274 HEAD AND NECK 



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. Whilst these movements are going on, the 

 mandible rotates around a transverse axis which traverses the bone in 

 the neighbourhood of the mandibular foramen. This is the point, there- 

 fore, of least movement, 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 chiefly confined to the 

 upper compartment of the joint, and the condyle of the mandible with 

 the articular disc glides anteriorly and posteriorly upon the temporal 

 articular surface. In the lateral movements of the jaw the mandible is 

 carried alternately from side to 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) lateral movement is produced by the muscles of 

 opposite sides acting alternately. 



Reflection of External Pterygoid. The condyle of the mandible must 

 now be disarticulated and thrown anteriorly with the attached external 

 pterygoid muscle. It is well to detach the discus articularis 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 dis- 

 articulation is complete, the muscle may be displaced anteriorly by gently 

 pushing the condyle under the internal maxillary artery. 



The reflection of the external pterygoid muscle brings into view, after 

 a little dissection, the mandibular division of the trigeminal nerve, 

 emerging from the foramen ovale, and breaking up into its branches of 

 distribution. The slender chorda tympani will be found proceeding down- 

 wards and anteriorly to join the lingual nerve ; and the middle meningeal, 

 tympanic, and accessory meningeal arteries may be traced to the points 

 where they leave the infratemporal region. 



Arterise Meningea Media et Tympanica and Eamus Menin- 

 geus Accessorius. The middle meningeal artery has already 

 been seen arising from the first part of the internal maxillary 

 artery. It proceeds upwards, medial to the external ptery- 

 goid muscle and lateral to the tensor palati, and disappears 

 from view through the foramen spinosum, by which it enters 

 the cranial cavity (p. 212). It is usually embraced by the 

 two roots of the auriculo-temporal nerve. 



The accessory meningeal artery and the tympanic artery generally arise 

 from the middle meningeal. The accessory meningeal inclines anteriorly and 

 upwards, and enters the cranial cavity by passing through the foramen 

 ovale ; the tympanic runs upwards and posteriorly, and reaches the 

 tympanum by passing through the petro-tympanic fissure (O.T. Glaserian). 

 In the tympanic cavity it anastomoses with the styloid-mastoid branch of 

 the posterior auricular artery. 



