DEKAKGEME:efTS OF THE GRIXDEP.S. 141 



nearly straight lines, which converge as they descend 

 toward the symphysis of the chm. These (the curves 

 and lines) may be, owing, in some cases, to congenital 

 conformation, very irregular. Sometimes, in fact, the 

 curves of the upper jaw are effaced ; at other times, 

 and most frequently, the lines of the lower jaw are 

 incurvated within the upper arcades. The deformities 

 may exist singly or together. The result is that, in 

 the approach of the jaws, the relation is not identically 

 established between the surfaces of friction, and the 

 result of this, in turn, is an irregularity of wear and an 

 abnormal development of the borders of the tables (the 

 crowns of the teeth), within in the lower jaw, without 

 in the upper. 



''3. Exuberance of ixirticular imrts of the Dental 

 Apparatus.— {A.) The upper grinders are wider than 

 the lower, so that in order to cause friction in their 

 entire thickness, a lateral movement of the lower jaw 

 is required. Sometimes, perhaps because the move- 

 ment is not effected throughout the entire limits of 

 the segment of the circle, the outer borders of the 

 upper teeth do not wear sufficiently, and therefore 

 become elevated and sharp. At other times it is the 

 inner borders of the low^er teeth that project. In 

 the former case the cheeks suffer; in the latter, the 

 tongue. 



" In rare cases the tables, which present a normal 

 inclination inverse in the two jaws, at length form 

 planes very oblique. The obliquity is sometimes so 

 great that the internal borders of the lower teeth are 

 very elevated, while the external is almost level with 

 the gums. The inverse effect manifests itself at the 

 uj)per jaw. The consequence is that the half-masti- 

 cated food slips into the pouch of the cheek. 



