MICROSTRrCTURAL CHANGES IN EARLY CARIES 181 



cules bigger than those of water, whereas the largest will accept 

 octanol and quinoline with ease. 



This zone is graduallv transformed into the body of the lesion by 

 the steady growth of the spaces in the dark zone. The bodv of the 

 lesion contains at least 5 per cent of spaces and may contain up to 

 25 per cent or possiblv 50 per cent or more. All these spaces are 

 large enough to admit octanol and quinoline and possibly even 

 larger molecules. Thus the zone of translucence described by Nishi- 

 mura (1926) and Gustafson (1961) just between the dark zone and 

 the body of the lesion would be explained in the same way and 

 would be distinguished because the more highly demineralized parts 

 of the bodv of the lesion probablv have an altered refractive index 

 which mav produce some form birefringence in media of R.I. 1.62 

 (Crabb, 1962). 



Using this knowledge of the spaces and their form birefringence, 

 it is possible to demonstrate something of the pattern of spread 

 (Fig. 8) (Darling, 1961). This begins as fine streamers spreading 

 into the normal enamel along the so-called interprismatic markings. 

 From these the cross striations are involved, and soon afterward the 

 whole of the prism seems to be affected. 



By combining the evidence from microradiography and observa- 

 tions of form birefringence, it is now possible to give a description 

 of the various stages as follows. 



The translucent zone shows no evidence of demineralization, 

 though occasionallv it may be hypermineralized. When the latter 

 condition is not found, i.e. in the great majority of cases, this zone 

 is characterized by the sudden development of large spaces in the 

 sites where normal enamel has minute spaces (Fig. 7). 



These spaces of the translucent zone persist in the dark zone but 

 are overlain bv the development of minute spaces penetrable only 

 bv the molecules of water. These grow throughout the dark zone 

 until they become large spaces, big enough to admit octanol, quino- 

 line, and balsam. When all the spaces reach this size the dark zone 

 is transformed into the body of the lesion. The body of the lesion 

 and to some extent the dark zone show demineralization. There can 

 be little doubt that the minute spaces of the dark zone growing into 

 the body of the lesion represent the results of demineralization, but 



