162 A MANUAL OF DENTAL ANATOMY. 



view, that dentinal fibril, dentinal sheath, and matrix, are 

 but three ages of the same tissue. 



For under the influence of caries the walls of the tubes, 

 invisible, or almost so, in perfectly healthy dentine, become 

 apparent. 



As I have elsewhere expressed it, the most external 

 portions of the odontoblasts undergo a metamorphosis into 

 a gelatigenous matrix, which is the seat of calcification, 

 while their most central portions remain soft and unaltered 

 as the fibrils. Intermediate between the central perma- 

 nently soft fibril and the general calcified matrix, is that 

 portion which immediately surrounds the fibril, namely, the 

 dentinal sheath ; as expressed by Dr. Lionel Beale they are 

 protoplasm, formed material, and calcified formed material. 



That the whole of the dentine is derived from a conversion of the 

 odontoblast cells is not agreed to by all writers. Thus Kolliker and 

 Lent believe that while the canals and their contents are continua- 

 tions of the odontoblasts, the matrix is a secretion either from these 

 cells or from the rest of the pulp, and so is an " intercellular " sub- 

 stance. Their view is therefore intermediate between the excretion 

 and conversion theories; and Kolliker goes on to say, "since the 

 dentinal cells are immediately drawn out at their outer ends into 

 the dentinal fibres, and do not, as was formerly thought, grow out 

 in such a manner that the dentinal fibre is to be regarded only as 

 the inner part of the cell, so it is not possible to derive the dentine 

 Immediately from the cells." But is not Professor Kolliker think- 

 ing and writing of those aged, spent cells which his pupil Lent 

 figured ? No one could speak of a young, active odontoblast as 

 " drawn out into the dentinal fibril." A good section of young 

 developing dentine shows that the cells are square and abrupt 

 towards the dentine ; they do not taper into the dentinal process in 

 the smallest degree, and there is no room for any intercellular sub- 

 stance whatever. 



Hertz coincides with Kolliker in regarding the matrix as a " se- 

 cretion from all the dentinal cells in common which stands in no 

 definite histological relation to the individual cells," but his figure 

 also I believe to be representative of an adult inactive surface of 

 pulp, in which dentine formation had almost ceased. 



Kolliker and Lent are of opinion that a single cell is sufficient to 

 form the whole length of a dentinal fibril, not having seen evidence 

 of active cell growth in the subjacent layer of the pulp, from which 

 they would infer that the membrana eboris was supplemented by 



