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nerves. This structure is surrounded in both the root and crown by the dentine or tooth bone which is nourished from within. The dentine of the root is covered by cementum which receives nourishment from the membrane which attaches the root to the jaw bone. The dentine of the crown or exposed part of the tooth is covered with enamel. Tooth decay proceeds slowly through the enamel and often rapidly in the dentine, always following the minute channels toward the pulp, which may become infected before the decay actually reaches the pulp to expose it; nearly always the decay infects the pulp when it destroys the dentine covering it. When a tooth has a deep cavity of decay, the decalcified dentine has about the density of rotten wood. With an adequate improvement in nutrition, tooth decay will generally be checked provided two conditions are present: in the first place, there must be enough improvement in the quality of the saliva; and in the second, the saliva must have free access to the cavity. Of course, if the decay is removed and a filling placed in the cavity, the bacteria will be mechanically shut out. One of the most severe tests of a nutritional program, accordingly, is the test of its power to check tooth decay completely, even without fillings. There are, however, two further tests of the sufficiency of improvement of the chemical content of the saliva. If it has been sufficiently improved, bacterial growth will not only be inhibited, but the leathery decayed dentine will become mineralized from the saliva by a process similar to petrification. Note that this mineralized dentine is not vital, nor does it increase in volume and fill the cavity. When scraped with a steel instrument it frequently takes on a density like very hard wood and occasionally takes even a glassy surface. When such a tooth is placed in silver nitrate, the chemical does not penetrate this demineralized dentine, though it does rapidly penetrate the decayed dentine of a tooth extracted when decay is active
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