Xl INTRODUCTION. 



It is, therefore, obvious that any inference as to the structure of the 

 pulp's surface, or the nature of its previous connection with the tran- 

 sitional cells and the superincumbent layer of dentine, which may be 

 founded on appearances observed under the circumstances above men- 

 tioned, is liable to the objection that the natural relations of the parts 

 observed have been destroyed. If the dentine be the ossified pulp, 

 as Dr. Schwann was disposed to believe, then the calcified part of 

 the growing tooth has been violently displaced from the uncalcified 

 part, and the part of the pulp which thus presents itself for examina- 

 tion is a lacerated and not a natural surface. 



But to the observer who regarded the dentine as a secretion 

 from the pulp's surface, every modification which he might detect 

 on that surface after the displacement of the dentine, would appear 

 natural, and be perhaps described as such with the view to the eluci- 

 dation of the secreting process. Thus the cells which might be 

 observed in progress of ossific transition into dentine would appear as 

 independent parts, and the products of a secreting property; their de- 

 tached condition being, all the while, a necessary result of the artificial 

 displacement of the new-formed cap of ivory, and the consequent 

 laceration of the pulp's substance. 



In the terms of the ' excretion theory ' the exposed surface of 

 the pulp over which the cells lie scattered is a ' formative surfaces ; 

 the nucleated cells are naturally ' detached,' and the ivory or den- 

 tine resulting from their calcification and metamorphosis, is, in 

 respect to the pulp, ' altogether a distinct formation, and by no 

 means an ossification of the pulp.' 



Such is the interpretation which an advocate of the excretion- 

 theory has given to the true phenomena of dental development first 

 observed and described by Purkinje, Raschkow and Schwann. 



