This name is applied to a condition which is not analogous to caries of bone, except in so far as in both there is destruction of the dense calcified structure. Caries of bone is related to inflammatory processes, but caries of the teeth has no such origin. It consists of a softening, usually progressive, of the enamel and dentine and their subsequent disintegration. The process appears to begin very commonly in places where the enamel shows normally rather deep furrows, and is therefore thinner and more easily destroyed than elsewhere. When the caries reaches the dentine it advances more freely, so that the enamel may to some extent be undermined. The lime salts are first absorbed, and then the organic basis is broken down. The caries advances in the direction of the dentinal tubules, as shown in Fig. 386.
Various views have been held as to the nature of the process. At one time it was regarded as inflammatory, but this view may be set aside, chiefly on the ground that a process exactly similar occurs in teeth which have been drawn and re-inserted, as well as in artificial teeth made of the ivory of the hippopotamus. The caries consists, in fact, of a gradual solution of the lime salts, and for this solution we must infer the existence of an acid. The secretion of the mouth is naturally alkaline, but in carious teeth an acid reaction has been detected. The acidity is often connected with derangements of the stomach, but it may have a more local origin, as when the secretion of the gums is abnormal, or perhaps when food is undergoing acid fermentation in contact with the teeth. It may be that before the teeth yield to an undue acidity they have already an abnormally weak power of resistance, and this may be related to personal peculiarities, inherited or otherwise.
At the advancing margin of the caries a widening of the dentinal tubes is visible, and in these widened tubes, as well as in the carious cavity, bacteria and Leptothrix threads are to be found. By some the process has been ascribed to the action of these organisms, especially by Klebs.
Fig. 386. - Section of tooth with caries. There are two cavities, one on each side, and in the pulp cavity a new-formation of secondary dentine. The secondary dentine is not exactly on the same level as the cavities, but at the extremities of the dentinal tubules leading from these. (After Salter).