ON LATERAL PRESSURE. IGl 



source or manner of being produced, or of the 

 circumstances under which it may arise. 



The immediate action upon the teeth to which 

 this appellation is commonly applied, is merely 

 the expression of some other and more remote force. 

 There are three separate and distinct sources 

 whence this force may be derived. 



The first, and most obvious, is that arising 

 from the force of growth — the " vis incrementiy 



The second is the " vis extensionis,^^ or pressure 

 caused by the muscles of the cheeks and lips upon 

 the outer curve of the dental arch, and by the 

 expansion of the tongue in sucking, &c. within 

 the arch. 



And, thirdly, the '' ?*i9 occlusionis/^ or under- 

 pressure, caused by an imperfect " bite." 



In the first division, in which the pressure may 

 arise from the growing teeth, its effects will of 

 course be principally confined to a limited period, 

 and generally terminate with the maturity of the 

 denies sapientice, although in some few instances 

 the retardation of particular teeth may cause it to 

 be delayed for a much longer time. Passing over 

 the deciduous teeth, the crowns of the first per- 

 manent molars are the first to become affected by 

 it ; for when these make their appearance through 

 the gum, immediately beyond the distal surface of 

 the second temporary molar, the bulbous protu- 

 berance of the latter projects over the intermediate 

 transverse septum of the alveolus, and catching 



M 



