THE INCISORS. 



617 



more and more acute in proportion as the plane of meeting of the 

 jaws, which is being displaced above and below parallel to itself, is 

 gradually separated from the primitive diameter. Consequently, the 



Fig. 288.— Incisive arcades exposed to show the progressive inclination of the teeth in relation to 



the plane of meeting of the javv's. 



tangents ««', bb', cc' , drawn through the new points, a, b, c, of contact 

 of the arcades, will no longer be perpendicular to this plane, but will 

 tend, on the contrary, to be more and more parallel to it. 



This permits the conclusion that the incidence of the incisors aug- 

 ments in obliquity with age, and that the horizontality of the arcades 

 indicates fairly the age, save, of course, in certain exceptional cases 

 which are sometimes met. 



Consequent on this change of direction there are certain modifica- 

 tions which appear at the inferior extremity of the head. The latter, 

 voluminous and rounded in young animals, becomes thin and pointed 

 in the old, whose teeth, often very long, project so much that the lips 

 can scarcely meet and occlude the buccal opening. 



