The author has examined not uncalcified histological sections (prepared by wear and tear) of normal teeth and teeth which presented carious processes in an initial stage, in order to make a comparative study of the microscopie structure of normal and pathological dental tissues. Starting from the principle that the elements constituting the tissues absorb silver salts inversely to their degree of mineralisation, the author has remarked that both in the enamel and in the normal dentin the degree of impregnation of silver nitrate differs according to the area under consideration. The silver impregnation is relatively more noticeable in the areas in which the organic substances prevail. By this method of coloration the author has observed that not all the enamel prisms do present the same chemical composition and that the organic mass is distributed in a irregular manner, depending on the area. In the sections of teeth affected by initial caries, the author has observed that the process of infiltration follows the areas mineralized to a lesser degree, which constitute real areas of minor resistence, through which the distructive process is more easily propagated. On the other hand, threfore, the topographic characteristics of the caries focus depend on the distribution of such areas. Besides the above-mentioned conditions which favour the intrusion of caries, the author considers the action of acidogenic agents, probably preceded by proteolytic germ action, which might lead, through a process of dissolution, to the destruction of the enamel prisms. From the above observations follow these prophylactic principles: a) prevent hypoplasty of the enamel by conrolling the diet of the breastfeeding and the child; b) avoid dietary factors which favour the production of fermenting acids; c) increase the resistence of the enamel apatites against decalcification.