190 E. JOHANSEN 



The chemical studies were performed on samples of enamel and 

 dentin prepared by mechanical separation of the tissues using dia- 

 mond wheels, burs, or hand instruments. This procedure has been 

 found to give enamel and dentin samples of high puritv. In studies 

 of the inorganic phase the organic matrix was eliminated by ashing 

 at temperatures of 550 to 600° C. Dry weights of the samples were 

 obtained by drying in partial vacuum for 24 hours at 105° C. 



For specific elements and for carbonate determinations the follow- 

 ing procedures were used. Calcium and magnesium combined were 

 determined by complexometric titration with EDTA, using erio- 

 chrome black T as indicator (Biedermann and Schwarzenbach, 

 1948). Magnesium was then determined separately according to a 

 colorimetric method (McCann, 1959), For phosphorus the colori- 

 metric method of Chen et al. (1956) was employed. Carbonate de- 

 terminations were performed on the standard VanSlvke manometric 

 apparatus (VanSlyke and Folch, 1940), modified for solid samples 

 (Sobel etal., 1943) with solutions prepared according to MacFadyen 

 ( 1942 ) . In the determination of fluoride the traditional steam dis- 

 tillation was employed ( Willard and Winter, 1933 ) , and fluoride in 

 the distillate was determined by spectrophotometric methods (Me- 

 gregian, 1954). (Specifics of procedures will be published in de- 

 tailed reports on these studies.) 



Results 



Ultrastructure 



A conspicuous feature of many sections of carious enamel in an 

 advanced stage of destruction was the occurrence of distinct struc- 

 tureless areas of varying width delimiting enamel rods ( Fig. 1 ) , and 

 corresponding to rod sheaths in location (Figs. 4 and 11). Another 

 characteristic was the sparse distribution of crystallites within the 

 carious tissue (Figs. 1 and 2). These findings indicated extensive 

 demineralization within rods and in rod sheath and interrod areas, 

 resulting in spaces which appeared to form a system of microchannels 

 in the tissue. The occurrence of such a system was readily demon- 

 strated by immersing a carious lesion of enamel in silver nitrate 

 solution. Within a few minutes the electron-dense silver was found 



