On certain Appearances occurring in Dentine, dependent on its Mode of Calcification

Czermak, among other interesting matter, has been the first, in this communication, to give a correct explanation of those curious appearances of globular, conglomerate formations in the substance of dentine, which have been so long an enigma to some of our most indefatigable microscopists.

My object in the present communication has been partly to give a summary and confirmation of Czermak’s paper, in reference to certain points in the anatomy of dentine, and partly to add some further observations of my own on the same subject and in the same direction.

The peculiar markings on dentine, known as the “Contour lines,” and their appendages, the irregular patches of small interspaces which limit the outer extremities of the contour lines, have never received a rational explanation until the publication of Czermak’s paper ; and the latter appearances especially have previously been the subjects of the most farfetched and untenable interpretations.

The peculiar patches of opaque interspaces, and the globular masses of dentine which bound them, have been shown by Czermak to be dependent on the mode in which the animal material of dentine is calcified, and to a certain extent (though 1 think not sufficiently) he attributes the contour lines to the same cause. At all events they are obviously associated and produced at the same time and under the same conditions of nutrition, and must be considered together.

The appearances in question are found variously disposed in different teeth and in different proportions in different specimens. It may be remarked, however, that they are always most conpicuous in those teeth whose enamel exhibits irregular development ; and in making sections this may be remembered for the purpose of selection. It will be found, moreover (as I shall presently explain), that there is an obvious relation in number and position existing between the contour markings in the dentine and the grooves and irregularities on the enamel—a fact which I have not seen mentioned, though I believe it to be uniform. Czermak has, however, pointed out another circumstance not previously described, by which this condition may be recognized in the entire tooth—it is the appearance of opaque white lines around the fang, forming white rings : this can be best seen by moistening the tooth for a minute and then holding it obliquely by the side of a bright light. These rings, which vary in breadth from the l-50th to the 1-100th of an inch, are scattered in succession from the neck to the apex of the fang : their white opacity contrasts remarkably with the darker, semi-clear, yellower intermediate portions of the fang. These rings are, of course, abnormal : their number varies : I have one tooth—an inferior canine—in which there are thirteen rings strongly marked. Of the import of these rings I will speak hereafter.

When, then, a well marked specimen (such as I have represented at Pl. V., fig. 1) is examined with a low power, the arrangement of the contour markings^* will be seen as follows : —I would, however, first observe that the section should be vertical, and of an entire tooth ; for if it be only of a portion of a tooth, the relative position and direction of the markings in the different parts of the organ cannot be seen, and these circumstances are of the first importance for comprehending the import of the appearances in question.

The term “contour lines” originated with Professor Owen, I believe, and implies the general similarity which these have with the contour of the tooth. The contour of the two, however, is not identical, for the markings (in whatever part examined) are more divergent than the outline of the tooth ; and, passing from within outwards, abut in succession upon the external surface of the dentine. In comparing the absolute contour of any tooth, it will be found that the angle formed by its sides is more acute at its summit, or the summit of any particular cusp, than the contour markings within.

In viewing the specimen with transmitted light, it will be seen that a series of dark opaque granular patches are arranged immediately within the enamel and crusta petrosa in the outer portions of the dentine. In the crown of the tooth they are usually more distinct and definite than in the fang : they are not continuous and do not form an uninterrupted layer, but are separated from each other by intermediate portions of normal and well-formed dentine. These patches are more or less club-shaped, with the butt-end of the club towards the surface, and the pointed or attenuated end stretching obliquely inwards and upwards towards the pulp cavity. These patches are so irregularly defined that, unless viewed with very low powers, their outline can scarcely be said to have any describable shape. When, however, they are slightly magnified, or seen without a microscope at all, they appear to be convex on the outer and upper margin, and straight on the opposite side. The upper margin and outer extremity are the darkest and most defined ; the lower margin and inner extremity are jagged and ill-defined. The little elements of the patch are very interrupted towards the inner extremity, but are scattered in a direction upwards and inwards and are lost. The contour marking is then taken on by a dark linear streak, which passes nearly in the same direction as the patch, with a slight curve, towards the pulp cavity. The nature of this line does not appear with low power ; it merely looks as an opaque streak, more or less dark and defined. The patch, as I have observed, is most marked near the surface of the tooth : the line, near the pulp cavity ; and where they join they are both indistinct, and sometimes not to be discerned. The relative direction of the contour markings is curious and interesting, and the examination of it in the different regions of the tooth is particularly important. Czermak does not notice this : indeed, his figure, drawn to display the general arrangement, only exhibits a very small portion of the tooth—not enough for the purpose—and what is shown is faulty, and only approximative.

For the particular description of the course of a contour marking, I will select the region just below the neck of the tooth ; they are there peculiarly indicative of their character and meaning. In passing from the surface of the tooth to the pulp cavity, the contour marking makes a double curve, like the letter f and precisely resembling the primary curves of the dentinal tubes in that region. In passing from without inwards the first curve presents a convexity outwards and upwards, then bending in the opposite direction, the convexity looks inwards and downwards, and as the line almost reaches the cavity within, the curve still continuing, it passes almost perpendicularly up the side of the pulp cavity, sometimes apparently joining the line above it. Now it will be seen that the curves of the contour marking not only resemble the primary curves of the dentinal tubes in a general way, but that they are exactly the same in amount at any particular spot, and always opposite in direction : as the tubes bend in one direction the contour markings bend in the opposite, so as to cross the former almost strictly at right angles ; and this may be stated as a rule—that the curves of the contour markings are in proportion to the primary curves of the dentinal tubes at any particular spot, and cross them at right angles. This may be seen by reference to fig. 1. In the crown and fang the contour markings are simpler and less curved. In the crown they are more horizontal and, passing above the pulp cavity, meet over its summit : here the markings are very short. In the fang they are almost vertical, and nearly parallel with the inner and outer surface of the dentine. The lower ones are usually very ill marked, and though they can be traced for a long distance up the fang, as they slowly approach the pulp cavity, they are indistinct and interrupted. Contour markings vary in intensity and number : they are most abundant in the root and most marked in the crown. In the root, though very numerous, they are often scarcely visible. In the crown, when the line is well marked, it is always bounded externally by the opaque patch ; but between these there are frequently others less marked—in such instances, lines without patches at their extremities.

In teeth, with more than one cusp, the upper contour markings are confined to their own cusps, and their extremities abut against the sides of those cusps ; but the succeeding ones join the markings of the contiguous cusps.

The contour markings are also well seen in a transverse section of a tooth, especially about the neck. Here they are represented by a series of concentric rings—a horizontal section cutting the successive markings at different distances from the pulp cavity. In this view Czermak likens them, not inaptly, to the year-rings in wood.

I would here observe that the contour markings are dark by transmitted light, and opaque white by reflected. When mounted in Canada balsam, with continued heat, so as to allow the specimen to soak in the fluid resin for some time before it cools, or when mounted in some liquid, the reverse is the case. The same also happens when decalcified specimens are mounted wet. It is the white opacity of the extremity of the contour markings that produces the appearance of rings on a tooth fang, already referred to.

Decalcified specimens exhibit further points concerning the contour markings. In preparing these specimens 1 first make the section accurately, as though for mounting in the ordinary way ; I then decalcify it by submersion in dilute muriatic acid. It is impossible to make even and regular sections by cutting the softened tooth, and I therefore always make the section before I decalcify it.

A vertical section thus prepared will be seen to exhibit the contour markings strongly. If such a specimen be hooked about with needles, so as to break it up, it will be found to tear in successive portions in the direction of the contour markings, the tear usually being the line of the marking itself. By this means the specimen may be broken up into a series of triangular portions—the triangles being formed by the external surface of the dentine and any two neighbouring contour markings ; the base of such triangles is outwards, and the attenuated apex is inwards, and drawn up the sides of the pulp cavity. These triangular slips thus produced are the intermediate portions of normal dentine, situated between the contour marking. Even the well formed dentine tears readily, but it is parallel with, and in the same direction, as the contour markings. Transverse sections decalcified break up into a series of concentric rings, beautifully and exactly regular : it is not easy to tear out complete rings, but they are partially separable, and are indicated in great numbers ; indeed their number seems to be limited only by the mechanical means employed to isolate them. Such a specimen is exhibited at fig. 2. Now, the breaking up of a vertical section into triangular imbricated slips, and of a transverse section into a series of rings, is tantamount, in the entire tooth, to a stratified or laminated arrangement : indeed, considering these circumstances, as they bear upon the solid tooth, they indicate its composition (in one point of view) as a series of hollow cones adapted one upon the other.

Czermak notices the stratification of the dentine, and speaks of the strata being loosened from each other : he says, “1 have succeeded in breaking off whole layers of tooth-substance, which had perfectly smooth surfaces.” He considers that the splittings of the tooth-substance are by no means a resolution of the structure into its original elementary parts. He further says,—” The ground substance has certainly a stratified composition, but this is usually latent, as it were.” It is fair to observe that the lamellation of dentine, as exhibited in decalcified teeth, was first pointed out by Dr. Sharpey,^* in the tooth of the Cachalot whale. It is most readily seen in the teeth of large mammalia (elephant, hippopotamus, &cc.), but its import is better understood in smaller teeth, where its relation to the entire organ can be contemplated at once.

I may here point out a fact which, if it have been noticed, has, I believe, hitherto escaped recording ; at least it has never been distinctly expressed. It is, that the contour markings, as well as the fracture lines, which so readily occur in the intermediate normal dentine, and are parallel to them, exactly correspond to the pulp surface in the progressive formation of the dentine—are identical in fact with the junction line of the pulp and internal dentine surface at any particular time of growth. In contrasting such a section as is exhibited at fig. 1, with a series of one-cusped teeth in different stages of advancement, this will easily be recognised : the portions of dentine seen between the contour markings in viewing the tooth from above downwards are, as it were, the successive increments by which the organ is built up, and by which the original expanded cap of dentine as it first appears, is converted into an elongated cylinder with a tube up the centre. I have dwelt thus much upon this point, as I have presently to show how the contour marking is produced by a condition common to the entire growing surface of the dentine at one time.

Having said thus much of the general arrangement of the contour markings, I will describe their anatomy as displayed by higher microscopical scrutiny.

Two hundred diameters will suffice for the magnifying power. When the patches are thus examined they are seen to consist of globular masses of dentine more or less isolated by interspaces, as they are less or more confluent one with another. The dentine globules (“ tooth-substance-balls,” as Czermak calls them) are spheres, hemispheres, or partial spheres, usually of normal dentine, and traversed in the usual way by dentinal tubes. Their size varies immensely—from l-400th, 1-300th, or even l-250th of an inch, down to particles of granular dimensions ; even 1-10,000th of an inch in diameter ; indeed there seems no limit to their minuteness. When cleanly mounted the outline of the globules is beautifully sharp. The interspaces between them vary in form according to the number and size of the globules that bound them. In contemplating a large interspace one sees the globules and partial spheres bulging into it, some bright and clear, others looming indistinctly out of focus. Sometimes the interspaces are reduced to mere semi-lunar lines of extreme tenuity. These appearances are represented in fig. 3. Dentine globules are largest in the crown of the tooth, and smallest in the fang, especially near the cemental surface : indeed, in the latter situation, in passing from the surface towards the pulp cavity they regularly enlarge, but while they increase in size they become more fused together, the globules are less spherical, and the interspaces proportionally smaller.

I would here remark that the interglobular spaces (as ordinarly observed in teeth extracted, allowed to get dry, and subsequently cut into sections) are truly hollow and filled with air. This Czermak has enforced.

The relation which the dentinal tubes have to the dentine globules and the interglobular spaces, is interesting and remarkable. The globules are permeated by tubes exactly as the other dentine : the face of a large globule sometimes exhibits as many as five or six tubes traversing it. Now, in following an individual tube across a mass of globules, one observes it follow a regular course, just as if there were no interspaces : one follows the tube across one globule, then, skipping the interspace, one finds it crossing the next globule in a line with its position in the first, and so on. There seems an evident continuity. In specimens, in which the interglobular spaces have been filled with Canada balsam, I have seen (as I have believed) the dentinal tubes collapsed upon the sides of the interspace, establishing the continuity. Kölliker has seen more than this ; for be says, that in decalcified specimens the interglobular spaces are sometimes filled with a soft substance, which is traversed by tubes, and “these may be entirely isolated like the dentinal tubes.” Though I have looked for these I have not seen them, but of the fact I cannot doubt when stated by such an authority, especially as I have observed what amounts to the same in a different phase.

The linear portion of the contour marking is explained in four ways :—First, by a series of secondary curves in successive dentinal tubes ; secondly, by the dentinal tubes being locally widened ; thirdly, by interglobular spaces. The two first, though producing the same general appearance when seen with low powers, are, I believe, essentially different from the contour marking dependent on abnormal calcification.

The interglobular spaces, which form the contour line, are usually a few scattered semilunar streaks, when seen with high power : sometimes these become confluent, and they then form a narrow linear interspace, not wider than a dentinal tube. This latter I have not seen described. Not unfrequently, however, it is impossible to find any anatomical change, even when examined with the highest power, in the dentine that exhibits a contour opacity ; and I can only imagine that a difference in density of such a layer relative to contiguous layers, probably dependent on its composition in the amount of earthy and animal matter it contains respectively, may possibly produce it. Such an explanation is quite in keeping with the rationale of the other element of the con-’ tour marking.

The explanation of globular dentine, and indeed of all the circumstances of the contour markings, is to be sought, and is to be obtained, by examination of the pulp, or inner surface of the dentine, especially of growing dentine. This is the great point of Czermak’s paper.

To obtain a specimen for examination, Czermak directs that a tooth (not completely formed) should be split, and then the section ground from without inwards until sufficiently thin, the pulp surface never being allowed to touch the stone ; the preparation is then to be mounted, with the inner, unrubbed surface supine. The appearance of such a specimen is thus graphically, and, as I can testify, most accurately described by Czermak :—

“ The tooth-substance appears then on its inner surface, not as a symmetrical whole, but consisting of balls of various diameter, which are fused together into a mass with one another in different degrees, and on which the dentinal tubes, in contact with the germ cavity, are terminated. By reflected light one perceives this stalactitc-like condition of the inner surface of the tooth-substance very distinctly, by means of the varied illumination of the globular elevations, and by the shadows which they cast. Here one has evidently to do with a stage of development of the toothsubstance, for the older the tooth is the less striking in general are these conditions, and the more even is the surface of the wall of the germ-cavity. In very old teeth considerable unevennesses again make their appearance ; these, however, aro not globular but have a cicatrized, distorted appearance. It is best to make the preparation from a tooth, of which the root is not perfectly completed. With such preparations one is readily convinced that the ground substance of the last-formed layer of the toothsubstance appeal’s, at least partly, in the form of balls, which are fused among one another and with the balls of the penultimate layers ; and one also perceives that in general their diameter becomes less and less, somewhat in the form of a point, towards the periphery of the tooth-substance. The majority of these balls is pierced through by one or more tubes, crosswise, from within outwards. Very frequently, however, they appear homogeneous, and contain no tubes.”

I will only add, from my own observation, that the globular surface is hardly so general as Czermak implies. Often one sees a considerable area that is even and flat, and destitute of globules. The large globules are always traversed with tubes ; those not traversed by tubes are always small. The globules, on the germ surface of secondary dentine, are small and tubeless ; they are often very minute,

I have found it much easier to obtain specimens than the plan proposed by Czermak, by procuring a tooth of which the fang is half grown, then introducing the point of a penknife into its open extremity, and scraping the inner surface. Small portions may be detached, which exhibit the globules admirably.

Another method of obtaining specimens, which further illustrate the internal surface of the dentine, is the following. In rubbing down a section of a tooth, as the operator approaches the pulp cavity, the last film of dentine frequently bulges into the unresisting cavity, and, instead of grinding up into particles, comes away in a little sheet, a little film of dentine parallel with the pulp-cavity’s surface, the innermost layer, and the one last formed. This should be carefully preserved and mounted. On viewing such a specimen by transmitted light, one sees the globules scattered about—some isolated, others more or less confluent ; and between them a pale, rather indefinite structure, uniting the whole into a sheet.

Now, in Czermak’s specimen one sees only the stalactitelike surface of the pulp cavity, and the prominent inner giobules do not appear connected ; whereas, in specimens obtained as I have just described, they are seen to form part only of the innermost layer of dentine. Upon close inspection, by transmitted light it is found that the globules are composed of well-formed consistent dentine traversed by patent tubes, the open extremities of which are presented to the eye. The tissue between the globules has a somewhat similar aspect, but the tubes appear shrivelled and collapsed, or are not indicated (see fig. 4). The innermost layer of secondary dentine, viewed under similar circumstances, presents the same general aspect : but the globules are tubeless, and the intermediate tissue appears homogeneous (see fig. 5).

But the most instructive specimens are to be obtained from the very thin cap of dentine found upon the fœtal pulp. The thin edge should be cut off, and examined on the inner surface ; it should be moist, and never allowed to get dry. In such specimens the globules are very apparent, but, as Czermak observes, they do not appear superficial but in the substance of the dentine. This he has not explained, but I have observed that, by gradually depressing the focus of the microscope, the first object that meets the eye is the ends of the columnar pulp-cells adherent to the surface of the dentine. As the focus is carried deeper, these appear more or less fused together, and more remotely the dentine assumes a consistent and definite structure. It is here, in the moist specimens, that the focus reaches the globules, and, consequently, there is no superficial stalactite-like bulgings of globules : it is only in dry specimens that that is seen. Now, if such a specimen be steeped in dilute muriatic acid so as to remove all the earthy materials, the globules instantly vanish, and the dentine, where they were seen, assumes the same aspect as that where they were not seen. No other change is produced. The existence of the globules, therefore, seems dependent upon the presence of earthy material. This suggested to Czermak the idea that the organic material of dentine is, during the calcifying process, impregnated with earthy salts in globular forms, and that, by a deeper degree of calcific impregnation, the whole tissue is imbued with the hardening element, and the globules are fused. Such a doctrine is capable of explaining all the circumstances of the case ; and we have only to imagine an arrest of calcification at the globular stage, over the surface of the pulp as it exists at any one time, to explain all the phenomena of the contour markings.

In teeth which have been allowed to get dry, one would imagine that those portions of dentine which have calcified would retain a consistent form, while the uncalcified animal material would shrivel up : hence the stalactite-like appearance of the pulp-cavity’s walls, and hence also the interglobular spaces in a dried tooth. I have shown that, in the innermost layer of dry dentine, the globules are held together by an attenuated tissue, in which the ends of dentinal tubes are indicated. Again, the contour lines, which exhibit no interglobular spaces, but which are continuous with imperfectly calcified globular patches, may be imagined to be themselves dependent on deficient lime impregnation. The integrity of such deficiently-hardened dentine would be seriously interfered with by getting dry ; hence, probably, one reason why decalcified dentine so readily tears along these lines.

Now, the idea that the contour markings are produced by an imperfect supply of calcarious material is consistent with other collateral circumstances. Upon that idea one would imagine that other tissues besides dentine, dependent for their maturation on lime impregnation, and the other teeth, would suffer at the same time ; that is, believing the effect to be produced by a general vice of nutrition : and such, indeed, is the fact. The enamel almost always suffers at the precise spot where the globular patch abuts upon the surface, rendering it irregular and rocky ; and it will constantly be found that these appearances are observable on many teeth of the same individual ; not at the same spot on all the teeth, but at places corresponding with the different degrees of development which the various teeth would have attained at one particular period.

Why the dentine should be thus aborted, so to speak, at successive periods of its growth, and why during intermediate intervals it should mature perfectly, are questions which can only be explained by imaginary successive periodic conditions of depressed and healthy nutrition in the individual during the formation of such teeth.