DENTAL HARD TISSUEJdESTRUCTION 



139 



Fig. 28. Mandibular rat molars (in quadrants of three) were exposed, 

 after preliminary dr}'ing, to a 5 per cent solution of HNO;^ for a few minutes 

 in vitro (A) and in vivo (B). In the living animals, the molars partly resumed 

 normal glossy surfaces 6 hours after the etching ( C ) , more fully 33 hours after 

 the etching (D). Quadrants of three untreated maxillary molars (A to D, 

 top) served as controls. For details, see text. (X 7.) 



than the 2 : 1 known to exist in whole enamel. Possibly the reason for 

 this ( in this writer's view ) may be the presence of different propor- 

 tions of phosphate and carbonate within the interprismatic regions, 

 which are late to calcify, and which appear to be primarily involved 

 in early subsurface demineralization. 



Secondly, in our own experiments it was found in several of the 

 "remineralized" specimens that whereas the microdensity appeared 

 to be recovered within a week after application of the acid, there 

 nevertheless appeared to be a greater microscopic dye-binding ca- 

 pacity within these areas of "rehardened" enamel. Moreover, in trans- 

 mitted light ( without any stain ) the pronounced Retzius lines were 



