326 DISTRIBUTION OF BACTERIA 



Dental Caries. Among the bacteriological processes 

 which take place repeatedly in the mouth of all individuals is 

 caries or the decay of the teeth. The subject of caries has 

 been studied by physicians and dentists for centuries. There 

 are many theories as to the cause of this condition. The 

 foremost is the chemical and parasitic theory, which may be 

 briefly stated as follows : the theory holds that decay of the 

 teeth results primarily from the action of the organic acids 

 produced by bacteria, first, on the enamel, and second, on the 

 dentine and pulp, and that as soon as the dentine is softened 

 the various putrefactive bacteria infect the tooth and cause 

 rapid disintegration and decomposition. 



It will be recalled that the crown of the tooth is covered 

 with enamel, which is the hardest tissue in the body. Under- 

 neath the enamel is the dentine, which is the principal con- 

 stituent of the tooth. This substance is traversed by small 

 microscopic tubulae, which run almost parallel to the long 

 axis of the tooth. These minute tubes are filled with pulp and 

 empty into a central perpendicular pulp cavity. The acids 

 produced by the bacteria dissolve the enamel and change it into 

 a cheesy mass. About 92 per cent calcium salts and 42 per 

 cent of organic material are lost on account of the acids se- 

 creted by the bacteria (Fischer). 



When the soft mass produced by the action of the bacteria 

 upon the enamel is washed off, as it is by the saliva, there 

 is usually a slight excavation produced in the underlying 

 dentine, which serves as a lodging place for the saprogenic 



