TEETH. 325 



one and the same tissue, the osseous ; while 011 the other, it is 

 evident that the enamel is a modification of the epithelium. 



Mr. Nasmyth describes the cementum as passing over the 

 crown of the tooth and surface of the enamel, in a thin layer 

 composed of hexagonal cells and fibres ; this layer exists only 

 on the surface of the enamel of the young teeth of the human 

 subject, and it is not composed of dentine, but consists of 

 either a few of the unelongated cells of the enamel pulp, or, 

 as Mr. Tomes considers, of the inner surface of the sac of the 

 tooth. 



Nature of Caries of the Teeth. 



Various opinions have been entertained in reference to the 

 nature of the peculiar decay denominated caries, to which 

 the teeth are so liable. Some have supposed that it is a 

 vital process resulting from inflammation. The fact that dead 

 teeth, that is, teeth which have been removed from the jaw 

 and are again employed as artificial teeth, undergo a similar 

 decay to that which affects the living teeth, proves that it is 

 not essentially a vital action, although it cannot be ques- 

 tioned but that the condition of vitality and the state of 

 development of the teeth must exert a powerful influence 

 over the progress of the decay. Other observers regard the 

 decay of the teeth as a purely chemical phenomenon, the 

 earthy matter of the teeth being removed by the action of 

 free acid in the saliva : this view of its nature certainly ex- 

 plains many of the circumstances connected with dental 

 caries, and is supported by the fact already cited, viz. that 

 dead teeth are susceptible of the change. 



Two facts, however, require to be determined before the 

 chemical theory of the decay of the teeth can be considered 

 to be proved ; first, that the saliva is in every case of dental 

 caries really acid ; and second, that the portion of the tooth 

 which is subject to the carious action is really dead : upon 

 both of these points considerable doubts may be entertained. 



For myself I have long entertained the idea that the real 

 and proximate cause of the decay of the teeth was to be 

 found in the presence of some parasitical production, and 



