pear to be of good order. Fig. 3 is that of a central incisor with a dense labial surface into which the silver nitrate penetrated but a very short distance. The

dark lines in the body of the enamel are not stain but are due to the lines of Retzius. Fig. 4 is that of a molar with a rough and poorly organized surface, into which the silver nitrate penetrated considerable distance.

It was seen that in cases of abrasion or partial wear of the enamel very little silver salt was taken up. This is very imperfectly shown in Fig. 5, in which abrasion had occurred in the tip of an incisor. The abraded portion took up no silver nitrate, the only color in the enamel being due to the lines of Retzius. Two very fortunate specimens were obtained, both upper cuspids which had obviously been under prophylactic treatment for some time. The surfaces of the enamel were highly polished and opalescent in appearance. Upon subjecting them to silver nitrate it was found that there was practically no silver stain in the enamel of either case. Fig 6 is a rather imperfect picture of one of these. All of the coloration in the enamel of this secion is due to the lines of Retzius as no silver nitrate is to be seen in the original.

From the foregoing data we are led to believe that the process of consolidation of the external surface of the enamel is more completely done under the action of wear or abrasion either by mastication or by prophylactic measures. This, to us, is a very significant finding as it throws light upon the production of the dense and polished enamel surface which is seen in cases of abrasion, and also the highly opalescent appearance of the enamel which results from continued prophylaxis. This change in opacity and even in color of the tooth under prophylactic treatment has long been seen by workers in that branch, and many of them have accredited it to a molecular and metabolic change in the structure of

the tooth. It is possible that such an abrasive action may produce consolidation of the dentin due to a filling up of the tubuli with calcium salts from the pulp, but experience teaches us that this seldom occurs as a result of abrasion unless the wear of the tooth has extended to the region of the enamo-dental junction and this does not occur in ordinary prophylaxis. From the evidence which is before us it is more reasonable to believe that the change in the color and optical appearance of the enamel in such cases is due to a more or less complete consolidation of their surfaces by the saliva which has been induced to continued polishing in the process of prophylaxis.

In a further consideration of this question we selected a number of teeth and polished them with disks and wheels until the enamel surfaces took on a high lustre. They were then split and onehalf of each was put in a solution of calcium carbonate and calcium phosphate in water and rocked in a tippler for several weeks. At the end of this time they were removed and, together with their controls, were subjected to silver nitrate. We found upon examination of the ground sections that the peripheral area of porosity had not been removed by polishing. That is, the polished enamel surface was still porous to a certain extent. Also that the half which had been in the calcium salt and water was unchanged from that of the control.

We then made a similar experiment in which we used calcium acid phosphate and tricalcium phosphate in a solution of saliva. This series was carried in the tippler for about one month, the salivacalcium solution being changed frequently. Upon staining and sectioning the teeth it was found that the halves which had been kept in the saliva and calcium salts were considerably less penetrated by the silver nitrate than were their controls. The experiment was not carried long enough to completely fill the exter

nal enamel spaces but such a result might reasonably have been expected had the process been continued.

It is very evident that the process of enamel consolidation cannot take place without the presence of saliva, which indicates that the mucins or globulins of that fluid have a part in the process. In this connection we observed a very curious phenomenon when making these experiments. In every case in which calcium salt and saliva was employed, concrescences were formed on the roots of each tooth which bore a striking similarity to tartar. They grew to considerable size and completely covered the roots of the teeth but did not form on the enamel in any case. In those tests in which water and calcium salts were used without saliva no excrescences were formed. Two teeth are shown in Fig. 7 which have characteristic formations.

For the purpose of determining whether it is possible for salivary calculus to form upon the root of a tooth some distance beneath the gum similar to that which is so commonly known as “serumal calculus," the following experiment was made. A single rooted tooth was imbedded in modeling compound and the material trimmed to simulate the gum margin. The compound was then warmed and the tooth moved slightly labially and lingually, thereby creating a narrow space on these two surfaces similar to pyorrhetic pockets. These pockets were not wider than the point of a small knife blade. After chilling, the whole piece was put in a test tube, covered with saliva and calcium salt and then rocked in the tippler for several weeks. The tooth and modeling compound were then removed and the labial plate of compound was split off to expose the root surface. Fig. 8 which was taken of the case at that stage shows that a thin plate-like formation of tartar was formed over two-thirds of the length of the root from the gingival downwards. This tartar formation is very similar in appear

ance to many forms of "serumal calculi" and might easily be mistaken for it if sufficiently stained by putrefaction. It is a very striking specimen and has led us to consider whether or not some of the similar formations which are so commonly seen are not more largely derived from the saliva than has formerly been supposed.

TOOTH POWDERS AND PASTES.

Our study of enamel surfaces led us to look into the various methods and materials made use of in the care of the teeth both by the operator and the patient. We found that among prophylaxis workers there exists rather a wide difference of opinion as to the methods to be used in the care of the enamel surfaces, but all agree as to the end to be sought, that of high enamel polish and a continuous cleansing of the tooth surfaces by the patient. We found, perhaps, the widest diversity as regards the matter of dentifrices which should be recommended to the patients. Of the proprietary tooth-powders and tooth-pastes which are most largely used and prescribed by the dental profession there is little information at hand regarding the ingredients of which they are composed or the specific action which they have upon the teeth. It seemed to us therefore that a study of the most commonly known varieties of mouth preparations might be of value. We therefore examined a number of dentifrices with special reference to the grits and abrasive substances which they contain and their effect when used upon the teeth. In making our report of this portion of our work we wish it to be clearly understood that the work which we have done or the report of same is not made with the intention of condemning or discriminating against any one or any group of these preparations. Our purpose is rather that of classifying their action upon the teeth in order that they may be more intelligently prescribed by the profession.