164 ON LATERAL PRESSURE. 



and cheeks when the jaws are distended. This has 

 been termed the vis extensionis, because it exists 

 principally during the action of these muscles. 

 We are all conversant with the effects of applied 

 pressure upon a misplaced tooth, and constantly 

 have opportunities of witnessing how readily a 

 tooth may be moved in almost any direction, and 

 that too with but a very small amount of force. 

 We also frequently see the anterior portion of the 

 upper denture rendered unnaturally prominent by 

 sucking the finger or thumb, or by biting the 

 lower lip ; and therefore it may be fairly inferred 

 that should any other part of the mouth, under 

 any particular circumstances, be in a position to 

 exert a pressure upon the teeth, it would be fol- 

 lowed by corresponding results. 



That such an effect does in reality occur, and 

 that the form of the palate, in combination with 

 the size and shape of the tongue, is the means 

 of producing the particular form of the dental 

 arch by which they are accompanied, has, to me, 

 long ceased to be a matter of doubt. 



In his work on the '' Anatomy, Physiology, and 

 Diseases of the Teeth," Professor Bell states, in 

 the chapter headed "Observations on the pretended 

 Prevention of Irregularity," pp. 89, 90, that " a 

 favourable prognosis may, with tolerable certainty, 

 be formed of the ultimate regularity of the teeth, 

 as far as it depends upon the relative proportions 



