MICROSTRUCTURAL CHANGES IN EARLY CARIES 183 



ing in size to the prism cortices, zones beneath striae of Retziiis, and 

 Hnes parallel to the cross striations are not affected until late in the 

 process of demineralization, but there is considerable difference of 

 opinion as to why this should be. 



Gustafson (1961) speaks chiefly of variations in the degree of 

 mineralization as the cause of this differential demineralization. 

 Brudevold (1961) has suggested that it is the chemistry of the struc- 

 tures and in particular the distribution of fluoride which is responsi- 

 ble. Poole ( 1962 ) believes that it may be largely a matter of the 

 accessibility of the structures to the pathways of attack, and Little 

 (1959) and Darling (1958) have suggested that it is related to dif- 

 ferences in the organic matrix of the structures. 



There seems no reason why all these possible causes should not 

 play some part, but the marked difference in susceptibility between 

 the surface zone and the deeper enamel makes it unlikely that this 

 difference will depend to any great extent on the degree of mineral- 

 ization or the accessibility of the structure to the pathways of attack. 

 It seems that here at least the resistance to attack is more likely to 

 depend on fundamental chemical or structural differences. 



To recognize such differences as exist between these two groups 

 of structures it is necessary to delineate them anatomically. Here 

 again there is some difficulty. Most workers seem to agree that the 

 early attack is at the margins of the structures known as prisms, but 

 whether it is truly interprismatic or in the prism cortex or just out- 

 side or just within this structure, or even along spaces at the junction 

 of the prism and the so-called interprismatic structure, seems to be 

 open to argument. Similar arguments seem to apply to other stages 

 in the anatomv of the attack. 



Probably the best and most acceptable description which can be 

 given at present is that the attack appears to take place first along 

 the interprismatic or border region of the prisms, from which it 

 spreads to involve the cross striations and later what seems to be 

 the prism core, leaving stripes parallel to the cross striations, and, 

 more obviouslv, what look like the cortices of the prisms, relatively 

 unaffected. 



In the surface attack there seem to be two possibilities which 

 appear to conflict. Guzman et al. ( 1957 ) believe that entry is along 



