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HISTOLOGWAL REQUIREMENTS IN ENAMEL WALLS. 119

break out later. The arrangement of enamel rods in such positions is
to be borne in mind, especially when extending approximal cavities in
incisors toward the lingual side and in large pit cavities in incisors. A
similar condition is found over the points of the cusps. Fig. 99 shows
a bucco-lingual section of an upper bicuspid. It will be noticed that
the rods forming the point of the cusp are not in the axial plane, and

Fi(i. 101.

Till 'jf '"I iuciMir. I About 50 X.)

do not reach the tip of the dentin cusp, but reach the dentin a little
way down on the outer slope. The enamel covering the tip of the
dentin contains many short rods, and they are very much twisted about
each other, so that the area from A and B to the point of the cusp is an
area of weakness for the cavity margins. If the margin reaches this
area, the cusp must be cut away and the enamel wall carried out in the
horizontal plane. Fig. 100 shows this area more highly magnified, and