THE MANDIBULAR JOINT 



217 



from the articular disc above to the condyle of the jaw below, extending somewhat 

 further down behind than in front. When the disc is perforated, the two sacs 

 communicate. 



The spheno-mandibular ligament (long internal lateral) (fig. 257) is a thin, 

 loose band, situated some little distance from the joint. It is attached above to 

 the spine of the sphenoid and contiguous part of the temporal bone, and is inserted 

 into the lingula of the lower jaw. 



Ifc covers the upper end of the mylo-hyoid groove, and is here pierced by the mylo-hyoid 

 nerve. Its origin is a httle medial to, and immediately behind, the origin of the medial por- 

 tion of the capsule. It is separated from the joint and ramus of the jaw by the external ptery- 

 goid muscle, the internal maxillary artery and vein, the inferior alveolar {dental) nerve and 

 artery, the auriculo-temporal nerve, and the middle meningeal artery. It is really the fibrous 

 remnant of a part of the mandibular (Meckelian) bar. 



The stylo -mandibular ligament (stylo-maxillary) (figs. 256 and 257) is a 

 process of the deep cervical fascia extending from near the tip of the styloid proc- 

 ess to the angle and posterior border of the ramus of the jaw, between the masseter 

 and internal pterygoid muscles. It separates the parotid from the submaxillary 

 gland, and gives origin to some fibres of the stylo-glossus muscle. 



Fig. 258.- 



-Sagittal Section through the Condyle of Jaw to show the Two Synoviai: 

 Sacs and the Articular Disc. 



Articular disc 



Section through condyle. 



Posterior portion of ^^!f^ 

 capsule ' vv^T^ 



Spheno-mandibular ligament 



Stylo-mandibular ligament -V% 



The arterial supply of the mandibular joint is derived from the temporal, middle meningeal 

 and ascending pharyngeal arteries, and from the latter by its branches to the Eustachian tube. 



The nerves are derived from the masseteric and auriculo-temporal. 

 I Movements, — The chief movement of this joint is of (i) a ginglymoid or hinge character, 

 ! accompanied by a slight gliding action, as in opening or shutting the mouth. In the opening 

 ■ movement the condyle turns like a hinge on the articular disc, while at the same time the ar- 

 ticular disc, together with the condyle, gUdes forward so as to rise upon the eminentia articularis, 

 reaching as far as the anterior edge of the eminence, which is coated with articular cartilage 

 to receive It; but the condyle never reaches quite so far as the summit of the eminence. Should 

 the condyle, however, by excessive movement (as in a convulsive yawn), glide over the summit. 

 It slips into the zygomatic fossa, the mandible is dislocated, and the posterior portion of the 

 capsule IS torn. In the shutting movement the condyle revolves back again, and the articular 

 disc ghdes back, carrying the condyle with it. This combination of the hinge and ghding 

 motions gives a tearing as well as a cutting action to the incisor teeth, without any extra muscu- 

 lar exertion. 



There is (ii) a horizontal gliding action in an antero-posterior direction, by which the lower 

 teeth are thrust forward and drawn back again: this takes place almost entirelv in the upper * 

 compartment because of the closer connection of the articular disc with the condyle than with 

 tne squamosal bone, and also because of the insertion of the external pterygoid into both bone 

 and cartilage. In these two sets of movements the joints of both sides are simultaneously and 

 ,, similarly engaged. 



1 • J-^^ *^*^ ^^^^ ^^ movement is called (iii) the oblique rotatory, and is that by which the 

 1 grinding and chewing actions are performed. It consists in a rotation of the condyle about 



^D 



