THE FACE. 



69 



FIG. So. Fracture of the lower jaw, showing 

 the line of fracture proceeding downward am: 

 backward, favoring displacement. 



Displacement. The displacement of the fragments will depend on the line of 

 fracture ; and the line of fracture may be determined by the direction and character of 

 the fracturing force. The line of fracture is oblique. It may be oblique from above 

 down or from without in. An examination of the muscles attached to the mandible 

 will show that the elevators of the jaw are attached to it posteriorly and its depres- 

 sors anteriorly. On this account, when the fracture runs obliquely down and 

 forward there is little or no displacement, because the depressors and elevators tend to 

 press the fragments together. When the fracture 

 runs downward and backward (see Fig. 80), the 

 depressors and elevators tend to separate the 

 fragments. The depression of the anterior frag- 

 ment is particularly marked when the fracture is 

 double, involving both sides of the jaw. The 

 muscles which tend to depress the anterior frag- 

 ment are the geniohyoglossus., geniohyoid, mylo- ^a? /-*" 

 hyoid (anterior portion), digastric, and platysma. 

 The muscles which elevate the posterior frag- 

 ment are the temporal, masseter, buccinator, and 

 internal pterygoid. 



The displacement may not only be up and 

 down, but may also be lateral. The line of frac- 

 ture may run from the outside either inward 

 and backward or inward and forward. The jaw is held in place by its own rigidity 

 when intact ; when broken, the smaller fragment is liable to be pulled inward by the 

 muscles passing from it toward the median line. These muscles are the internal 

 pterygoid and the mylohyoid. The influence of the former is more marked than of 

 the latter, because the fracture frequently divides the mylohyoid, leaving a part of 

 it attached to each fragment. When the fracture passes from without inward and 



backward, then there will be little or no 

 displacement, because the internal ptery- 

 goid and mylohyoid draw the fragments 

 together. (See Fig. 81.) When the 

 line of fracture is from without inward 

 and forward, the internal pterygoid of 

 the injured side and the mylohyoid draw 

 the posterior fragment inward, while the 

 internal pterygoid of the opposite side 

 draws the anterior fragment outward 

 (Fig. 82). 



From a consideration of the forego- 

 ing facts, we see that when there is dis- 

 placement it is because the fracture runs 

 from above downward and backward, 

 and from without inward and forward. 

 The anterior fragment is displaced down- 

 ward and the posterior fragment is dis- 



FIG. 81. Fracture of the jaw, showing absence of dis- placed inward, 

 placement when the line of fracture runs from the inside TT 4.1, U tt. t 4.U 



forward and outward. Fractures through the region of the 



molar teeth are not particularly uncom- 

 mon, and this is likewise the case with fractures obliquely downward and outward 

 through the angle of the jaw. In these injuries the firm attachment of the masseter 

 on the external surface of the jaw and the internal pterygoid on its inner prevent 

 displacement. 



Fractures of the coronoid process are exceedingly rare. In them displacement 

 is prevented by the attachment of the temporal muscle, which passes much farther 

 down on the inside than on the outside. 



Fractures of the neck of the jaw are particularly serious. Inserted into the 

 condyle and neck of the jaw is the external pterygoid muscle. When a fracture of the 

 neck occurs, this muscle pulls the upper fragment anteriorly and tends to tilt its 



