PHYSICAL CHEMISTRY OF ENAMEL DISSOLUTION 229 



2 (Fig. 3h). Thus the decalcification mediums not only reproduce 

 the zone differences of a natural incipient carious lesion, but also 

 offer a means of studying these peculiar and little-understood 

 changes. 



An additional comparison of the natural and in vitro lesions was 

 made by examination with electron microscopy (Figs. 20, 21, and 

 22 ) . Diamond knife sections cut from undemineralized samples were 

 used (Neal and Murphy, in preparation). Loss of substance from 

 the interrod region was clearly apparent in both the natural (Fig. 

 20) and the chemically produced lesions (Figs. 21 and 22). The 

 higher content of matter in the relatively sound outer layer was 

 also demonstrable, as were the open pathways between the rods in 

 this region. The advancing front of the lesion deeper in the enamel 

 could be seen as an enlargement of the space between the rods that 

 gradually disappeared in the normal enamel, while the body of the 

 lesion consisted of large spaces with only scattered remains of the 

 original enamel structure. The attack could be seen as beginning at 



resulted from the increase in acidic buffer capacity of phosphate, although 

 the presence of phosphate further reduced attack on the enamel surface. ( X 

 200.) 



Figs. 16 and 17. A section of an incipient carious lesion produced by a 

 24-hour exposure at 4°C to a medium consisting of 0.10 m lactic acid and 0.03 

 M CaCl2 plus 6 per cent hydroxyethyl cellulose can be compared with Figs. 8 

 and 9 to see the efiFect of increasing lactate buffer concentration. 



Fig. 16. In the light micrograph a very sharply defined relatively sound 

 outer layer can be seen above the body of the lesion. ( X 200. ) 



Fig. 17. The microradiograph confirms the presence of the relatively 

 sound outer layer and the loss of subsurface mineral. ( X 200.) 



Figs. 18 and 19. A section of an incipient carious lesion produced by five 

 24-hour exposures at 4°C to fresh samples of a medium consisting of 0.05 m 

 lactic acid and 0.03 m CaClo plus 6 per cent hydroxyethyl cellulose adjusted to 

 pH 3.5. 



Fig. 18. A light micrograph shows the depth of the lesion and some bands 

 within the lesion. ( X 200.) 



Fig. 19. A microradiograph proves the presence of a very wide relatively 

 sound outer layer, amounting to 20 microns, and the loss of material from 

 deep within the enamel. Interrupting or cycling the attack led to an increase 

 in the sound outer layer, deeper penetration of the lesion, and slower de- 

 calcification of the body of the lesion. (X 200.) 



