rotation at the atlanto-(!pi.str<)i)lii(; joints an; t liose \vhi(;h take origin from near the mesial plane 

 either in front or licliind and which arc atta(;hed al)ove either to the atlas or the skull, lateral 

 to the atlanto-epistrophic joints. When the muscles which lie at the back of the joint on one 

 side act thoy will turn the head tf) the same side and will be aided by the muscles in front on the 

 opposite side. If tht; muscles in front aiul behind on the same side act simultaneously, they will 

 pull down the head to that side and will be aided by muscles which pass more or less vertically 

 from the transverse process of the atlas to points below. 



