On the Relation of Merves to Odontoblasts and the Growth of Dentine.*

BY W. G. AITCHISON ROBERTSON, M.D., D.Sc., F.R.C.P.E. PLATE III.

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LINICAL and pathological observation both show that the dentine of the tooth is very closely connected with the nervous system, and is in consequence highly sensitive. Upon what structures does the sensibility of the dentine depend? In what manner is the dentine connected with the nerves of the pulp so as to become so sensitive to external stimuli ?

Perhaps there is no other structure in the body which is so largely supplied with nerves as the pulp of the tooth; even in the smallest fragment we find many nerve fibres. If we take the pulp from the incisor tooth of an ox and examine it after having allowed it to lie in a solution of osmic acid for a few minutes, we can see clearly through the darkened semi-transparent tissue a large blackened nerve trunk passing up the centre of the pulp, giving off on its way innumerable lateral branches, and dividing in a brush-like manner near the upper part of the pulp. All the fine branches are directed towards the periphery of the pulp. In longitudinal sections of the pulp we can see the same in greater detail; many large bundles of medullated and non-medullated nerve-fibres running longitudinally near the centre and giving off lateral branches, which are found in great numbers near the periphery, and divide into single nerve fibres just under the odontoblastic layer, being specially numerous at the apex of the pulp. The separate nervefibres enter the layer of odontoblasts and are lost in it. Teased specimens of osmic-acid preparations of pulp also show its richness in nerves. I may note in passing that in many of the teased specimens the axis cylinders of medullated fibres were often found. projecting a long way beyond its medullary sheath (Pl. III. Fig. 1). It appeared as if the sheath had broken across at one of the nodes of Ranvier and had been pulled off the axis cylinder. In these fibres, however, the broken end of the sheath was sharp and abrupt; whereas, had it given way at a node, we should have expected to * From the Dental Record, to the editor of which we are indebted for the use of the engravings on Plate III.

find its end inverted, owing to the natural constriction in the sheath at the node. The presence of these long, isolated axis cylinders seems to me to completely refute the idea of Professor Leydigthat the nerves are tubes filled with a semi-fluid substance, and also the assertion of Engelmann that the axis cylinder is not perfectly continuous, but is, like the medullary sheath, interrupted at Ranvier's nodes.

Non-medullated nerve-fibres are also seen in such preparations, though not so numerous as the white. In some preparations they may be separated from the medullated fibres, and can be recognised by the nuclei at intervals in the neurilemma (Fig. 2).

How do these terminal filaments of the nerves end in the odontoblastic layer? Why should there be such a multitude of nerves in the pulp if they have not a special function and definite termination? In all other parts of the body the function of nerves is motor or inhibitory, sensory, secretory, or trophic, and the fibres mostly end in special terminal structures at the periphery. Do the nerves of the tooth end in the odontoblasts themselves; or, as Magitot affirms, in a layer of cells beneath the odontoblasts? Do they pass between the odontoblasts and accompany the dentinal fibres into the dentinal tubules, as stated by Waldeyer; or do they pass between the odontoblasts and occupy a special set of tubules in the dentine, as described by Boll? To find the true answer to these questions was the object of this rather difficult inquiry, and the conclusions I have arrived at I will now detail.

I shall in the first place describe the odontoblasts as I have seen them in the pulp of the ox tooth. On examining the microscopic sections of the pulp, I was surprised to find odontoblasts were absent from their periphery. At first I thought they had merely fallen away during the process of mounting, but their absence was so constant that another explanation had to be sought. At first I fractured the tooth with a hammer, and on separating the fragments the pulp was usually found lying almost free; and it was these pulps which showed an entire absence of odontoblasts when cut into sections. Professor Haycraft one day happened to pick up a fragment of the tooth thus fractured, and noticed on its surface a shining membrane, which was found to consist of many cells, often closely aggregated, and having a round, oval, or pyriform

shape (Fig. 3). Attached to many of the oval cells there was a long process embedded amongst the other cells. Other fusiform or tailed cells lay free, each having a large nucleus situated in the centre of the round or oval cells and at the broad end of the pyriform cells. These were the odontoblasts that were missing in the sections. The greater number had remained attached to the dentine when the pulp was separated, showing that their connection with the dentine is stronger than with the pulp which they invest, and also demonstrating the actual existence of the membrana eboris of Kölliker. The tail or elongated process of these cells is not the dentinal fibre, for it is directed into the substance of the pulp, but is the root, central, or pulp process of the odontoblast. The dentinal fibre or peripheral process had been torn off the greater number of the odontoblasts by the act of scraping, though in many a small stump of it remained attached to the opposite extremity of the cell, and in some cases this proximal part of the dentinal fibre could be seen entering a dentinal tubule. These two processes— the long pulp process and the dentinal fibre-are the only processes in the odontoblasts of the ox. These cells have no lateral processes.

To see the odontoblasts in situ I sawed through some teeth with great care and removed the pulp by means of a sharp knife. I made sections of these pulps after embedding in paraffin, and found that although not all, yet a very large proportion of odontoblasts remained attached to the pulp. These were seen to be arranged in two layers. Many dentinal fibres could be seen projecting from the periphery of the pulp to varying distances. These had been drawn out from the dentinal tubules and now lay free (Fig. 4). In one case the dentinal fibre could be seen springing from its odontoblast, and apparently giving off its lateral branches, the latter, however, being mere rudiments. In other situations, odontoblasts were seen which had become separated from the other cells and had drawn out along with them their internal or root process. This was in some cases of great length, and could be traced for some distance into the pulp. In other cases part of the dentinal fibre still remained attached to one extremity of the separated odontoblast, while from the other extremity the long internal root process was seen extending into the pulp.

I have not, in the course of my reading, observed any allusion to the great length of this internal process of the odontoblast ; on the contrary, Waldeyer's description is generally adopted, according to which it is very short and constantly connected with one of the cells lying immediately beneath membrana eboris. According to Hertz the process does not exist. I would therefore again direct attention to its great length, to the absence of lateral branches, and to the oval, fusiform or pear shape of the odontoblasts in the tooth of the ox. In many of these pulps masses of lymphoid tissue are seen. As a rule, these are situated close to the periphery, and just under the odontoblastic layer.

On snipping off small pieces of the outer surface of the pulp and teasing them in a one per cent. solution of osmic acid, the specimens showed clearly the nerve fibres isolated and running amongst the odontoblasts, but the latter adhered closely to one another, and could not be separated by teasing, and thus it was impossible to trace the nerve fibres to their ultimate destination. In order to render the cells more easily separable, some pulps were placed in a o'6 per cent. solution of potassium anhydrochromate for twenty-four hours. Fragments from their outer surface were then teased in picrocarmine, with the result that the long central process of the odontoblast was rendered very evident (Fig. 5). These central processes ran into the pulps towards the nerves, and could often be traced inwards to a distance greater than six to twelve or more times the length of the odontoblast itself. From the opposite extremity of each odontoblast the dentinal fibre proceeded, and in many cases this was also exceptionally long. The central process arises from each odontoblast gradually, the proximal end of the cell gradually tapering till it becomes the pulp process. The distal extremity of the odontoblast, on the contrary, rapidly narrows down to a fine fibre, which is continued onwards as the fibre of Tomes. The nucleus of each cell seems to be swollen up, and is in these preparations a large oval body filling up a large part of each odontoblast. When examined by a high magnifying power, each odontoblast appears almost as if a mere fusiform enlargement of the continuous fibre, formed by the long root process and the dentinal fibre, with a large nucleus in the dilated part.

JOURNAL OF MICROSCOPY, 3RD SER., VOL. 4, PL. 3.

1.- Teased portion of pulp of tooth of Ox. Shows the long projecting axis cylinder process; on one a piece of the white sheath is seen separated from its nerve fibre.

2.-Shows medullated and non-medullated nerve fibre.

3.-Scraping from inner surface of dentine. Shows odontoblasts with their long central processes. 4.-Surface of pulp. Distal processes of odontoblasts seen projecting from surface. Odontoblasts are seen pulled away from the surface, though still attached to it by their central process. In one, both distal and central processes are seen springing from the same odontoblast. 5.-Portion of surface of pulp teased in potassium anhydrochromate solution. Shows very long central process belonging to each odontoblast, and entering surface of pulp. The odontoblast has fallen off in many cases, and leaves the central process projecting like a fine hair or nerve fibre.

6. Apparent direct continuation of root process of odontoblast with axis cylinder of nerve.