equal that of the cast metal. In wide-mouthed cavities the filling is secured by being built upon plugging carefully introduced into the cavities of the roots, and also by lateral pins of the gold filling made to enter from this into little holes or grooves drilled for the purpose into the walls of the tooth. In order to destroy the sensitiveness of the exposed nerve, Dr. Spooner of Montreal first recommended the use of arsenic, and this was made public in a treatise of his brother, Dr. S. Spooner of New York. For killing the nerve and for deadening the sensitiveness of the bone of the tooth, when but a thin shell of this covers the nerve, it is now used in combination with sulphate of morphine and tannin, in the proportion of 3 parts of each of the last two named to 1 part of arsenic. A portion of a grain weight of the mixture is introduced into the cavity, and allowed to remain long enough to accomplish the object. It has been generally considered impracticable to preserve a tooth when the decay has reached into the internal or pulp cavity. In this condition inflammation often takes place at the root, and matter collects, forming an ulcer between the periosteum of the tooth and the bone. If the discharge of this be stopped by filling the cavity, the matter will find its way through the gum, causing a gum boil near the root; or it produces inflammation of the face, often attended with great suffering, which is relieved only by the removal of the tooth. The modern treatment is to perforate the sac at the root by a fine drill passed through the cavity; and if the nerve be sensitive, this is cut out and removed by a delicate steel wire furnished with a hook at the end, so small that it can pass freely into the nerve cavity. A weak solution of nitrate of silver is then forcibly injected into the cavity, the curved beak of the syringe being surrounded with wax, which is crowded into the tooth to prevent the escape of the caustic solution. This is immediately washed out, and a piece of the solid caustic is introduced into the lanced gum boil, which connects with the diseased fang. The unhealthy portions are thus cauterized, and after creosote introduced into the cavity has remained a few days, and healthy action has taken place, the tooth may be safely filled, with the liability of further trouble from the same cause greatly reduced. -The only unobjectionable material for filling teeth is gold foil or the sponge gold specially prepared for this purpose. The latter excellent material is produced by dissolving gold free from copper in nitro-hydrochloric acid, placing the solution in a flat-bottomed vessel, and heating and precipitating by strong solution of oxalic acid. In a few hours the gold is wholly deposited, and the supernatant liquid may be decanted off, taking care not to disturb the gold at the bottom. The vessel is then several times filled with boiling water and decanted, until the last washings contain no more oxalic acid. The gold is now carefully slipped on to a piece of filtering paper, and by means of a spatula gently pressed into the form of the desired cake, but a little

thicker. It is then removed to a porcelain crucible, and heated for a short time, somewhat below a red heat, when it shrinks and becomes coherent. This process was described to the London chemical society by Mr. D. Forbes in 1857. Tin foil may be used, and its malleability and cheapness well adapt it for large and badly shaped cavities, and for temporary fillings in sensitive teeth, but it is liable to oxidize and produce discoloration. The mineral cement, or amalgam of mercury, silver, and tin, has been found by some dentists a very convenient material for inserting in its soft, fresh state into cavities, particularly when these are large, badly shaped, and the tooth very sensitive. It very soon sets, forming an imperishable and exceedingly hard filling. Many dentists approve of its occasional use, provided it is composed of chemically pure materials. Others loudly condemn it, asserting that it is liable to occasion salivation, inflammation of the periosteum, and of the gums and mucous membrane.-The extraction of the teeth is an important branch of dental practice-safe and easy with good instruments in skilful hands, but, as practised by the unprofessional operator, not a little hazardous. This is apparent from the cases recorded of the extraction in one instance of 6, and in another 5 teeth at once, together with the tearing away of the anterior and posterior plates of the cavity called the antrum in the roof of the mouth, which occurred in one case in Virginia, and in the other in North Carolina, a blacksmith being the operator in each case. Ambroise Paré particularly notices the danger of inexpert operators attempting to extract teeth. Speaking of his "tooth mullets," he observes: "Unless the person knows readily and cunningly how to use them, he can scarcely so carry himself but that he will force out 3 teeth at once." The improved instruments of modern times, however, have greatly lessened this risk, and pain is avoided by the use of anaesthetic agents, if necessary, or by the application of the galvanic current. In this process, which is a late American discovery, the current from the common electro-magnetic machine used for medical purposes is caused to pass through a wire, one end of which is held in the hand of the patient, and the other is attached to the instrument. This end, however, is broken, and is closed only when the operator has fixed the instrument and is ready to draw the tooth. He then presses with his foot upon a board which closes the circuit, causing a slight shock to pass through the jaw. Unless the gum is inflamed and sore, the effect is almost always to benumb the nerves, so that the tooth is removed with little or no pain. The last department of dentistry to be noticed, is the construction and application of artificial teeth. These were formerly carved from ivory of the tusk of the elephant or the tooth of the hippopotamus. They were obtained also by altering the shape of the teeth of neat cattle, of sheep, &c.; and the crowns of human teeth were often conveniently ingrafted upon the roots

of the original front teeth. All these materials are objectionable from their susceptibility to the action of the fluids of the mouth; ivory soon becomes offensive from being saturated with these fluids, and all of them are liable to decay, inducing at the same time disease in the sound teeth remaining. Porcelain teeth, well named incorruptible, perfectly resist the corrosive action of the fluids of the mouth; and as they are now manufactured, they imitate so perfectly in color and animated appearance the natural teeth, that they are often not easily distinguished from them, unless their superior beauty excite suspicion of their true character. Though of French invention, they owe their present perfection in great part to the skill of American dentists. But their manufacture being a distinct art from that which properly pertains to dentistry, and requiring peculiar skill and experience, it is now mostly carried on by persons not belonging to the profession. Single teeth are thus prepared of all the varieties that may be required, and sold to the dentists at very low prices. The first experiments in this branch in the United States appear to have been made by Mr. Charles W. Peale of Philadelphia, who, compelled at an early age to use artificial teeth of ivory, and seeing in the newspapers about 1807 an account of mineral teeth, procured pure qualities of clay and silex, and, constructing his own furnace and muffles, moulded the artificial teeth and gums in wax, made casts in plaster and brass, and swaging the gold plates, fitted them with platinum wire and gold springs of his own manufacture, and produced sets for himself and some of his friends. He made a set for his son, Rembrandt Peale, in 1808, and gave instructions in his methods to Mr. Barabino, a dentist then practising in Philadelphia. The first regular manufacturers were Greenwood, Woffendale, and Parkhurst, who were engaged in the business about 1825. French artificial teeth were made in Philadelphia from 1827 to 1830, by Plantau and McHenry. In 1829 Mr. Villers came to New York from England, bringing with him mineral teeth, which he claimed to have made by a process he discovered in 1819. The proportions of the ingredients he used in 1830 are similar to those employed at present, viz.: 4 ounces of feldspar, 1 of crystallized quartz, and 12 grains of kaolin. The manufacture of mineral teeth for the supply of dentists was first undertaken by Mr. Daniel W. Stockton, in Philadephia, about the year 1835; and to him, together with Neal and Alcock, is due the credit of establishing this branch of business in the United States. Other persons followed their example, until in the different cities of the United States there are no less than 9 manufactories devoted to this business, the aggregate annual production of which is over 2,000,000 teeth. The consumption of platinum for the little pins which fasten these upon the plates is estimated at about $40,000 worth annually. They produce both single teeth and teeth in blocks of several.-Various methods of securing artificial teeth in their places have been

in use. So long ago as 400 years before the Christian era, they were fastened by ligatures of flax and of silk, and with wire of gold and of silver, to the natural teeth that remained. The insertion of pivots of gold into the roots of the natural teeth for attaching to these artificial crowns was long since practised, and platinum and hickory wood have been substituted for gold. In modern times metallic clasps, spiral springs, and fastenings of gutta percha and of caoutchouc, have been used for this purpose; and a dentist of London has lately contrived a covering of vulcanized caoutchouc to slip over the wires, to protect the natural teeth from their wearing action. But the most perfect method is to secure the teeth, either in whole or partial sets, to a plate of gold or other metal, which is so accurately fitted to the gums that it is firmly retained by atmospheric pressure. Plate-work involves several distinct operations. The first object is to obtain, in some hard metal, an exact model of the mouth in which the plate is to be fitted. For this purpose, yellow or white wax, free from mixture of grease, and softened by warm water, is placed in a shallow vessel, called an impression cup, which may be introduced into the mouth. Plaster of Paris made into paste may be substituted for the wax. The contents of the cup are firmly pressed around the gums, and, if for the upper jaw, are made to cover the roof of the mouth as well. With care, an experienced operator thus obtains in a few minutes an exact mould of the parts to which the material is applied. The teeth, if any are present, leave their forms faithfully impressed in their true positions, and the cavities between are represented by corresponding projections in the wax or hardened plaster. The impression removed from the mouth serves to furnish a model of the jaw, which may be taken in plaster of Paris also. This is used as a pattern in moulding sand, and a cast is then obtained in any metal, as, for instance, zinc; and by pouring melted lead upon the zinc, which is turned over upon its face and surrounded with a brass or iron collar for retaining the lead, a mould in this metal is obtained precisely like the original one in wax; or the same may perhaps be more surely secured by pressing the cast into the surface of melted lead, and holding it till the lead cools. By means of the zinc cast and lead 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, from the plaster model, the teeth have been cut off before making the metallic casts. A duplicate plaster cast serves to give the position of those teeth to which the plate is to be finally fitted. The edges of the plate are cut to fit between the teeth, and where there are none, they bend over the rounded gum with its exact curve. The metal commonly used for the plate is gold, more or less alloyed; for cheap sets silver is sometimes

substituted. When it has received its proper shape, a ridge of wax is laid around the lingual side, and it is placed in the mouth, in order that the patient, by opening and shutting the jaws, may indent the impressions of the teeth in the opposite jaw, which will indicate the arrangement and length of the artificial ones to be attached to the plate. The plate with the wax is then taken out of the mouth, and brushed over with sweet oil. It is then laid upon its face, and plaster is poured over it, imbedding in it a wire frame, which is bent around somewhat in the form of the jaw, and projects behind the plaster. When the plaster is hardened, another portion is poured over the other side, and in this a similar wire is introduced. The oil prevents adherence of the plaster to the plate and wax. The two frames being hinged together, and set at the right distance apart, represent the two jaws; they are called the articulator, and are used for temporarily holding the artificial teeth, while they are selected and placed in their correct positions, with reference particularly to their articulating properly with those in the opposite jaw. A little wax laid upon the alveolar ridge of the plaster model causes the teeth to adhere as they are pressed back upon it. In the same way they are held when transferred to the plate, upon which the ridge of wax lies behind them. To secure them and admit of the removal of the wax, the front portion of the plate is imbedded in a mixture of plaster of Paris and either sand or asbestus, which mixture entirely covers the fronts of the teeth. The wax can then be taken away, and the backs of the teeth be exposed without displacing them. The next step is to fasten these by a lining of the same metal as the plate attached to each tooth by the little pin in its back, and to the plate by soldering. This being securely done, the plate and teeth thoroughly cleansed, and the metal polished, it is ready for the mouth. -The numerous joints and open spaces, which were liable to catch and retain particles of food, formerly rendered this work very imperfect. The improvements which have brought it to its present state of perfection are due to the skill of American dentists. A variety of materials 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 employed in the same way. These prove to be important auxiliaries in mechanical dentistry, especially for temporary sets of teeth. They do not, however, readily take the colors which may be applied to more suitable substances. A method has recently been invented by Dr. A. A. Blandy of Baltimore, and called by him the cheoplastic, by which the teeth are secured in an alloy of great fusibility, and not liable to change its form in cooling. By the specification this alloy may consist of 10 to 20 parts of tin. 1 to 5 of bis

muth, 1 to 4 of antimony, and 1 to 2 of silver; to these 5 to 15 parts of cadmium may be added to increase the strength of the alloy. In case an unpleasant taste should be experienced from the mixture, it may be electro-gilded. This recommendation implies a possibility of the alloy being affected by the action of the fluids of the mouth, and time is required for its capability of resisting this action to be fully tested. The alloy is to be run into a mould of the gums, and the plate is thus cast instead of being shaped by swaging. The teeth, previously arranged in the mould, are secured in their places by this melting of the alloy around them.Another process lately introduced is to dispense with metallic plates, and mould the mineral paste into complete sets, as in making small blocks of teeth. This method is objectionable from the difficulty of procuring a perfect fit, and the liability to entire loss of the set by accidental fracture.-In 1851 the process called continuous gum was invented by Dr. John Allen, late professor in the Ohio college of dental surgery. In this a silicious compound, similar in composition to that of which the teeth are made, but more fusible, is applied in the form of a paste over the fastenings at the back of the teeth, and also in the front, so as entirely to bury the ends of the teeth, as the natural ones are buried in the gums. To withstand the high degree of heat requisite for baking this upon the plate, platinum is substituted for gold. Platinum has beside the advantage of forming at a high heat a close union with the silicious compound, which is spread over the lingual side of the plate as well as over the bases of the teeth. When thoroughly dry, the work is baked at a white heat in the muffle of an assaying furnace. A new application of the paste is then made to fill all the crevices caused by shrinking, and upon this coating are made numerous ridges 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 veins, rugæ, &c., seen upon the natural gums and roof of the mouth. The baking is repeated, and after this the coating of coloring matter, called the gum enamel, is applied, when a third baking completes the process. The process claimed by Dr. Hunter is similar to that above described. The compositions made use of are empirical mixtures of pure silica and feldspar, with suitable flux to produce a fusible compound, possessing sufficient strength, hardness, and permanency 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 composition of

the paste for the gums has been perfected by Dr. E. A. L. Roberts of New York, who has largely manufactured it for Dr. Allen, and given it increased density and strength, overcoming in a great measure its brittleness, and causing it most effectually to resist the action of the fluids of the mouth. In the application of these artificial substitutes various kinds of professional talent and mechanical skill are called into play; and the operator, in order to give the natural expression to the mouth, imitating the true colors and proportions of the teeth and of the gums, must even possess a certain degree of that genius and taste which guide the pencil of the artist or the chisel of the sculptor. The mechanical operations connected with the work have led to increased knowledge in the use of plastic compounds, and introduced improved methods of treating the metals employed. (See BLOWPIPE, FURNACE.)

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 5th week of fœtal 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 3 weeks papilla developed at the bottom of the groove become the germs of the future milk or temporary teeth. In the progress of development the papillæ are enveloped in open follicles, 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 dentine, of which the body of the tooth is composed, while the enamel is formed from a separate pulp connected with the opercula of the sacs. (See DENTISTRY.) As teeth are required before the jaws have attained their growth, and yet from their structure 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 membrane may be discovered above and at the inner part of the opercula of the milk teeth; these depressions soon become converted into minute compressed sacs, which gradually sink behind and below the sac of the milk teeth, and in these sacs are developed the first 10 permanent teeth of each jaw; the other 6 are developed in sacs placed posterior to those of the last milk teeth, which are formed in a manner precisely similar to those of the milk teeth themselves. The ossification of the permanent teeth commences a little before birth with that of the first molar, and proceeds during the first 3 years of infancy successively in the incisors, the canines, and the bicuspids. The approach of

the time for the eruption of the temporary teeth is announced by an increased secretion of saliva. In the earlier months of infancy the mouth is comparatively dry, but as the teeth shoot into the gums the mouth becomes moist and the child begins to drivel. The progress of dentition is not apparently continuous, but after the eruption of each successive pair a pause of one or two months generally follows. The central incisors commonly pierce the gum in the course of the 7th month after birth, those of the lower jaw preceding the upper ones by a short interval; between the 7th and 10th months the lateral incisors make their appearance; from the 12th to the 14th month the anterior molars, and between the 14th and 20th the canines are cut; and the first dentition is completed between the 18th and 36th months by the protrusion of the posterior molars. Both the time and the order of appearance of the first set of teeth admit of a good deal of variation, their progress being hastened or delayed sometimes for a period of 6 or 7 months, by a lateral incisor, or even a molar or canine tooth, cutting the gum before the appearance of the central incisors. The period of primary dentition is one looked forward to with much anxiety, and is justly regarded as attended with increased risk to the life of the infant. Statistical inquiries show that during its continuance the proportionate mortality becomes much increased, and in the bills of mortality numerous deaths are ascribed to teething alone. It must be remembered, however, that at this time all the functions of the young being are in a state of great activity, and that teething is but one in a series of changes by which the infant is prepared to substitute for the milk provided by its mother, food suitable to the conditions of its future existence. In a healthy infant dentition in itself is attended with little inconvenience and no danger; when the teeth come to distend and stretch the mucous membrane lining the gums, there is probably a little tenderness and pain, some fretfulness, and perhaps slight febrile excitement; but in the absence of other causes of disease, this soon passes over; if, however, the nervous system is unduly excitable, dentition may seriously complicate other maladies. When the process of dentition is advancing normally, it should never be interfered with; when the gum is red, swollen, and painful, scarification may be resorted to with advantage, and may be repeated if necessary, the trifling loss of blood affording relief to the inflamed gum. When the tooth is evidently about to pierce the gum, if the child appears to suffer, it may be freed by cutting down to it with the gum lancet. In cases where convulsions supervene suddenly without an evident cause, if dentition is proceeding actively and the gums are tense and swollen, the gum lancet may be resorted to. In the convulsive affections which take place in children whose nervous systems have been rendered irritable by improper diet, or an impure or vitiated atmosphere, the late Dr. Mar