1899] 



MICROSCOPICAL JOUBNAL. 



341 



ting such teeth for minute examination, I have found the 

 outside layer of enamel and dentine to be stained with 

 various compounds. The exact chemical nature of these 

 compounds I have not been able to prove to my entire 

 satisfaction. Ou scraping off this protective stain, the 

 the microscope reveals it to be chiefly bacteria, and be- 

 cause of this, I am inclined to believe that certain bac- 

 teria have the power of protecting the teeth, just a lichen 

 growing on rocks protects them from disintegration. 



This shows penetration of tobacco stain in the enamel. 

 Tooth lateral incisor from man aged 53. Has used tobacco 

 since he was a boy. Mesio-approximal cavity with tooth- 

 structure black from infiltration of stain. Cavity has been 

 there for years, never became any larger, so he never had 

 it filled. Tooth evidently immune to the action of bacteria. 



The bacteria causing this peculiar discoloration I have 

 observed growing near the gum margin, and in places diflB- 

 cult of cleansing, especially on the lingual and approximal 

 surfaces. Their growth on the teeth will be noticed as a 



