MICROSTRUCTURAL CHANGES IN EARLY CARIES 179 



formed the basis for the explanation of Darhng's ( 1956) observation 

 that the dark zone disappeared in aqueous media and his later 

 demonstration ( 1958 ) that in an aqueous medium with a refractive 

 index similar to that of enamel all the form birefringence of the 

 carious lesion was abolished for all practical purposes. This per- 

 mitted direct observations of the intrinsic birefringence of the enamel 

 in the various zones as a measure of demineralization, and these 

 were found to correspond to the general pattern of the microradio- 

 graphic observations ( Darling, 1956 ) . 



Using the same principles of form birefringence, it was found 

 possible to estimate the amount of spaces in each zone (Darling, 

 1958). Later, with the demonstration by Poole et al. (1961) and 

 Darling et al. (1961) that the spaces varv considerably in size so 

 that they vary in their capacit\' to receive the molecules of mounting 

 media, it became possible to assess the size of the spaces also, though 

 this could only be expressed in terms of the molecules which they 

 would admit. It is obvious that the new evidence disturbs many 

 previous theories, but so far there has been little objection to it. The 

 zones of dispute previouslv mentioned seem to be susceptible to 

 explanation on the same basis, with the reservation that the trans- 

 lucent zone may at times be hypermineralized, when presumably it 

 would show few if any spaces. On the basis of this evidence Darling 

 ( 1961 ) has described normal enamel and the zones of the carious 

 lesion (Fig. 7). 



He states that "normal" enamel contains approximately 0.1 per 

 cent of spaces of minute size penetrable only by molecules whose 

 maximum size is similar to that of the molecule of water. These 

 spaces are distributed chieflv in the interprismatic markings, cross 

 striations, and striae of Retzius. 



The translucent zone contains approximately 1 per cent of spaces, 

 all of which are large enough to admit the molecules of octanol and 

 quinoline. These spaces are also distributed chiefly in the interpris- 

 matic markings, cross striations, and striae of Retzius. They do not 

 appear to grow bv enlargement of those spaces found in normal 

 enamel, but rather replace them abruptlv. 



The dark zone contains approximatelv 2 to 4 per cent of spaces of 

 a great variety of sizes. The smallest are impermeable to all mole- 



