66 APPLIED ANATOMY. 



so that as they glide forward the last lower molars strike the second upper ones. 

 The incisors likewise can be kept in contact as the jaw moves backward and forward. 

 It is this movement in the rodent animals which keeps their edges sharp. In chew- 

 ing, the jaw is depressed, the teeth separated, and the food held between them by the 

 tongue and buccinator muscle. The teeth are then approximated by the lower jaw 

 closing and the condyle sliding upward and backward from the eminentia articularis 

 into the glenoid cavity, carrying with it the articular cartilage. 



The hinge motion takes place between the condyle and the interarticular carti- 

 lage. The anteroposterior motion takes place between the interarticular cartilage and 

 the eminentia articularis: the cartilage is carried forward with the mandible. A rotary 

 movement occurs when, in chewing, the condyle of one side remains in the glenoid 

 cavity while that of the other rises on the articular eminence. The radius of rotation 

 is a line passing from one condyle to the other. In widely opening the mouth, as 

 in yawning, the condyles are tilted forward while the angles of the mandible are 



External lateral ligament 



Internal pterygoid muscle 



FIG. 76. Dislocation of the lower jaw ; the zygoma and part of the masseter muscle have been cut away. 



carried somewhat backward. As the axis of this motion passes from side to side 

 through the mandibular foramina, this portion of the bone moves but little, and the 

 inferior dental vessels and nerve are not put on the stretch. 



Dislocation of the Lower Jaw. The forward dislocation is practically the 

 only one to which the jaw is subject. Dislocations in other directions are apt to be 

 accompanied by fractures. An understanding of the mechanism of the production 

 and reduction of this dislocation requires a knowledge of the movements of the jaw, 

 and the influences which the ligaments and muscles exert in limiting them. The 

 normal movements of the jaw have already been discussed. 



The ligaments which limit the movements of the jaw are those forming the ca,p- 

 sular ligament. This is made up of four parts: anterior, posterior, internal lateral, 

 and external lateral. The anterior is very weak, hence pus in the joint is most apt 

 to make its exit forwards. It is readily ruptured in dislocations. The posterior 

 ligament, though stronger, may also be torn. The two lateral ligaments, the outer 

 being the stronger, become tense when the condyle slips forward on the articular 

 eminence. In dislocation they remain attached to the mandible and are not rup- 

 tured (see Fig. 76). 



