ERUPTION OF THE TEETH. 201 



much too large for it, a very small force is sufficient to 

 deflect it. And, indeed, a very slight force, constantly 

 operating, is sufficient to materially alter the position of a 

 tooth, even when it has attained to its full length. 



Along the outside of the alveolar arch the muscular lips 

 are exercising a very symmetrical and even pressure upon 

 the crowns of the teeth ; so also the tongue is, with equal 

 symmetry, pushing them outwards : between the two forces, 

 the lips and the tongue, the teeth naturally become moulded 

 into a symmetrical arch. That the lips and tongue are tho 

 agencies which mainly model the arch is very well illustrated 

 by that which happens in persons who have from childhood 

 suffered from enlargement of the tonsils, and are conse- 

 quently obliged to breathe through the mouth, which is thus 

 pretty constantly open. This causes a slight increase in the 

 tension of the lips at the corners of the mouth, and is im- 

 pressed upon the alveolar arch as an inward bending of the 

 bicuspids at that point ; thus persons with enlarged tonsils 

 will be found, almost invariably, to present one of the forms 

 of mouth known as V-shaped. 



But Dr. Norman Kingsley attaches far more importance 

 to disturbed innervation than to any mechanical causes, and 

 refers most dental irregularities to unhealthy conditions of 

 the child's nervous system. 



f\ When the crowns of the teeth have attained such a level 

 as to come in contact with their opposing teeth, they very 

 speedily, from readily intelligible mechanical causes, are 

 forced into a position of perfect correspondence and an- 

 tagonism ; and even at a somewhat later period than that 

 of eruption, if this antagonism be interfered with, the teeth 

 will often rise up so as to readjust themselves in position. \/ 



