288 STRUCTURE, RECAY, ANT) PRESERVATTON OF THE TEETH. 



widening of the tubules of the dentine and the constant 

 presence there of micrococci and bacteria. The microbes 

 commonly found in the month, introduced soon after birth, 

 inhaled and taken in with food and drink ever afterwards, 

 and always teeming along the whole alimentary cana\ had 

 been shown to be intimately associated with dental decay : 

 twenty-two different kinds had been described by Miller, of 

 Berlin, as existing in the mouth, and sixteen of those were 

 acid-producing, therefore aiding the disintegrating action 

 of the acid fermentation of the saliva. The two funda- 

 mental conditions associated with caries : 



a. Decalcification by acids, 



6. Growth of microbes, 

 were then discussed. It was pointed out that the saliva is 

 easily rendered acid by food decomposition; and the food, 

 acting as a sponge, absorbs and retains the acid in the 

 cracks and flaws on the surface of the enamel. This tissue, 

 having little power of resistance to chemical influences, be- 

 comes destroyed by a process of mechanical and chemical 

 erosion ; this is the first stop in dental caries, which must 

 of necessity commence at the surface ; but the tubular 

 structure of the dentine will render it an easy prey to germ 

 disintegration when once a way has been found for germ 

 penetration through the covering of impermeable enamel. 

 It was mentioned that caries can be produced in hippo- 

 potamus ivory under the ordinary conditions found in the 

 mouth ; but that acids alone will not produce it if germs be 

 excluded. 



As regards the great and increasing prevalence of dental 

 caries, it was stated that the incidents and accidents of a 

 lifetime are more than sufficient to account for tooth de- 

 struction before the advent of senile decay ; the wonder is 

 that teeth last as well as they do. Even if no congenital 



