THE FACE. 



FIG. 74. Illustrating up-and-down or pure hinge motion of 

 the mandible. 



Movements of the Jaw. The jaw has four distinct movements. It can be 

 moved directly forward or backward; up and down, a pure hinge motion; a rotary 

 movement on a vertical axis through one of the condyles; and rotation on a trans- 

 verse axis passing from side to side 

 through the mandibular or inferior 

 dental foramina. The muscles of mas- 

 tication are the temporal, masseter, 

 and pterygoids ; these are supplied by 

 the motor branch of the fifth nerve. 

 To these we may add the buccinator, 

 which is supplied by the seventh nerve, 

 and the depressors of the jaw, the 

 digastric, geniohyoid, geniohyoglos- 

 sus, mylohyoid, and platysma. The 

 posterior belly of the digastric receives 

 its nerve supply from the facial ; its 

 anterior belly from the mylohyoid 

 branch of the inferior dental from the 

 fifth. The mylohyoid is supplied by 

 the mylohyoid branch of the inferior 

 dental. The geniohyoid and genio- 

 hyoglossus are supplied by the hypo- 

 glossal nerve. The platysma is sup- 

 plied by the inframandibular branch 

 of the facial nerve. The upward move- 

 ment is produced mainly by the mas- 

 seter and temporal muscles. It is the 

 principal movement in carnivorous 

 animals ; therefore, these muscles in 



them are well developed, and the joint is a pure hinge joint. The internal pterygoid 

 and buccinator likewise aid in closing the mouth ; the depressors already mentioned 

 open it. The lateral or rotary movement around a vertical axis passing through one 



condyle is used in chewing ; therefore, we 

 find the muscles most concerned, the ptery- 

 goids, best developed in herbivorous ani- 

 mals, or those which chew the cud. The 

 external pterygoid is especially efficient in 

 pulling the jaw forward ; superficial fibres 

 of the masseter help in this. The posterior 

 fibres of the temporal muscle pull the jaw 

 back, as do likewise the depressor muscles 

 of the jaw. In this rotary movement one 

 condyle remains back in its socket while the 

 other is brought forward on the eminentia 

 articularis. 



The up-and-down movement of the 

 jaws, when limited in extent, is a pure hinge 

 movement without any anteroposterior dis- 

 placement, and takes place between the con- 

 dyle and the interarticular cartilage (Fig. 

 74). The anteroposterior movement is 

 necessarily accompanied by a slight descent 

 of the jaw, as the condyle glides from 

 the glenoid cavity (Fig. 75) onto the emi- 

 nentia articularis. It goes nearly, but not 

 quite, to the highest point of the articular 

 eminence. If the jaws are kept closed during this anteroposterior movement, some 

 of the teeth of the upper and lower jaws will still be in contact, the number varying 

 in different individuals. The last molar teeth are usually higher than those in front, 

 5 



FIG. 75. Illustrating direct anteroposterior move- 

 ment of the mandible. The condyle is resting on the 

 eminentia articularis. 



