The lacunæ present all sorts of peculiar and irregular forms, particularly in the thicker portion of the cement. They are elongated in the direction of the lamella or long axis of the tooth.

The canaliculi or, as they are sometimes called, processes of the lacunæ, are most abundantly given off at right angles to the lamellæ, and are for the most part directed towards the outer surface than towards dentine. Many of the canaliculi are of great length and of large diameter. The canaliculi of neighbouring lacunæ anastomose freely with each other, and establish a network of communication throughout the whole body of the cementum. And, furthermore, the canaliculi are occasionally connected with the terminal branches of the dentinal tubes.

In the living condition these lacunæ are not lacunæ, or spaces, literally, neither are they so in bone; but they are filled or occupied with the remnant of the formative osteoblasts (see Fig. 3). In bone, these osteoblastic remnants occupying the lacunæ are protoplasmic; whereas in cementum - the majority of them are protoplasmic, while in many instances those remnants partake of the nature of uncalcified formed material. There are, therefore, no spaces or "lacunæ " in the living state, and those negative histological elements are to be found only in sections of dried bone and cementum (see Fig. 4).

FIG. 3.-This is from a drawing of human cementum, showing the protoplasmic contents of the "lacunæ." These are identical in appearance with the contents of the lacunæ of the adjacent bone (not shown). At P is seen a portion of the periosteum with its protoplasm-osteoblasts. The specimen is a portion of a tooth and jaw, and, being decalcified, the canaliculi and laminæ are not visible in the section.

FIG. 4 represents a section of dried human cementum. The black lacunæ with the radiating canaliculi are distinctly shown, so are the laminæ. D, dentine; c, cementum; P, periosteum.

The lamination of the cementum is parallel to the long axis of the tooth. In transverse section this lamination is seen to constitute a series of concentric rings around the dentine or tooth substance. The outer layer is denser than the subjacent portions, and is quite devoid of lacunæ. On the surface the tissue is finely nodular, and like unto an infinite number of minute and perfectly fused globules. When we come to the development of the tooth-tissues the cause of this will be made apparent.

The inner surface of the cementum is closely applied to, and inseparably connected with, the dentine of the root.

Where the cementum is thin, as where it approaches the neck of the tooth, neither lacunæ nor canaliculi are present. Indeed, the lamination is scarcely visible, so that it has an almost structureless appearance.

Sharpey's fibres are also to be found in cementum. This fact, their origin, significance, and the manner of demonstrating them, I pointed out in Lecture II.

The foregoing description of human cementum may be regarded as that of a type tissue. But, as already stated, there are to be found numerous modifications of that structure in the animal kingdom; and in many instances the analogous and homological tissue has little histological resemblance to the "type-tissue"

Dental Surgery and Medicine.

SECONDARY HARD FORMATIONS IN PULP CAVITY— THEIR PHYSIOLOGY AND PATHOLOGICAL SIGNIFICANCE.

A paper read before the Odonto-Chirurgical Society of Scotland on February 10th, 1881.

By GEORGE W. WATSON, L.D.S. Edin.,

Surgeon to the Edinburgh Dental Hospital, and Lecturer on Dental Surgery and Pathology at the Edinburgh Dental Hospital School.

On the completion and calcification of a tooth, the central vascular and nervous mass called the pulp is completely enclosed to the extremity of its root in hard tissues; but, nevertheless, this pulp contains in itself all the elements (in the odontoblast cells) for the future development of secondary

dentine, and we can therefore readily understand the frequent occurrence of these formations in the pulp cavities of teeth. These new growths differ somewhat in their anatomical characters from that of normal dentine, and are divisible into varieties. They were first recognised by anatomists of the previous century. J. Hunter (Natural History of the Teeth') says in regard to them :-" In teeth which are worn away by attrition that portion of the pulp cavity adjacent to the abraded surface becomes filled with a new substance, which occupies the centre of the part worn away, and is generally softer than the rest of the tissue of the tooth." Bell, Rousseau, Bertin, Owen, and others, met with these hard formations, both in human teeth and those of animals, but no one gave a minute anatomical description of them till Mr. Tomes took the subject up. Mr. Salter, in 'Guy's Hospital Reports' for 1853, treats of osteo-dentinal formamations in addition to simple calcification of the pulp and granular calcareous deposits, and subdivides them, according to the anatomical characteristics they exhibit, into dentine of repair, dentine excrescence, and osteo-dentine, which excellent arrangement I shall keep by in describing them (diagram shown explanatory of this arrangement):-1st. Dentine of repair. As we have already seen, this variety was recognised and known more than 100 years ago, but only as a result of abrasion, and is very interesting to us as showing the vis medicatrix naturæ exhibited by such hard tissues as dentine. This particular form of secondary dentinal deposit can be studied by making careful sections of teeth, incisors, canines, or premolars, affected by chronic caries, abrasion, or erosion. From the tabulated list of 1000 teeth which I have cut up and examined (teeth, I may remark, obtained from the Dental and other hospitals, the ages of patients from whom they were extracted varying from youth to old age) I find dentine of repair to be pretty prevalent, especially in teeth of a hard texture. Soft, carious teeth, unless prepared for and protected by a single filling, owing to the rapidity of their disintegration, get very little chance to deposit a layer of secondary repair tissue. The filling, on the other hand, while protecting from the oral fluids, sets up a slight irritation in that part of the pulp opposite the lesion, which, in the majority of cases, results in the renewed activity of the odontoblast cells, and the formation of a layer of secondary dentine, thus giving the tooth some chance to recover itself. Dentine of repair, then, is produced when, as a result of injury or disease, a portion of the protecting cap of enamel is removed, thereby exposing the underlying highly sensitive dentine to irritation-this

impression being, doubtless, conveyed to the pulp by the bioplasm contained in the dentinal tubules exciting the odontoblast cells to renewed productivity and the formation of compensatory tissue. This is not, however, invariably the case, as you will see from the specimens handed round The amount of this tissue deposited is proportionate to the extent of the injury at any particular part, and registers its progress by distinct laminations. It is always adherent to, and in direct continuity with, the primary dentine, and always forms on that part of the pulp cavity next the lesion, is separable from the pulp, distinguishing it from osteo-dentinal deposits, which are invariably found entangled in the pulp, and, like excrescence, generally commence towards the radical end of the tooth. Repair tissue was thought for a considerable time to occur only in teeth of elderly persons, very much worn. This however, has been proved to be incorrect by Mr. Salter, who has shown that this pathological condition may be present as a result of disease or injury at any age, which accords with my own observations on the subject. That peculiar condition of the dentine of the roots of some teeth, called horny dentine, and produced by chronic periostitis, is also an indication of the formation of similar tissue, consisting, as it does, of a filling up of the dentinal tubules with secondary dentine, giving it a yellowish and semi-transparent appearance (microscopic section shown). In teeth worn down by attrition, and in the translucent zone of caries, the same condition exists. As you will observe from the specimens passed round, the amount deposited varies considerably. When the amount is large, and there are present other varieties of secondary dentine, coalescence takes place, and the pulp cavity becomes obliterated. Dentine of repair, as an effort of nature to limit diseased action, is extremely interesting to us in view of pulp capping, which I think should be more universally adopted, instead of devitalising the pulp on the slightest exposure, as is too often done I am afraid, and thus very considerably limiting the usefulness of the tooth for future work. The subject of repair tissue compensatory of disease or injury to the teeth might, I think, form a very good and profitable subject for discussion at some of our future meetings.

Dentine excrescence is a much rarer condition than the previous, and, by rights, should be classified under odontomes or tumours of the hard tissue of the teeth. This variety consists of little nodules of secondary dentine, varying very much in form and number, attached to the walls of the pulp cavity of teeth, which may be otherwise healthy and free from disease. This condition is present likewise in the roots of

some teeth, the crowns of which are the subject of some lesion, whether the result of injury or disease. Their presence in the pulp cavity seldom causes any inconvenience, but, as Mr. Salter and others have shown, they are sometimes associated with neuralgia of a very severe character. Mr. Salter figures and describes (in his Dental Surgery and Pathology') a very good specimen of this lesion, which is very similar to one I have been fortunate enough to come across. It occurred in an upper wisdom tooth, the history of which I shall just detail. The patient, a military gentleman, aged about fifty, called on me, suffering from severe neuralgia, supraorbital and temporal, which he thought might be caused by a third upper molar. It was carious, but not to any great extent, on the masticating surface, and painful on tapping with a steel instrument. He wished it extracted, which I did. This did not cure it at once, as he experienced great pain of a neuralgic character for a considerable time after extraction. On making a longitudinal section of the tooth, I found the pulp cavity very small, and an oval excrescence growing from the side of pulp chamber, and still further narrowing its space and pressing aside the pulp. As you will observe, gentlemen, from the tooth which I herewith hand round, there is a carious cavity in the crown, which has not penetrated very far, having been arrested by the calcification of the contents of tubules, producing caries carbonacea of old writers. The excrescence is semi-transparent, and of a yellow colour; the tubules nearly to the full extent of the excrescence pass upwards towards the neck of the tooth, presenting the same appearance and colour, which is due to calcification of their contents. That this excrescence, pressing on the highly sensitive pulp, was the cause of the neuralgic pain I have not the slightest doubt, as the pain ceased entirely an hour or two after the tooth's removal. Any inflammation of the pulp, however slight, we can readily understand, would, by the enlargement thereby produced, crush the delicate fasciculi of nerve fibres against the sharp and prominent excrescence, and lead to the reflex nervous pain complained of. Another specimen of excrescence, occurring in an inferior premolar, I hand round for your inspection. It was extracted about a month ago from the mouth of a young woman, aged about twenty-three, and was associated with severe intermittent neuralgia, which had continued for a considerable time, though various remedies had been tried. As you will observe, the inner cusp and a considerable portion of the crown is destroyed by caries exposing the pulp, which I found infiltrated with pus. On examining the pulp cavity, there was found growing from the lingual wall a flattened and sharp