;)L'4 DR W. G. AITCHISON ROBERTSON ON THE RELATION OF 



difficult to make out, on account of the extreme delicacy of the processes and the manner 

 in which they are obscured by overlying cells and neighbouring processes. Nevertheless, 

 several demonstrations (as in Fig. 7) were obtained, showing that the pulp processes do 

 pass into groups of nerve fibres, amongst which they seem to run for some distance 

 before they acquire a medullated sheath. The long central process seems to become the 

 axis cylinder of a nerve fibre, which gradually acquires a primitive sheath in which the 

 medullary or white substance slowly accumulates, till an ordinary medullated nerve 

 results. 



In other cases the medullary substance seems to develop to its maximum amount at 

 once as soon as the odontoblastic central process becomes continuous with the axial band 

 of the nerve. By the potassium bichromate the axis cylinders and grey nerves are 

 rendered evident as fine silky yellow fibres, and can be traced as such to the central 

 processes of the odontoblasts. It is very difficult to say whether all the odontoblasts 

 send in their long pulp processes to join the nerve fibres. Processes are, however, seen 

 to pass inwards from both superficial and deep layers of odontoblasts. 



I have been unable to find any evidence in support of the view advanced by the late 

 Professor Boll, that nerve fibres pass up between the odontoblasts to the dentine. It 

 seems to me probable that the nerve fibres figured by him as passing direct to the 

 dentine were central processes of the odontoblasts that had been drawn out from the 

 pulp and from which the odontoblasts had fallen off. Certain of my own preparations, 

 where this has happened, closely resemble the drawings made by Boll (Fig. 5). Owing 

 to the extreme slenderness of the processes connecting the odontoblasts with the nerves 

 of the pulp, the former are extremely liable to fall off, and leave fine threads which might 

 be mistaken for independent nerve fibres. I cannot agree with Boll when he states that 

 some dentinal tubules contain dentinal fibres, while others contain nerve fibres. Nor yet 

 can I at all agree with the statement of Waldeyer, that the nerves accompany the 

 dentinal fibres into the tubules. What I have already stated I believe to be the true 

 explanation, viz., — that the axis cylinders of medullated nerves gradually lose their 

 medullary sheath, and after running through the pulp for a longer or shorter distance in 

 this way, they become continuous with the central processes of odontoblasts, and that the 

 odontoblasts and dentinal fibres are the terminal organs of the nerve fibres. This would 

 explain why the dentine is so sensitive. We may regard the odontoblast with its 

 peripheral process as an end-organ, which, if not itself sensitive, at once transmits sensory 

 impulses to the nerve with which it is connected, as the odontoblast is with the axis 

 cylinder of a nerve by means of its long pulp process. Whether all odontoblasts are 

 connected with nerves by their root process is a question still unanswered, and con- 

 sequently I cannot say whether all are to be considered as end-organs. Reasoning by 

 analogy, it is not necessary that each odontoblast should be in direct communication with 

 a nerve fibre in order that sensibility may be conferred on the dentine. It is only 

 certain cells in the epidermis which are specialized to receive sensory or tactile impres- 

 sions, and these, being connected with the axis cylinders of nerves, confer sensibility to 



