166 ON LATERAL PEESSUEE. 



allied to the former peculiarity. But when it can 

 be shown, that with this particular form of the 

 palate vault, the tongue in sucking or in mastica- 

 tion, by its pressure is capable of producing the 

 expansion of the jaws, and that when the palatal 

 vault is differently formed, so that no pressure can 

 be exerted by it upon the teeth during mastica- 

 tion, &c., the expansion of the jaws does not take 

 place, it will scarcely be doubted but that they 

 are in reality to each other as cause and effect. It 

 is, moreover, in the absence of the possibility of 

 this pressure by the tongue within the arch, that 

 nearly all cases of irregularity occur. 



Fig. 17. 



The above diagram. Fig. 17, is a life-size outline 

 of the section of a cast between the first and 

 second bicuspides on each side. 



The dotted line is an imaginary section of the 

 tongue at the same part in the position assumed 

 when in the act of sucking or of deglutition. The 

 muscular side of the tongue coming thus in contact 

 with the lingual surface of the teeth, has a ten- 

 dency to press them outwards, and which, as the 

 result of the tongue growing with the increasing 

 age of the child, fully explains the " disjwsition of 



