1899] 



MICROSCOPICAL JOURNAL 



339 



spots of beginning decay is treated with a solution of ni- 

 trate of silver, the extent of decay can be readily seen. 

 This partially decomposed surface containing millions 

 of bacteria, I believe, can be made to take up substances 

 capable of forming a layer immune to bacterial growth. 

 So I have given the term "immune layer" to any layer of 

 enamel or dentine that has become sterile to the growth 

 of bacteria. This layer is partly due, in some mouths, 

 to a protective film of bacteria that has become stained, 

 and remains adherent to the surface of the teeth. 



Section of enamel showing immune layer that has been 

 produced by the action of formaldehyde or formalin upon 

 the enamel. Hardened in alcohol 50-per-cent formalin 

 10-per-cent while still warm. Stained with Rubin S 

 eosin and oil of cinnamon. The staining brings out the 

 depth of penetration or the fact of bacteria. Under di- 

 rect sunlight the bacteria can be seen penetrating the 

 enamel at points indicated. 



