. Dental ngth be- ing at right angles to it. The rods are the shape of irregularhexagons and are cemented together by a bond which has muchless resistance to stress, and slightly less resistance to chemicaldissolution than the rods themselves. In fact, primary decay ofthe enamel begins by the dissolution of this cementing sub-stance. The rods themselves are broken away or fall out, thusallowing an entrance of the micro-organisms of decay to thedentin through the breach thus made, as in Figs. 8 and 9. Wefind these rods in two conditions, namely, straight rods lying instraight rows, wh
. Dental ngth be- ing at right angles to it. The rods are the shape of irregularhexagons and are cemented together by a bond which has muchless resistance to stress, and slightly less resistance to chemicaldissolution than the rods themselves. In fact, primary decay ofthe enamel begins by the dissolution of this cementing sub-stance. The rods themselves are broken away or fall out, thusallowing an entrance of the micro-organisms of decay to thedentin through the breach thus made, as in Figs. 8 and 9. Wefind these rods in two conditions, namely, straight rods lying instraight rows, which are easily split apart much like a straight-grained pine stick, and irregular and twisted rods, very hardto cut or split, more like a pine knot. There seems to be a gen-eral idea that there is a material difference in the density ofenamel of different teeth. This, however, is erroneous, the ap-parent hardness being due to the interlacing of the enamelrods, making them more difficult to cut with instruments, but. Fig. 11 6 THE DENTAL SUMMARY no more resistant to decay. The rods lying in the gingivalthird of a tooth are almost always in direct line with the shortaxis of the tooth. In the middle third they decline to the oc-clusal, and in the occlusal or the incisal thirds they incline quitesharply so that at the summit of the cusps in the molars andbicuspids and the incisal surfaces of the front teeth they are inline with the long axis of the tooth. (Figs. 10, II, 12). In themolars rounding over the cusp they again diverge until the de-velopmental groove is reached. Thus you will see that acavo-surface angle in the occlusal or incisal third of any of theaxial surfaces would require a much longer bevel than a mar-gin on the occlusal surface or in the gingival third, in order that no enamel rods will be left with their dentinal ends cut offand short peripheral ends left unsupported. If such short rodsare not fractured by the plugger, they are sure to be brokenduring mast
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