344 



A scheme of classification may be examined in two ways ; we may 

 scrutinise its theoretical claims for adoption, or we may apply it to 

 practice and see whether the result to which it leads us is in accord 

 with knowledge already existing. Now with every respect for Dr. Rose's 

 well earned authority, it appears to me as though his classification 

 did not come satisfactorily out of either of these tests, even though 

 we travel little beyond the illustrations of his own paper for our 

 arguments, and I therefore wish to briefly point out some few instances 

 in which it appears to me to break down. 



He would define True Dentine (Dentin, Orthodentin) as a hard tissue 

 with a smooth surface, which grows on one side only, and which is 

 developed under an epithelial sheath (enamel organ) and under 

 this he groups as subdivisions Normal Tubular dentine, Vitrodentine, 

 in which no protoplasmic processes occur, and Vasodentine (using this 

 latter term in the restricted sense, as I myself have applied it, and 

 distinguishing it as the Vasodentine of Tomes). 



As he himself points out in another part of his paper, where he 

 is criticising my -old attempt at classification, any criterion which re- 

 lates to development is not altogether convenient, as we do not always 

 know about the development of the tissue which may be under observ- 

 ation, and he has unconsciously given an instance of this incon- 

 venience in his paper, when he tells us inferentially, how the tooth of 

 an extinct creature, Sclerocephalus labyrinthicus, was developed, arguing 

 backwards from the assumption, with which others may not agree, 

 that his proposed criteria are quite reliable. 



But I do not lay any stress upon this, as I do not see how any 

 good classification of the sorts of dentine can be framed without 

 reference to developmental questions. 



To take first his definition of a true dentine, that it is developed 

 under an epithelial sheath; this is quite true of a large number of 

 cases, but it breaks down when any attempt is made to use it as of 

 universal application, and is therefore not good as a definition. 



For instance the tooth of Carcharias is made up of a body of 

 fiue tubed dentine, in all its appearances and in the manner of its 

 calcification exactly like other fine tubed dentines, in fact a very typ- 

 ical ordinary dentine. 



Outside this there comes a layer the nature of which has been 

 the subject of some little discussion, most authors having however 

 regarded it as enamel, though it has characters which are unusual in 

 enamels, such as being laminated and being penetrated by outrunning 

 processes of dentine matrix. 



