376 e. sprawson. - royal...

376 E. Sprawson.

(11) Dakin, 1909. Pecten, ‘ L.M.B.C. Memoirs No. 17,’ London.(12) Dakin, 1910. ‘ Mitt. Zool. Stat. Neapel,’ vol. 20, heft 1.(13) Drew, 1906. ‘ Univ. Maine Studies,’ No. 6.(14) Winton, 1930. {In the Press.)(15) Gasser, 1930. ‘ Physiol. Rev.,’ vol. 10, p. 33.(16) von Uexkull, 1912. ‘ Zeit. Biol.,’ vol. 58, p. 305.

6 n . 314.018 :599.2

On the Histological Evidences of the Organic Content and Reactionsof Marsupial Enamel, with a Note on Human Enamel.

By E velyn Spraw son , M.C., L.R.C.P.Lond., M.R.C.S., L.D.S.Eng.

(Communicated by Prof. W. Bulloch, F.R.S.—Received March 27, 1930.)

[Plates 33-36.]

CONTENTS.Page

I.—Introductory and Historical........................................................................................ 376II.—Nature of Material used and Technique of Investigation ................... ............... 377

III. —Histological Appearances and Inferences drawn therefrom................................... 379IV. —Conclusions and Note ................................................................................................ 383

Part I.—Introductory and Historical.During histological investigation of the organic content of human enamel it

was noticed that portions of young human enamel (Plate 33, fig. 1) which had been permeated by stain from the dentine aspect were permeable through about three-fourths of their thickness and so looked structurally very similar to marsupial enamel treated in the same manner (fig. 2), the chief differences being the direction of the prisms, and the degree and regularity of patency of the inter-prismatic substance. As histological structure is more readily appreciated in marsupial than most other mammalian enamels, owing to its tubularity, it was decided to make a brief examination of the enamel of young and old marsupials of the genus Macropus, for comparison with similarly made preparations of human enamel. Macropods were chosen because they exhibit the greatest degree of tubularity among marsupials.

Sir John Tomes (1) in 1849 first described that tubular enamel occurs in marsupials and was “ common to the teeth of at least the great majority of

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Marsupial Enamel. 377

marsupials, if not all, except the wombat.” He also showed (2) that when it was decalcified the tubes could be seen maintaining their continuity with the dentine after the lime salts had been removed. He, too, first noted (1) that “ in the human teeth the dentinal tubes are in small numbers, occasionally only continued for a short distance into the enamel,” and so drew attention to a resemblance between human and marsupial enamels at the amelo-dentinal junction. The observations of J. H. Mummery (3) in 1914 confirmed this.

Sir Charles Tomes (4) believed that these tubes were due to persistence of the process (described by and named after Sir John Tomes) of the ameloblast cell, which he thought remained uncalcified in the central portion of the finished enamel prism in marsupials, but became calcified almost at once in human and most other mammalian enamels.

Von Ebner (5) in 1890 described and figured that the tubes lie between the prisms; this has since been confirmed by others and is now the generally accepted view.

Part II.—Nature of Material used and Technique of Investigation.As it was desired to examine the enamel under conditions as near those

obtaining during life as possible only recent material was used. This was obtained from a young female Aru Island wallaby (Macropus and anold male black-tailed wallaby ( Macropusualabatus); newly erupted teeth were obtained from a young museum specimen of the latter species.

The method of investigation was to permeate the enamel by stains, which were used in two ways :—

I. For permeation of the enamel from the pulp canal, so that stain had totraverse the dentine before it had access to the deep aspect of the enamel.

II. For lateral permeation.

In addition a few specimens were permeated with ammoniacal 5 per cent, silver nitrate, and the silver then precipitated in situ by adding a solution of slightly alkaline sodium hydrosulphite. This, however, did not cause precipitation to occur throughout tissues, and it was ultimately found better to grind the section from the permeated tooth, using xylol, as described later, and then precipitate the silver by exposing to sunlight. In some teeth the protein content was digested with 1 per cent, trypsin before permeating with the silver salt, as it was thought that the protein might be coagulated by the salt and so interfere with permeation, but in practice no appreciable difference was



378 E. Sprawson.

noticed in sections prepared in this manner. Further experience with this technique is desirable. I t was hoped that the smaller molecule of the crystalloid salt would penetrate better than the stains and show greater detail, as indeed it does.

For method I, the root portion of each tooth was cut off transversely and the pulp removed. Solid stains soluble in aqueous media were then introduced into the pulp canal and moistened with normal salt solution ; the opening was then closed with oxyphosphate of zinc cement and the tooth placed in normal salt solution. Only the procumbent lower incisor teeth were suitable for this method.

For method II, the teeth were cut into thick sections and placed in a strong solution of stain in normal salt for several days before being ground thin. This was done to see, by comparison with method I, if additional stain would penetrate the enamel from its now exposed lateral aspects. Cheek teeth were used for this purpose.

Methylene blue and acid fuchsine were used for permeation ; both permeated well, but it was found difficult to take photomicrographs in colour which showed accurately the area of permeation by methylene blue ; moreover, this stain is not always fast in Canada balsam.

It was hoped that these stains would find their way through uncalcified spaces in enamel by the physical process of diffusion ; should such diffusion occur under method I, one would infer that they had permeated a space occupied in life by some vital fluid, since it is believed there are no vacuous spaces in body tissues. I t is known that such vital fluids occupy uncalcified portions of dentine, and stains would have to traverse these before they could reach the enamel; moreover, these “ spaces ” in dentine can be seen in direct continuity with the enamel. Should diffusion appear in continuity from the pulpal aspect of the dentine to the external aspect of the enamel, one would infer that this was due to traumatic injury, since it is inconceivable that there should be physiological leakage of vital fluids into the mouth.

It was found that all the stain that was going to diffuse into the enamel did so within a few hours of introduction. The teeth were not allowed to become dry, as artefact cracks frequently occur in all calcified tissues of dry teeth ; this is seen in many museum specimens (fig. 3). For this reason specimens were kept in aqueous saline during the permeation process, otherwise permeation might have been due to capillary attraction and not diffusion. Alcoholic stains were not used, in case such hygroscopic medium might produce artefacts by abstraction of water.




A difficulty presented itself in grinding sections. When grinding hard tissues it is usual to keep the carborundum wheel used for this purpose wet, to avoid the heat of friction and consequent damage to tissues ; the stains used, and the silver salt, being freely soluble in water, would, of course, dissolve out when the wheel was wetted, and this occurred even when reagents reputed to fix the stains were used.

The difficulty was overcome by using for this purpose a fluid in which the stains were insoluble. Xylol was tried, and met requirements very satisfactorily ; no stain is lost by this method and a stain-diffused tooth may be taken direct from a watery medium, dried on a cloth and ground. When ground sufficiently thin it is washed in xylol and mounted in Canada balsam. The absence of water made the tissues rather more brittle than when making a section in the usual manner. The teeth used were not removed by forceps, so one may conclude that no artefact cracks were produced before permeation.

Part III.—Histological Appearances and Inferences drawn therefrom.

Young Marsupial Enamel ( Macropusbrunii).—The cheek teeth of this animal showed only slight signs of wear. In teeth prepared by method I the enamel was found to be freely permeable via the dentine (fig. 4) to three-fourths or more of its extent (fig. 2). This permeability appeared to be the inter- prismatic substance, though occasionally prism substance seemed to have absorbed some stain. That permeability was inter-prismatic wTas confirmed by making a section transverse to the long axis of the prisms (fig. 5). This section was made from a cheek tooth by method II.

Old Marsupial Enamel ( Macropusvalabatus).—The cheek teeth of this animal showed considerable signs of wear, enamel being entirely worn away from large areas of the occlusal surfaces. In sections prepared by method I enamel was hardly permeable, or only so to a slight degree (fig. 6). The tubes appeared to be fewer and narrower (fig. 7), and though not so permeable their continuity with the dentine was still quite evident; one infers that as age and wear progressed, increased calcification of the inter-prismatic substance took place, till it became impermeable and a more or less solid mass.

The above are, however, general statements, because examination of the cheek teeth of the young animal showed (fig. 8) that though the enamel on some aspects was freely tubular through almost its whole extent, on others it was not so, and in these usually showed no trace of its original tubular structure. These latter portions (fig. 9) therefore closely resembled the bulk of human enamel in structure and presented a large number of markings (“ laminae”)



380 E. Sprawson.

which, there can be but little doubt, are mostly cracks, produced by the trauma of mastication. As, however, in human enamel there is evidence that some laminae may be developmental in origin, the same may be true of marsupial enamel; but when the greater number of laminae are found on those aspects where the enamel is thin and which are more exposed to trauma, and all stages of reaction, ultimately rendering them impermeable, are observed, it is strong evidence that most are traumatic in origin.

These laminae or cracks vary considerably in extent and appearance, and inferences of some importance may be drawn from these facts. Some of them extend through the whole thickness of enamel and into the dentine, and of these :—

(i) In some (fig. 10) the dentine is found to have taken in more stain at the point of injury than elsewhere ; in these the stain penetrates the extreme extent of the enamel crack, so that there is a potential leakage of lymph into the mouth ; one infers that the damage was recent and so gave more ready ingress to stain.

The minute anatomy of the damaged area is seen in better detail in silver preparations and presents remarkable appearances (fig. 11). In these the dentine tubes are larger, more branched and more patent than in the surrounding dentine ; it appears as if the initial dentine reaction were analogous to that of inflammation in soft parts, and that this increased patency is for the purpose of bringing an increased amount of calcium salts to the damaged area for its repair, to result ultimately in the production of a scar of increased calcification, known as a translucent zone.

In the enamel the inter-prismatic substance admits more stain in the immediate vicinity of the crack than elsewhere, and in view of subsequent findings (see figs. 13 to 20) it seems certain that this greater patency is a reaction to injury.

If these inferences are true they are of great interest, because they are conclusive evidence that the exchange of calcium salts goes on in enamel and dentine, governed from the pulp. Till now, such exchange has been hypothetical, though it has been recognised for many years that additional calcium salts may be deposited in dentine, and recently in human enamel also, under pathological conditions (6).

(ii) In a second variety (fig. 12, crack 1) a definite reaction is found in the dentine,- which has resulted in the production of a translucent zone of



hypercalcification, so that at that point it is impermeable, except centrally, where a few dilated tubes are seen : these are in direct continuity with the enamel crack, which also is permeable. This indicates a reaction of repair, appearing primarily in the dentine (by this method of preparation).

(iii) In a third variety (fig. 13) the enamel crack is permeable mesially for a certain distance and continued to the surface as an impermeable lamina or linear translucency ; the dentine shows a translucent zone permeable centrally in continuity with the enamel crack as before.

(iv) In a fourth variety (fig. 14) not only is the enamel crack completely impermeable but the adjacent enamel as well; it shows a definite zone formed by hypercalcification of the inter-prismatic substance, which thus renders it impermeable, and in extreme cases converts it into a non-tubular structure simulating human enamel. A translucent zone is present in the dentine. At this stage there are no centrally dilated tubes present; apparently the purpose for which they became dilated having been achieved, they also have become occluded.

In transverse section (fig. 15) one can show that hypercalcification in the repair of enamel cracks takes place in the inter-prismatic substance, because adjacent to cracks it is seen to be impermeable.

(v) In a fifth variety (fig. 16), possibly when the enamel crack was a small one, repair is of such perfection that a translucent line of hypercalcification is seen, like a linear scar.

After repair has fully taken place in enamel and dentine there is a possibility that translucent zones may disappear, because occasionally impermeable enamel cracks are seen unassociated with translucent zones either in enamel or dentine ; in the latter case this may be because the crack never extended into the dentine. Again sometimes, though translucent zones have obviously been present both in enamel and dentine (fig. 17), they are permeable, though not to the same extent as in adjacent uninjured tissues. There is, moreover, the evidence, as seen in such injured areas as in fig. 11, of dentine tubes opened up amidst an impermeable area and of the reopening of inter-prismatic spaces in enamel.

Other cracks, which involve enamel only, are seen, and these, too, may be found permeable in different degrees, as follows :—




382 E. Sprawson.

(i) Permeable to stain throughout their length, apparently recent cracks (fig. 12, crack 4). Silver preparations (fig. 18) show the opening up of adjacent inter-prismatic spaces, analogous to what is seen in newly damaged dentine and already shown in fig. 11, but the two cracks here seen are at a later stage. The peripheral portions show the commencement of the formation of the translucent zone after the opening up of the inter-prismatic channels, consequent on injury. This proves that repair takes place from without inwards and must therefore be governed from the pulp. In one of the cracks it can be seen that the violence causing it detached a small piece of enamel, and in the other the peripheral portion of the crack has already closed. These preparations also show that there is an accompanying local reaction in the dentine, even though the damage does not extend into it.

(ii) Permeable only through the mesial portion and continued to the periphery as an impermeable lamina or a linear translucency (fig. 12, crack 3, and fig. 19).

(iii) Obvious cracks, entirely impermeable to stain and apparently entirely healed (fig. 12, crack 2).

(iv) Linear translucencies, which are by analogy linear translucent scars (fig. 20).

It will be noticed constantly in the same section (fig. 8, see also figs. 9, 12, 16) that on those aspects of a tooth which present many cracks, the processes of repair and hypercalcification have obliterated the tubular appearance of the enamel. Probably this is aided by the conjunction of translucent zones ; whereas on aspects protected from trauma by contact with teeth in front and behind, there are few or no cracks, and the tubular system is typical and extends practically to the surface of the tooth. One inferred that the former aspects were more subject to the physiological trauma of mastication, and hence damage ; and in fact this is so. This section is made from a cheek tooth of the young animal, and it is the exposed buccal and lingual aspects which show many cracks and non-tubular enamel. Examination of the enamel covering these aspects of newly erupted cheek teeth of a young Macropus ualabatus showed that they are as freely tubular as any other part of marsupial enamel before they have been subjected to the trauma of mastication.

It is a significant fact that if any lamina can be traced through the whole thickness of the enamel, any part of it which is impermeable is always the peripheral portion, unless the damage at that point was rather extensive.



Marsupial . 383

Concurrent with the changes due to age and wear already tabulated there are, in addition, changes in the prism substance. That this becomes hypercalcified is evident, because in the more highly calcified specimens the undamaged portions admit no stain by any of the three methods used.

Part IV .—Conclusions and Note.

From the above data one may therefore conclude that in marsupial enamel

1. The tubes are inter-prismatic and consist of spaces of different and varyingdegrees of patency.

2. In the young animal these spaces are much more permeable than in theold animal.

3. As age and w ear progress the enamel becomes more highly calcified. Thisis brought about partly by deposition of calcium salts occluding the inter-prismatic channels. This, though not at first obliterating the tubular appearance, lessens the number and diameter of these spaces ; later, as calcification progresses, they may become obliterated.

4. The enamel is frequently cracked ; these cracks often extend into thedentine and occur more on the exposed aspects of teeth than others. Doubtless, these are due to the trauma of mastication.

5. The cracks, though often permeable to stain, appear to become occludedboth in enamel and dentine.

6. Since in partly permeable cracks, extending through the thickness of theenamel, it is the peripheral portion which is occluded, closure must be governed from the pulpal aspect.

7. When both enamel and dentine are injured, the dentine reacts locally byan increased patency of its tubes at the site of injury. Ultimately hypercalcification occurs there, but the central tubes leading to the enamel crack maintain their patency till the enamel crack is occluded.

8. The enamel adjacent to its site of injury appears to react in a manneranalogous to that of the dentine ; it shows increased inter-prismatic patency at first and hypercalcification later. This hypercalcification takes place from without inwards, and commences before the actual occlusion of the enamel crack.

9. Closure of cracks appears therefore to be brought about by calcium saltsconveyed via the dentine.



384 E. Sprawson.

10. The above facts indicate that the spaces permeated by stain can in life convey calcium salts for repair and remove them as required.

11. Exchange of calcium salts therefore can take place both in enamel and dentine, this exchange being governed from the pulpal aspect.

12. By analogy with human enamel, some laminae may be developmental in origin, but, in any case, only such as do not permit of escape of vital fluids into the mouth, since it is inconceivable that there should be such physiological leakage of vital fluids. One can, however, find no evidence of developmental laminae in marsupial enamel.

13. Marsupial enamel must therefore be regarded as a living tissue which reacts to injury and undergoes increased calcification as age and wear progress.

14. Inflammation is defined as the reaction of living tissue to injury. Apparently both dentine and enamel react to injury, but, being avascular, do not show the usual signs and symptoms of inflammation, as seen in vascular tissues.

Allowing for structural and calcific differences, similar reactions to those shown in marsupial teeth have been found by the author in human enamel and dentine. At present five figures only illustrating these conditions are shown (figs. 21, 22, 23, 24, 25). I t follows, therefore, that many of the conclusions concerning marsupial enamel must also be true of human enamel.

My thanks are due to the Zoological Society of London for supplying me with material as it became available, so expeditiously that only a few hours elapsed begween the deaths of the animals and the preparation of the teeth for examination ; also to the British Museum (Natural History) for specimens of newly erupted teeth. My thanks are also due to the Medical Research Council, under whose auspices this work has been done.

REFERENCES.

(1) Tomes, J., “ On the structure of the dental tissues of marsupial animals, and moreespecially of the enamel.” ‘ Phil. Trans.,’ vol. 139, pp. 403-413 (1849).

(2) Tomes, J., “ On the presence of fibrils of soft tissue in the dentinal tubes.” ‘ Phil.Trans.,’ vol. 146, pp. 515-522 (1856).

(3) Mummery, J. H., “ On the nature of the tubes in marsupial enamel and its bearingupon its development.” ‘ Phil. Trans.,’ B, vol. 205, pp. 295-313 (1914).

(4) Tomes, C. S., “ On the development of marsupial and other tubular enamels, withnotes upon the development of enamel in general.” ‘ Phil. Trans.,’ B, vol. 189, pp. 107-122 (1898).



Sprawson. Roy. Soc.Proc. B. vol. 106,

15 16




(5) Von Ebner, V., “ Strittige Fragen fiber den Bau des Zahnschmelzes.” ‘ Sitzungs-berichte d. K. Akad. d. Wissensch., Math.-naturw. Cl., Wien.,’ vol. 99 (abth. 3), pp. 57-104 (1890).

(6) Mummery, J. H., “ Translucent zones in enamel.” ‘ British Dental Journal,’ vol. 47.No. 9, pp. 473—482 (1926).

DESCRIPTION OF PLATES 33-6.

P late 33.

Lettering applicable to all figures :—a, enamel; b, dentine; c, lingual aspect;d, buccal aspect; e, mesial aspect ; / , distal aspect.

Fig. 1.—Young human enamel from a permanent molar, permeated from the pulp chamber by methylene blue ; longitudinal section ; permeation is seen to be inter-prismatic. X 235.

Fig. 2.—Young marsupial enamel from an incisor, permeated from the pulp chamber by acid fuchsine ; longitudinal section, a portion of fig. 4 ; permeation is seen to be inter- prismatic. x 235.

Fig. 3.—Indian elephant molar, old museum specimen; shows many large artefact cracks. X 2/5.

Fig. 4.—Young marsupial lower incisor, permeated from the pulp chamber by acid fuchsine, longitudinal section; shows degree of permeation in uninjured enamel, x 24.

Fig. 5.—Young marsupial enamel from a cheek tooth, permeated as a thick section from all sides by acid fuchsine, transverse section; the enamel is ground transverse to the long axis of its prisms ; permeation is seen to be inter-prismatic, x 235.

Fig. 6.—Old marsupial lower incisor, permeated from the pulp chamber by methylene blue : transverse section; shows degree of permeation in uninjured enamel, x 24.

Fig. 7.—A portion of the enamel of fig. 6. X 235.

Plate 34.

Fig. 8.—Young marsupial cheek tooth, permeated as a thick section from all aspects by acid fuchsine, transverse section; showrs many cracks and non-tubular enamel at c, d, and few or no cracks, and the tubular system throughout nearly its whole extent, at e ,/. x 11.

Fig. 9.—A portion of fig. 8, c, showing obliteration of tubular system, and so looking very like the bulk of human enamel. X 100.

Fig. 10.—See Plate 35.Fig. 11.—Young marsupial cheek tooth, permeated from all aspects by 5 per cent, silver

nitrate. The metallic silver has been precipitated in situ ; shows a crack extending through the enamel and into dentine permeable throughout its extent. There is increased patency of dentine tubes at the site of injury and opening up of inter- prismatic substance of enamel adjacent to the crack, x 125.

Figs. 12-14.—See Plate 35.Fig. 15.—Young marsupial cheek tooth, permeated as a thick section from all aspects by

methylene blue ; transverse section, with enamel ground transverse to the long axis of its prisms ; shows a crack in the enamel; the inter-prismatic substance adjacent is impermeable to stain. X 150.



386 E. Sprawson.

Fig. 16.—A portion of fig. 8, e, shows an occluded enamel crack which remains as an impermeable line of hypercalcification. There appears to have been a slight similar reaction in the immediately underlying dentine ; shows also the degree of tubular penetration on protected aspects. X 110.

Fig. 17.—See Plate 35.F ig. 18.—Young marsupial cheek tooth, from same section as fig. 11 ; shows two cracks

through the enamel. In one (right) the force causing the crack detached a small portion of surface enamel; this crack is permeable throughout its extent. Near the dentine the inter-prismatic substance adjacent to the crack has opened up and admitted silver; peripherally adjacent inter-prismatic substance has become occluded by hypercalcification and shows a commencing translucent zone. In the other (left), near the dentine, inter-prismatic substance adjacent to the crack has opened up and admitted silver. Peripherally, the crack is occluded and adjacent inter-prismatic substance is hypercalcified and impermeable and shows a more advanced translucent zone than does the other (right) crack. In both cracks the adjacent dentine shows local reaction, though neither crack extends into it. X 125.

Plate 35.

F ig. 10.—Young marsupial cheek tooth, permeated as a thick section from all sides by acid fuchsine; transverse section; shows a crack extending through the enamel and into dentine permeable throughout its extent. Photomicrograph in colour. X 105.

Fig. 12.—A portion of fig. 8, c. There are four cracks numbered 1, 2, 3, and 4.1. This crack extends through the enamel and into the dentine. The dentine has

reacted by forming a translucent zone of hypercalcification impermeable except centrally, where the tubes remain dilated and in direct continuity with the enamel crack, which is permeable throughout its length.

2. A crack extending through the enamel only, entirely impermeable to stain.3. A crack extending through the enamel only ; the peripheral portion is occluded.4. A crack extending through the enamel only and permeable throughout.

Photomicrograph in colour, x 105.Fig. 13.—Young marsupial cheek tooth, permeated as a thick section from all aspects by

acid fuchsine ; transverse section ; shows a crack extending through the enamel and into dentine ; the dentine has reacted as in fig. 12, crack 1 ; the enamel crack is occluded peripherally. Photomicrograph in colour. X 100.

F ig. 14.—Young marsupial lower incisor, permeated from the pulp chamber by acid fuchsine : longitudinal section ; shows a crack extending through the enamel and into dentine ; the enamel crack is occluded, the translucent zone in the dentine is complete and no longer permeable centrally. Photomicrograph in colour. X 24.

F ig. 17.—Young marsupial lower incisor, from same section as fig. 14 ; shows an occluded crack in enamel. There are permeable translucent zones in both enamel and dentine. Photomicrograph in colour, x 24.

Plate 36.

Fig. 19.—Young marsupial cheek tooth, permeated as a thick section from all aspects by acid fuchsine; transverse section; shows enamel crack permeable mesially and occluded peripherally as a linear translucency. X 70.



14



23




Fig. 20.—A portion of fig. 8 shows occluded cracks in the enamel, some of which are in continuity with permeable cracks ; the occlusions show up as linear hvpercalcification. The enamel is here ground transverse to the long axis of its prisms, x 125.

Fig. 21.—Young human enamel from a transverse section of a permanent molar, permeated by 5 per cent, silver nitrate ; metallic silver then being precipitated situ ; shows a crack extending through the enamel and into dentine, permeable throughout its extent; there is increased patency of dentine tubes at site of injury and opening up of inter-prismatic substance of enamel adjacent to the peripheral portion of the crack (compare with fig. 11). x 20.

F ig. 22.—Young human enamel from a transverse section of a premolar permeated with silver (as with fig. 21) after digestion with 1 per cent, trypsin ; shows a crack extending through the enamel into dentine. The dentine has reacted by forming a translucent zone of hypercalcification, in which the central tubes are dilated and in direct continuity with the enamel crack, which is permeable throughout its length (compare with fig. 12, crack 1). X 22.

F ig. 23.—Human enamel from a longitudinal section of a maxillary permanent incisor permeated with silver, as with fig. 21 ; shows three cracks each extending through the enamel and into dentine. In each the peripheral portion of the enamel crack is occluded to a different degree, and there has been reaction in the dentine, as in fig. 22 ; also to a different degree in each case. (Compare with fig. 13.) x 45.

Fig. 24.—Young human enamel from a transverse section of a premolar permeated from the pulp chamber by methylene blue ; shows a crack extending through the enamel and into dentine. The enamel crack is completely occluded and adjacent enamel hypercalcified; the dentine also has reacted and the translucent zone is no longer markedly permeable centrally. (Compare with figs. 14 and 17.) X 25.

Fig. 25.—Young human enamel from a transverse section of a permanent molar permeated from the pulp chamber by methylene blue ; shows a crack extending through enamel only. It is permeable mesially and occluded peripherally as a linear translucency. (Compare with fig. 12, crack 3, and fig. 19.) The author finds that some linear translucencies are developmental in origin in human enamel. X 25.



376 e. sprawson. - royal...

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