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Fig. 6. Ground section, unstained, through early stage caries showing bacterial film attached to surface of tooth and indicating parallel rod and filamentous organisms (1, 2) with growing ends and fruiting heads (3) at the surface of the pile. Photographed by transmitted light which causes the white chalky enamel to appear dark. Note lines of Retzius (4) enamel prisms (5) and feathery edge at deeper part (6).

Fig. 7. Section cut through compact bacterial film pad removed from over white spot caries lesion. Note rods and filaments extending outward (2) from the cuticle (1) to which they are firmly attached and (3) fruiting heads of Leptothrix racemosa projecting at the surface.

Such acids produced by bacteria at the surface of the bacterial film or within it, are carried, as if by a sponge or wick, through the film to the cuticle through which they pass to the enamel beneath. If such acids are produced at the particular location in sufficient strength and over sufficient length of time, decalcification of the enamel occurs. This decalcification is only partial, giving rise to the softened "white spot" caries lesion. The enamel is not completely dissolved by such weak acids in the same way as it may be, experimentally, by stronger acids. For instance, the enamel is completely dissolved and disappears (except for some remaining enamel matrix material) from a tooth immersed in 10 per cent HCl for an hour or two. On the other hand the enamel on a similar tooth immersed in 0.15 per cent HCl, even for much longer time, is not dissolved in the same way. Only partial decalcification occurs, similar to the partial decalcification of the "whit spot" caries. The enamel on the tooth becomes softer and may be broken up or crushed like a piece of chalk in much the same way as that of the early stage lesion may be. Anyone interested can confirm this observation by the simple procedure of immersing (suspending) tooth specimens in solutions of acid of the suggested strengths, for several hours (several days for weak acids).

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