. The principles and practice of dental surgery. ity acts well at first, and may be useful for thetemporary purpose of retaining a plate, until the changes ofwhich the mouth is capable, adapt it more perfectly to theplate; but for permanent adhesion, the only reliable applica-tion of the atmospheric-pressure principle, is, the vacuum ofcontact, to be found only in well fitting plates. In conclusion, we remark, that a vacuum cavity acting assuch, gradually draws the gum into it, and finally fills it by amore or less permanent enlargement; (or if the shape of thecavity is such that it is impossi
. The principles and practice of dental surgery. ity acts well at first, and may be useful for thetemporary purpose of retaining a plate, until the changes ofwhich the mouth is capable, adapt it more perfectly to theplate; but for permanent adhesion, the only reliable applica-tion of the atmospheric-pressure principle, is, the vacuum ofcontact, to be found only in well fitting plates. In conclusion, we remark, that a vacuum cavity acting assuch, gradually draws the gum into it, and finally fills it by amore or less permanent enlargement; (or if the shape of thecavity is such that it is impossible to fill it, the irritation is aptto excite morbid action,) and that, when thus filled, the plate isthen retained, solely, by the vacuum of contact. But when acavity, intended to hold up a plate, leaves no prominence ormark in the mouth, it unmistakably proves that it is exertingno force ; it then diminishes the force of adhesion by the pres-ence of air, and has no compensating advantage, except in TEETH RETAINED BY ATMOSPHERIC PRESSURE. 721. removing pressure from a hard central ridge, and thus lesseningthe tendency of the plate to rock. The engraving, Fig. 248, represents the appearance of adental substitute for the upper teeth. The difference betweenthe plate applied upon this principle p,Q 348 and one with spiral springs, is, thatthe former is rather wider than thelatter, covering more of the roof,so as to give a larger surface forthe pressure of the covers the whole of the outersurface of the alveolar ridge, anda considerable portion of the roofof the mouth ; but it should not goas far back or run so high up as -- some dentists are in the habit of extending it. If allowed tocover those parts of the membrane, which cover the insertion ofthe cheek muscles, on the outside of the ridge or the palatemuscles at the back of the mouth, the gums will be chafed orulcerated, the patient nauseated, and the piece rendered unstableby the action of the muscles. Unless it
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