BACTERIA OF THE MOUTH. 34! 



In thin sections of decalcified teeth, stained with fuchsin 

 and vesuvin, it is seen that bacteria are scattered, though 

 somewhat irregularly, throughout the dentine (the hard 

 substance of which the tooth is principally composed) that 

 is undergoing decay or softening. This is always most 

 marked near the surface, but it must be noticed that as the 

 bacteria travel along the dentine tubules (little canals that 

 run vertically through the dentine), there is usually a small 

 part of the softened area in which micro-organisms are not 

 seen, as it seems that these cannot make their way readily 

 along the fine transverse branches that connect the vertical 

 tubules, and, as Miller puts it, although the organisms " keep 

 up with the softening in the direction toward the pulp, they 

 fall considerably behind in the lateral directions, so that the 

 invasion, particularly in the lateral direction, is usually much 

 less extensive than the softening ; " and the tubules near the 

 surface always contain more bacteria than those deeper 

 down in the softened area. It is important to remember 

 that bacteria may be found in apparently normal dentine 

 canals, whilst a similar invasion seems to go on in the roots 

 of " milk " teeth, and also where abscesses have occurred 

 in the roots of " permanent " teeth. It is now held by most 

 of those who have given attention to the subject, that the 

 bacteria which are found in decaying teeth are only playing a 

 secondary part, though a very important one, in the process 

 of caries. In the first instance there appears to be a soften- 

 ing of the various parts of the tooth by acids, commencing 

 with the enamel ; in the case of people who take perchloride 

 of iron or nitro-hydrochloric acid this softening may go on 

 exceedingly rapidly. As we have already seen, lactic acid 

 is constantly present in the mouth, though often in very 

 small quantities ; if left to act on the lime it may give 

 rise eventually to softening at the margins of the gums 

 and to caries, acids preparing the way for the invasion of 

 various bacteria, by combining with the lime and softening 

 the tooth. Where once the lime salts have been removed, 

 bacteria can attack the basic substance most easily ; they are 

 now in a position to make their way along the dentine 

 tubules, and by the intertubular spaces, and once in this 

 position they attack the surrounding tissue with very great 

 vigour ; they use it as a food material, absorbing and 

 digesting it until they have made their way into the greater 



