172 ON LATERAL PRESSURE. 



degree of lateral pressure among tlie crowns of 

 the teeth, which causes them to decay at their 

 sides in contact. This is more especially felt 

 among the upper bicuspides, as well as the upper 

 incisors ; and it is a rare circumstance to find 

 the former in a perfect state, under these 

 conditions, long after they have become fully 

 developed. 



The entire absence of lateral pressure from this 

 source in the extreme forms of Fig. 18, renders the 

 teeth much less liable to decay upon their mesial 

 and distal surfaces. But they are more especially 

 the victims of destruction in another way. The 

 absence of pressure during the period of the 

 crown of the tooth being formed and its being 

 covered with enamel, has a marked effect upon 

 the character and outline of the tooth. Begin- 

 ning to form simultaneously at a number of 

 centres, the enamel continues to increase greatly 

 in thicJcness, but fails to be spread out sufficiently 

 to become thoroughly united at the sides and 

 angles, where two or more plates meet ; the con- 

 sequence of this is, that these imperfections are 

 the origin of that decay which so constantly ac- 

 companies this form of the mouth, in the fissures 

 upon the grinding surface of the molars, as also 

 the medial depression on their buccal surface, as 

 well as the division between the two cusps of the 

 bicuspides. 



In the form Fig. 1 7 these circumstances rarely. 



