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116 DENTAL HISTOLOGY AND OPERATIVE DENTISTRY.
not he increased wilhont making the margin of filling material too thin
;
the rods forming the margin shonld therefore be protected by bevel-
ling the cavo surface angle. At the gingival wall the rods are inclined
apically from the horizontal plane about 6 centigrades (20*^). 'J'he
wall should bo siiaved in that plane, increasing the angle a little, and
the cavo-snrface angle should be bevelled. Fig. 97 shows the occlusal
enamel wall alone, after cleaving and trimming into form. Such
enamel walls may be taken as typical of axial surface cavities, the

Fig. 98.

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structure of enamel about a fissure : B, buccal side ; /., lingual side. (About 70 X.)

angle of the enamel with the dentin wall being determined by the
direction of the enamel rods in the position where the margin is laid.
Grooves, fissures, and pits are always positions of weakness, and
when a cavity appi^oaches a groove or pit a good margin, histologically,
cannot be prepared without cutting beyond it. Fig. 98 shows an
occlusal fissure in a bicuspid, which illustrates the conditions of struct-
ure characteristic of these positions. The rods are inclined toward
the fissure, and between the bottom of the fissure and the dentin are
very irregular. If a cavity wall were made to approach this fissure from
the lingual side, so as to come to the dotted line, the wall would have
to be inclined 6 to 8 centigrades (20° to 28°) from the axial plane toward