Lectures on operative dental surgery and therapeutics . rusion of moisture. On the selectionof the rubber, much will depend as to the comfort andconvenient manner in which the operation may be per-formed. If too thick, the collar will not be easilyreversed in its position and carried to the pointdesired, besides, it greatly complicates matters by anexcess of material which must necessarily encroach uponterritory that should be left free. On the other hand,should the rubber be too thin, the power of the distendedportion would be so abridged as to diminish the collar in size,and consequently its
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Lectures on operative dental surgery and therapeutics . rusion of moisture. On the selectionof the rubber, much will depend as to the comfort andconvenient manner in which the operation may be per-formed. If too thick, the collar will not be easilyreversed in its position and carried to the pointdesired, besides, it greatly complicates matters by anexcess of material which must necessarily encroach uponterritory that should be left free. On the other hand, should the rubber be too thin, the power of the distendedportion would be so abridged as to diminish the collar in size, and consequently its tenacity in resisting the aggressivefluids ; also, the substance of the material between theteeth would be so attenuated, that the slightest mis- 74 calculation in leaving a deficiency between the originalperforations, would greatly aggravate the case. The rubber, when adjusted over convex portions of atooth, extends in a direct line, from point to point, acrossany intervening concave surface, leaving a small segmentalspace unprotected (see Fig. 26.). Fig. 26. Tlie dotted line B points to an interval—a versed sine, as it were—between the rubber and tooth. When a cavity extends to or near the gum, it becomesnecessary to depress the rubber to the required curve by theuse of spring appliances. The depression being downwardsonly, and the tooth below the cavity concave upon its sur-face, the direct downward pressure will not fill this lateralretrocession, and an opening for the admission of the fluidsof the mouth will be left, unless an available compressagainst the base of the concavity is devised. In some cases, cotton moistened with sandrac varnish may be packedunderneath the rubber, and a spring appliance used topress upon the cotton, and against the cervical walls ofthe tooth. To illustrate such a cavity, Fig. 26 representsthe ante-proximate concave, cervical wall of a molartooth. Referring to Fig. 27, the difficulties to be surmountedare at once apparent, as the tendency o