THE DENTAL TISSUES. 91 



may be merely the outer part of the hard dentine, or may 

 be a thin layer of enamel. It is to be regretted that special 

 names have been given to this layer ; it is sometimes called 

 vitro-dentine, sometimes ganoin or fish-enamel ; but there is 

 no reason why it should have a special name at all. The 

 similarity of the channels of pulp in osteo-dentine to Haver- 

 sian canals in bone is very close ; in fact, when teeth con- 

 sisting of osteo-dentine become, as in many fish they do, 

 anchylosed to the subjacent bone, it becomes impossible to 

 say at what point the dentine ends and the bone commences ; 

 and this difficulty is intensified by the fact that the bone of 

 many fishes lacks lacunse, and is almost exactly like dentine. 



Osteo-dentine was defined by Professor Owen as dentine 

 in which the matrix was arranged in concentric rings around 

 the vascular canals, and in which lacunae similar to those of 

 bone were found. 



But neither of these characters are to be found in many 

 teeth, which, if the manner of their development is to be 

 taken into account, are unquestionably made of osteo-den- 

 tine ; and so they cannot be made use of for purposes of 

 definition, although lacunie and lamination of the matrix 

 are far more often present in osteo-dentine than in the other 

 varieties of dcntinal structure. 



The varieties of dentine may be grouped thus : 



(A.) Dentines developed upon the surface of a pulp, by 

 calcification of a specialised layer of odontoblast 

 cells. 



(i.) Hard, unvascular dentine, thoroughly per- 

 meated with dentinal tubes, which radiate 

 from a simple central pulp chamber. 

 Example Human dentine, 

 (ii.) Plici-dentine, permeated with dentinal 

 tubes, which radiate from a pulp chamber 

 rendered complex in form by foldings in 



