184 A. I. DARLINCx 



the prism cores, which can be seen on the outer surface of the en- 

 amel. Mortimer ( 1962 ) tends to agree so far as occlusal lesions are 

 concerned, but it is very difficult to understand how such an attack 

 can leave the surface zone apparently intact. It might be argued 

 that what they see is the late spread of the attack back into the sur- 

 face zone from its undersurface along the prism cores. The evidence 

 in both cases is microradiographic. Darling (1956, 1958), on the 

 other hand, both by birefringence and bv microradiograph\', shows 

 evidence of attack along the striae of Retzius passing through the 

 surface zone into the lesion beneath it. Admittedly, the whole of the 

 surface zone is involved in a process similar in some wa\'s to that in 

 the rest of the lesion, but it rarelv passes the stages of translucent 

 and dark zones before the whole of the surface breaks down. Even 

 dark zones are rarely seen in the surface zone except along the striae 

 of Retzius. 



It seems tremendously important that tlie pathwavs of attack 

 should be clearlv identified on an anatomical basis. 



As for the qualitv of the changes in the various zones, we know 

 that the evidence obtained from the dark zone and the body of the 

 lesion by various methods of examination suggests that the changes 

 in these zones are produced bv the progressive effects of selective 

 or differential demineralization. The translucent zone, on the other 

 hand, shows no exidence of mineral loss bv microradiography or in 

 birefringence. On the basis of work by Rowles (1955), Darling and 

 Mortimer (1959), and Stack ( 1954 ), Darling (1961) has suggested 

 that this zone mav be caused bv loss of organic material or carbon- 

 ate. Darling and Mortimer ( 1959 ) have claimed that similar zones 

 can be produced by removal of soluble organic material, though this 

 work is being reexamined bv them because of some doubts. The 

 theory on which they base this work suggests that the selective de- 

 calcification of the enamel structures is related to the presence of 

 predominantly "soluble" matrix in the "susceptible" structures and 

 of "insoluble" matrix in the "resistant" structures. Both types of ma- 

 trix are known to exist, but it is difficult to prove their precipe 

 distribution. 



There can be no doubt that we are approaching a real understand- 

 ing of the carious process in enamel, but the end is not yet and will 



I 



