362 HEAD AND NECK 



is necessary therefore to remove, with the bone forceps, the laminae of the 

 epistropheus and the posterior arch of the atlas. The squamous part of 

 the occipital bone also must be taken away by sawing it through, on 

 each side, immediately posterior to the jugular process and the condyle, 

 carrying the saw cut into the foramen magnum. The upper part of the 

 tube of dura mater, which still remains in the vertebral canal, must 

 next be carefully detached. A broad membranous band stretching upwards 

 over the posterior aspect of the body and dens of the epistropheus 

 is displayed. This is the membrana tectoria. 



The Membrana Tectoria (O.T. Posterior Occipito- axial 

 Ligament). This is a broad ligamentous sheet which is 

 attached below to the posterior aspect of the body of 

 the epistropheus, where it is continuous with the posterior 

 longitudinal ligament of the vertebrae. It extends upwards, 

 covering the dens and the anterior margin of the foramen 

 magnum, and is attached above to the superior grooved 

 surface of the basilar portion of the occipital bone. 



Dissection. Detach this membrane from the epistropheus and throw 

 it upwards upon the basilar portion of the occipital bone. By this pro- 

 ceeding the accessory ligaments of the atlanto-epistropheal joints and the 

 cruciate ligament are brought into view, and very little further dissection 

 is required to define them. 



Accessory Atlanto-epistropheal Ligaments (Fig. 145). 

 These are two strong bands which take origin from the 

 posterior aspect of the body of the epistropheus close to the 

 base of the dens. Each band passes upwards and laterally, 

 and is attached to the medial and posterior part of the 

 lateral mass of the atlas. To a certain extent they assist the 

 alar ligaments in limiting the rotary movements of the atlas 

 upon the epistropheus. 



Ligamentum Cruciatum (Fig. 145). The cruciate ligament 

 is composed of a transverse and a vertical part. The liga- 

 mentum transversum atlantis is by far the most important con- 

 stituent of this apparatus. It is a strong band which stretches 

 from the tubercle on the medial aspect of the lateral mass of 

 the atlas on one side to the corresponding tubercle on the 

 opposite side. With the anterior arch of the atlas it forms 

 a ring which encloses the dens the pivot around which the 

 atlas bearing the head turns. It is separated from the 

 posterior aspect of the dens by a loose synovial membrane 

 which extends anteriorly on each side until it almost reaches 

 the synovial membrane in connection with the median joint 

 between the dens and the anterior arch of the atlas. Indeed, 



