JOINTS OF THE NECK 275 



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). The tectorial membrane 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 the tectorial membrane from the epi- 

 stropheus and throw it upwards upon the basilar portion of the 

 occipital bone. By that proceeding the accessory ligaments of 

 the atlanto-epistropheal joints and the cruciate ligament will be 

 brought into view, and very little further dissection is required 

 to define them. 



Accessory Atlanto-epistropheal Ligaments (Fig. 104). 

 The accessory atlanto-epistropheal ligaments 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 corresponding 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. 104). The cruciate ligament 

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

 verse part is by far the most important constituent. 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 forwards, 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, in some cases a com- 

 munication exists between the two synovial cavities. 



The vertical part of the cruciate ligament consists of an 

 upper and a lower limb, whicti are termed the crura. Both 



