TEMPORAL AND INFRATEMPORAL REGIONS 273 



little is added to the strength of the joint by their presence. 

 The security of the joint depends not so much upon its liga- 

 ments as upon the strong muscles of mastication, which keep 

 the head of the mandible in its place. 



The discus articularis is an oval plate of fibro-cartilage, with 

 its long axis directed transversely. It is interposed between 

 the condyle of the mandible below and the mandibular fossa 

 (O.T. glenoid) and the articular tubercle (O.T. eminentia 

 articularis) above, and it divides the joint cavity into upper 

 and lower parts, each of which is provided with a separate 

 synovial lining. To expose the cartilage, the temporo-man- 

 dibular ligament must be removed. The disc will then be 

 seen to be adapted to the 

 two bony surfaces between 

 which it lies. Above, it is 

 concavo - convex in corre- 

 spondence with the tuber- 

 culum articulare and the 

 mandibular fossa of the tem- 

 poral bone ; whilst below, it 

 is concave, and fits upon 

 the upper aspect of the con- 

 dyle of the mandible. In 

 the centre it is thin, and in 

 some cases it is perforated. FlG II2 ._ Diagram of ' the different 



Its circumference is thick^ positions occupied by the head of the 

 more especially posteriorly. mandible and the discus articularis 

 _ , , , i , j i ,_i as the mouth is opened and closed. 



It should be noted also that 



the external pterygoid muscle is partly inserted into its 



anterior border. 



The synovial stratum which lines the capsule enclosing the 

 upper cavity of the joint is of greater extent and looser than 

 that of the lower compartment. This is in association with 

 the larger size of the articular surface of the temporal bone 

 as contrasted with the condylar surface. 



Movements. The movements which the mandible can perform at 

 the temporo- mandibular joint are the following: (i) depression; (2) 

 elevation ; (3) protraction ; (4) retraction ; (5) lateral or chewing move- 

 ments. When the mandible is depressed the discus articularis and the 

 condyle move anteriorly on the mandibular fossa, and the condyle finally 

 takes up a position on the tuberculum articulare. This forward gliding 

 of the disc and condyle in the upper compartment of the joint is accom- 

 panied by another movement in the lower compartment of the joint, 

 VOL. II 18 



