Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/113

Rh shaped as already described, the gold plug should be secured and consolidated piece by piece, until there is built up a mass filling every part of the vacant space with a uniform consistency of metal which, when finished, ought to present the feeling of being as hard as a piece of solid gold. The other fillings are more easily dealt with. The same careful preparatory steps are requisite in all fillings, but the insertion of the plug in amalgam and other stoppings being performed while the material is in a plastic condition, the process is rendered much more simple. The cavity should be completely filled, but not over-filled, and the amalgams ought to be used with as little mercury as is at all possible. A number of instruments are necessary for effecting all these various manipulations, but to describe them here would be as unintelligible as it appears unnecessary. Excavators, enamel cutters, burr head drills, points, pluggers, burnishers, &c., are only some of those required ; while their modes of use are either by the hand or by mechanical-apparatus, such as what are termed burring- engines, &c. Stopping may be regarded as one of the most valuable operations in modern dentistry ; and although it is no guarantee that the tooth stopped is ever after safe from the renewed attack of caries any more than its unstopped neighbours are from its original attack, yet it is surprising how few well-filled teeth are lost by cartes recommencing in the stopped cavity. Besides those already mentioned, the teeth and jaws are subject to a number of disorders and lesions which it would be out of place here to do more than enumerate. Fracture and dislocation of the teeth, ulceration and absorption of the gum, necrosis and exfoliation of the jaw, alteration in the secretions of the mouth, the deposit of tartar or salivary calculus on the teeth or in the salivary ducts, the effect of various medicines and poisonous agents on the teeth, jaws, and mouth generally, these and the like matters are all of much interest, and more or less connected with dentistry proper. But for information in regard to them the reader must be referred to the various excellent publications treat ing of them, which have appeared in considerable numbers since dental surgery has occupied more notice and taken a place as one of the recognized specialities of medicine.

Mechanical dentistry, properly so called, consists in the construction of artificial .substitutes to supply the place of lost teeth. Stopping and such like operations might also be classed with mechanical dentistry as contrasted with purely surgical treatment ; as yet, however, these matters are not quite decided ; and the day when the dental surgeon and the mechanical dentist like the ophthalmic surgeon and the optician should each occupy a separate sphere has not arrived. All that can here be given is a mere outline of the principles involved in mechanical dentistry. The subject is one comprehending a knowledge of many depart ments of mechanical science ; and to do more than indicate the nature of the various modes of construction, and the processes carried on in the manufacture of artificial teeth, would be useless and inexpedient. The removal of roots and stumps as a preparatory step in the fitting of artificial teeth is a matter to be decided by the circumstances of the case. The length of time which can be afforded for cicatrization and absorption of the alveolar walls and gum ; the presence of adjoining teeth to be left standing, especially front teeth ; the fitness of the patient for the operation of extraction, these and other circumstances must determine what amount of surgical preparation is to precede the supplying of false teeth. As a general rule, the clearer the gums are of stumps and decayed teeth the better ; but at the same time certain ad vantages, transitory as they may be, are in some instances to be derived from their retention. The jaw, gum, and teeth being then considered as in a suitable condition, the first step in the process is to obtain a plaster cast of the parts, &quot; the model,&quot; as it is termed. This is done by pressing softened beeswax or some similarly plastic composition against them until they are imbedded and leave an impression in it, giving an exact mould of the gums, remaining teeth, and all other parts on its removal. Plaster of Paris is now run into the mould so obtained, and when this is set and hardened a perfect facsimile of the structures to be fitted is the result. Any further proceedings now depend upon the mode and material in which the future artificial set is to be con structed. Every set of artificial teeth consists of represen tatives of the lost organs, modelled in a species of porcelain, and mounted upon a base adjusted to the gum and remain ing natural teeth. This base is manufactured in a variety of materials, the principal of which are (1) metal plate, of gold, platinum, silver, or different alloys; (2) vulcanized caoutchouc, or vulcanite, as it is called ; and (3) celluloid base, a composition of collodion and camphor, which has not been long enough tested as yet to rank with the other substances ; while (4) the teeth may be mounted merely with as much extraneous material as will support a pin or pivot by which they may be attached as new crowns to a root in which such pivot is firmly inserted. When it is intended that the base shall be of gold or other plate, a metal die and counter have to be made from the plaster model, between which dies the plate is embossed, and the requisite form obtained. The die and counter die are generally made the one in zinc or gun metal, the other in lead or tin; and unlike the dies from which jewellery patterns, &c., are embossed, and which may serve for thousands of times the dental dies, having served to emboss the plate for one patient, are of no further use for any other case. The plate being thus far advanced next requires to be adjusted to the mode in which the patient closes the opposing jaw or teeth against it in shutting the mouth in other words, the &quot;bite&quot; has to be taken, and the artificial teeth, which are to be mounted on the plate, arranged accordingly. Any fastenings supporting or steady ing the set have also to be adjusted; and after this, if every thing has gone well, the false set should be ready for placing in its destined locality and for use by the wearer. Should it be proposed to make the base of vulcanite, celluloid base, or a similar material, a different mode of procedure must be adopted. These materials necessitate a greater bulk of substance occupying the mouth than is the case where metal plate is employed. This, however, is in some cases an advantage since, for instance, where the gum has been greatly diminished in size through absorption, it requires some bulk of material to restore the parts to their normal size, and to give the former natural expression to the features. In preparing a vulcanite base no metal die is necessary. The base is built up in wax directly on the plaster model, and the porcelain teeth adjusted in their places, the bite and attachments being carefully attended to, as described in speaking of plate cases. The set thus made up, and presenting the exact counterpart of what the finished work is intended to be, is now, after testing it and finding it correct and perfect in the mouth, imbedded in Paris plaster as follows. A small box, or &quot; flask,&quot; as it is denominated, of iron or other metal, like one saucer inverted on the top of another, is opened and the model with the wax-built set on it is placed in the lower saucer, which is then filled up with plaster to the level of the wax set. This being allowed to harden is soaped or oiled all over its surface, and the lid of the flask, or what corres ponds to the upper saucer, is now placed upon the under poition of the flask. An opening in this covering portion enables plaster to be next poured into it till the inclosed

