TEMPORAL AND INFRATEMPORAL REGIONS 177 



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

 that of the lower compartment. The greater extent of the 

 synovial stratum of the upper cavity of the joint is associated 

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

 (3) protraction ; (4) retraction ; (5) side to side or chewing movements. 

 When the mandible is depressed the articular disc and the condyle 

 move forwards in the mandibular fossa, and the condyle finally takes 

 up a position on the tuberculum articulare. The 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, 

 which consists in a rotation of the condyle of the mandible on the 

 lower surface of the articular disc. Elevation of the mandible or closure 

 of the mouth is brought about by a reverse series of changes in both 

 compartments of the joint. There is some doubt about the position of 

 the transverse axis around which the movements of elevation and depres- 

 sion take place, but it is generally supposed to be situated at the level of 

 the mandibular foramina. Those are the points, therefore, of least move- 

 ment, and consequently in opening and shutting the mouth the inferior 

 alveolar vessels and nerves are not unduly stretched. In protraction and 

 retraction the movement is confined chiefly to the upper compartment 

 of the joint, and the condyle of the mandible, with the articular disc, 

 glides forwards and backwards upon the temporal articular surface. In 

 the side to side movements of the jaw the mandible is carried alternately 

 from one to the other side, as in the process of chewing. 



The muscles on each side which are chiefly engaged in producing these 

 movements are the following: (i) depressors the platysma, the mylo- 

 hyoid, and the anterior belly of the digastric ; (2) elevators the masseter, 

 internal pterygoid, temporal ; (3) protractors the external pterygoid, and 

 to some extent the internal pterygoid and the superficial fibres of the 

 masseter ; (4) retractor the posterior fibres of the temporal and the deep 

 fibres of masseter ; (5) side to side movement is produced by the muscles 

 of opposite sides acting alternately. 



Dissection. The condyle of the mandible must now be 

 disarticulated and thrown forwards with the attached external 

 pterygoid muscle. It is well to detach the articular disc with 

 the head of the bone, in order that it may be more thoroughly 

 examined. Care must be taken not to injure the auriculo- 

 temporal nerve, which lies in close proximity to the medial 

 aspect of the joint. When the disarticulation is complete, draw 

 the muscle forwards by gently pushing the condyle under the 

 internal maxillary artery. 



When the external pterygoid has been reflected and the 

 areolar tissue medial to it has been cleaned away the following 

 structures will be exposed. The middle meningeal artery ; the 

 mandibular division of the Irigeminal nerve and its branches ; 

 the chorda tympani branch of the facial nerve, and, in a well- 

 injected subject, the tympanic and accessory meningeal branches 

 of the internal maxillary artery may be seen. Follow the 

 middle meningeal artery upwards. Just before it enters the 

 foramen spinosum it passes between the two heads of origin by 

 VOL. Ill 12 



