THE DENTAL TISSUES. 53 



the outer layers of the enamel, but has no sharply denned 

 boundary, fading away gradually into the colourless tissue 

 lying within it. Some authors have supposed that the pig- 

 ment lay in a thin coating of cementum, or in a very dis- 

 tinct layer of enamel, but as has above been stated, such is 

 not the case. 



Cavities of irregular form sometimes exist in the enamel 

 near to the surface of the dentine, and when such spaces 

 exist the dentinal tubes sometimes communicate with them, 

 but these are perhaps to be regarded as pathological ; 

 Bodecker regards them as filled up by protoplasm. Irregular 

 fissures and cavities also occur upon the outer surface of the 

 enamel, which also have no special significance save as pre- 

 disposing causes of dental caries. 



In man, however, dentinal tubes may occasionally be seen 

 to enter the enamel, passing across the boundary between 

 the two tissues, and pursuing their course without being lost 

 in irregular cavities j though this appearance is seldom to be 

 found. As was pointed out by my father, the passage of the 

 dentinal tubes into and through a great part of the thickness 

 of the enamel takes place in marsupials with such constancy 

 as to be almost a class characteristic. 



The only exception to the rule amongst recent marsupials 

 occurs in the wombat, in which no dentinal tubes enter the 

 enamel ; those extinct marsupials which have been examined 

 present, as might have been expected, a structure in this 

 respect similar to that of their nearest allies amongst the 

 recent genera. 



The enamel of the wombat is peculiar also in another 

 respect, being covered by a strong and remarkably uniform 

 layer of cementum. 



The penetration of the enamel by dentinal tubes is not, 

 however, a peculiarity quite confined to the marsupials, for 

 it is to be found in some rodents (e.g. the jerboa), and in 

 some insectivora (e.g., the soricidse). 



