CAEIES OF THE TEETH. 509 



Caries commencing at the crown or table is due to a portion 

 of the dentine losing its vitality, and the power of resisting the 

 ^jhemical action of the fluids of the mouth ; a portion of the 

 enamel of the cro"svn may be fractured by the animal accidentally 

 biting a piece of hard stone or metal contained in his food. 

 Mere fracture of the enamel is insufficient of itself to lead to 

 caries of the teeth of the lower animals, for it is a substance that 

 is gradually worn off by the attrition of the teeth continually 

 going on during mastication ; but the pressure which has 

 been sufficiently great to cause fracture of the enamel, may 

 at the same time have caused such an amount of injury to 

 the subjacent dentine that it dies, and progressively becomes 

 decomposed. 



In man it seems there should be death of the dentine and 

 acidity of the oral fluids before caries can take place, test-paper 

 applied to a carious tooth invariably showing the presence of 

 free acid ; and a very smaU perforation in the enamel may co- 

 exist with a considerable amount of disease in the dentine. 



Mr. ToAes thus describes the process of destruction in tho 

 teeth, as weU as that conservative action which, as we have 

 ah'eady seen, takes place in caries of the bones. 



He says — " When a portion of dentine has become dead, it 

 is circumscribed by the consolidation of the adjacent living 

 tissue. The tubes become filled up, they are rendered . solid, 

 and the circulation is cut off from the dead mass by the con- 

 solidation of the tubes. Tliis consolidation does not go on 

 gradually from without inwards, keeping in advance of the 

 decay, but occurs at intervals. It would seem successive por- 

 tions of dentine lose their vitality, and that the contiguous living 

 tissue becomes consolidated." ^ 



"We thus see that nature retards the progress of tho disease by 

 throwing barrier after barrier of defence against its inroads. 



The act of consolidation is a vital one : but, according to Mr. 

 Tomes, it is probable that the dentine thus consolidated loses its 

 vitality. " This may be inferred from the place that it occupies, 

 its external protective position, its decreased capabilities for 

 capillary circulation, and its increased density." 

 • Another interesting exhibition of vital action is the production 



^ Dental Phytiolopy an4\Suisfry, ij Joas ToMsa, Surgeoa-Dentiat to th« 

 ^Mi(IdlesexJ3o8pitaL 



