.jl 1>KNTISTRY low vessel, which may he introduced into the mouth. i'la-ter of I'aris made into paste may
- the wax. The contents of
lp are firmly pressed around the gums, an. I, if for tin- upper jaw. are made to cover f of the month ai well. An experienced -r thus nhtains in a few minutes an exact mould of the parts to which the material is ap- The teeth, if any are pre-eiit. leave faithfully impressed in their true
- d the cavitie- U-twe.-n are repre-
- >ondiii!r projections in the
- hardened pla-t.-r. The impression re-
tVom the mouth serves to furnish a ..f the jaw, which may he taken in plas- ter of I'ari- also. This i- used as a pattern in moulding -and, and a cast is then obtained in any metal, as for in-tance zinc ; and by pour- ited lead upon the /inc, which is turned I.MII its face and surrounded with a brass or iron collar for retaining the lead, a mould in thi- metal is obtained precisely like the ori- ginal one in wax. By means of the zinc cast and leud mould, the exact shape of the parts is transferred to the sheet of gold or other metal, this being placed between the two, and made, by hammering and swaging, to assume all their irregularities of surface. The fit is the more readily made if the teeth have been cutoff from the plaster model before making the me- tallic casts. A duplicate plaster cast serves to give the position of those teeth to which the plate is to be finally fitted. A variety of ma- terials have been experimented upon, in which to securely imbed the bases of the teeth. Gutta percha has been used to contain them ; but its texture and strength were in a short time destroyed by the action of the fluids of the mouth. It was then applied vulcanized or mixed with sulphur; and caoutchouc is em- ployed in the same way. These prove to be important auxiliaries in mechanical dentistry, My for temporary sets of teeth. They do not, however, readily take the colors which may be applied to more suitable substances. i the process Catted continuous gum was ! by IM-. .liihn Allen, professor in the 'liege of dental surgery. In this a sili- mp"und, similar in composition to that of which the teeth are made, but more fusible, is applied in the form of a paste over the ' the hack of the teeth, and also Vi.nt. so as entirely to bury the ends of the natural ones are buried in the
- MIIII<. To withstand the hiirh degree of heat
hakit.-/ this upon the plate, plati- num is substituted for pold. Platinum has the advantage of forming at a high onion with the silieious compound Which is -;,rvad ov, r the lin-ual side of the - .-ll as over the l,a-es of the teeth. When thoroughly dry. the work is baked at a white heat in the muthV of an assayiii" furnace A new application of the paste is then made 11 "I* 1 " Mtted by shrinking and upon tin- coating are made numerous ridges DENTITION ' and depressions with the spatula, which, when afterward covered with the coloring enamel, cause this to assume different shades of the color, and present the appearance of the nat- ural gums. The baking is repeated, and after this the coating of coloring matter, called the irum enamel, is applied, when a third baking completes the process, by which a proper degree of hardness and a natural color are produced. The compositions used are empirical mixtures of pure silica and feldspar, with a suitable flux to produce a fusible compound, possess- ing sufficient strength, hardness, and perma- nency of character. The work can easily be repaired when broken, or alterations made when required by changes in the mouth, by building upon it more of the paste and again baking; in this way even the length of the artificial teeth can be increased and new ones introduced. In the same way the artificial processes called cheek restorers were applied by Dr. Allen, which are projecting portions built upon the artificial gums far back in the mouth, and serve to distend the cheeks when these are fallen in. The mechanical operations con- nected with the work have led to increased knowledge in the use of plastic compounds, and introduced improved methods of treating the metals employed. DENTITION. In all the higher animals the teeth are developed directly from the mucous membrane, and are therefore, like hair, nails, feathers, &c., appendages of the skin, and form no part of the true osseous system. As early as the fifth week of foetal life, according to the observations of Prof. Goodsir, a deep, narrow groove between the lip and the rudimentary palate in the upper jaw indicates the future situation of the teeth. Within the next three weeks papillas developed at the bottom of the groove become the germs of the future milk or temporary teeth. In the progress of develop- ment the papillae are enveloped in open folli- cles, and these again are converted into shut sacs; contemporaneously with these changes, the edges of the dental groove are themselves growing, so that by the 14th week they meet, enclosing the tooth sacs. Within the sacs the papillary pulp is gradually converted into den- tine, of which the body of the tooth is com- posed, while the enamel is probably formed by calcification of the epithelium abundantly pro- duced from the inner surface of the sacs. As teeth are required before the jaws have at- tained their growth, and yet from their struc- ture are incapable of enlarging pari passu with the bones in which they are placed, provision is made for a temporary set, which when they have served their purpose are replaced by the permanent teeth. As early as the 14th week minute crescentic depressions of mucous mem- brane may be discovered above and at the in- ner part of the opercula of the milk teeth ; these depressions soon become converted into minute compressed sacs, which gradually sink behind and below the sacs of the milk teeth, and
