160 A MANUAL OF DENTAL ANATOMY. 



period of formative activity being past, and I am inclined to 

 think that his views on the subject of development are open 

 to criticism, as being based upon the appearances presented 

 by such old cells. 



FIG. 75 ('). 



The dentine is, I believe, formed by the direct conversion 

 of the odontoblast cells, just as is the enamel by that of 

 the enamel cells, and is derived from them, and from them 

 alone. 



According to this view, which is supported by Waldeyer, 

 Frey, Boll, Dr. Lionel Beale, and many other writers, the 

 dentinal fibrils, the dentinal sheaths, and the matrix between 

 these latter, are alike derived from the metamorphosis of 

 the odontoblast cell. In other words, the three structures 

 in question may be taken as being three stages in the con- 

 version of one and the same substance : thus we have the 

 dentinal fibril in its soft condition, little more than the 

 unaltered protoplasm of the cell, then the dentinal sheath, 

 one of those peculiarly resistant substances which lie on the 

 borders of calcification; and lastly, the matrix, the com- 

 pleted, wholly calcified tissue. 



That some such relation exists seems to be indicated by 

 the fact that dentinal tubes once formed are capable of 



(') Odontoblasts in situ. After Waldeyer. 



