138 R. F. SOGNNAES 



seem materially altered. When, however, the etched teeth were left 

 in the mouth of the animals for 1 week after application of this acid, 

 the same increase in radiodensity could be demonstrated in the 

 enamel surface (Fig. 27H and I). In some areas, as shown in Fig. 

 27 I, this increased density did, in fact, exhibit itself to quite an 

 extent below the external enamel surface. 



The teeth of monkeys as compared with those of other animals 

 offer the advantage of greater similarity to human teeth. But it 

 seemed of interest to explore the usefulness of smaller rodents for 

 similar demineralization tests. Though the relatively small molar 

 teeth of rats do not lend themselves to isolated demineralization of 

 discrete areas within single teeth, it proved possible to expose jaw 

 quadrants of three molars to various agents, using other jaw quad- 

 rants for contrast. Such a scheme is illustrated in Fig. 28A to D. 



After preliminarv drying, the mandibular rat molars in quadrants 

 of three were exposed to a 5 per cent solution of HNO3 for a few 

 minutes in vitro (Fig. 28A) and in vivo (Fig. 28B) respectively, 

 littermates being compared at the outset of the experiment. In living 

 littermate animals, the etched molars resumed nearlv normal glossv 

 surfaces in 6 hours after the etching ( Fig. 28C ) , more fully normal 

 33 hours after the etching (Fig. 28D). In each case, the maxillary 

 quadrants with the three untreated molars served as controls (Fig. 

 28A to D, top). Thus, from a clinical and esthetic point of view it 

 would appear that the teeth of normal animals can tolerate an in- 

 cipient surface etching and regain the seeminglv normal glossiness 

 of a healthv tooth surface. 



It should not be assumed, however, that the partially demineral- 

 ized tooth substance can become completelv reconstructed to its 

 precise original physical and chemical state. 



In the first place, there is not necessarily a direct correlation be- 

 tween physical and chemical properties of teeth. The hardness and 

 solubility of identical areas of ground surfaces of enamel on a large 

 number of teeth were measured bv Swartz and Phillips (1952). The 

 enamel varied widely in hardness and solubilitv from tooth to tooth 

 as well as from area to area on the same tooth. No consistent corre- 

 lation between hardness and solubility was detected. The Ca/P ratio 

 of the enamel dissolved during the solubility tests was slightlv higher 



