§ xi.j Bacteria of tooth-caries. 121 



said perhaps that their invasion is hindered by the presence 

 of the characteristic dwellers in the mouth mentioned above. 



I repeat that I vsfould have these latter spoken of at present 

 only as forms which are actually present side by side, nor will 

 I enter further into the question how far they stand in genetic 

 relation to one another. From the impression which they give 

 us and from present investigations it seems to be highly probable 

 that we have before us several distinct social species. 



The dwellers in the digestive and respiratory passages which 

 have now been described, together with other near allies found 

 in mammals are, so far as our knowledge goes, almost without 

 exception harmless guests, lodger-parasites only, those which 

 live in the mouth being perhaps even useful as protectors 

 against an invasion of destructive ferment-forms. Certain forms 

 however are disagreeable exceptions, inasmuch as they cause 

 caries, the disease in which the teeth become hollow. Every 

 hollow tooth is penetrated throughout by Bacteria, and different 

 forms or species occur in different cases ; Miller (55), after ex- 

 amining hundreds of teeth, has distinguished five of these forms. 

 He has also shown by very thorough investigation that one of 

 them, a Micrococcus, forms lactic acid in a substratum contain- 

 ing sugar or starch. The salts of calcium in the substance of 

 the tooth are dissolved by the excreted acid, and the Bacterium 

 is thus enabled to force its way into the tooth ; as more and 

 more of the calcium is withdrawn the Bacterium passes into 

 the tubuli of the dentine of the tooth, and ultimately spreads 

 through and destroys the tooth. It can scarcely be doubted 

 that MilIer's_four other species produce the same eflfects. 



