In dentin, caries usually follows Tomes’ fibers; in interglobular spaces, it rapidly spreads parallel to the tooth surface. Globuli of calcification in these areas are often free from disease. In enamel, caries passes from the surface along lamellae to the dentino-enamel junction whence it spreads parallel to the dentin surface, thus undermining enamel. Striae of Retzius indicate borderlines between different enamel layers of calcification. The progress of enamel caries is halted by well-calcified layers, and then spreads parallel to the course of the striae.
Reference: J. Am. Den. Assoc., 18, 17, 1931.