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ENAMEL. 103
The apj>earaiice of striation is the record in the fully formed tissue
of the manner of ii^rowth, each dark stripe, or expansion, in a rod
representing a globule of partially calcified material. The ameloblasts
build up the rods by the addition of globule after globule, surrounding
them with a cementing substance and completing the calcification of
both. In this sense the striation of the enamel may be said to record
the growth of the individual rods.
While the enamel is a very hard substance when its .structure is
complete and perfect, its most striking physical characteristic is a ten-
dency to split or crack in the direction of its structural elements when
a break has been made in the tissue. While it is difficult to cut across
the rods or make an opening on a perfect surface, if a break has been
Fig. 84.

Enamel showing direction of cleavage. (About 70 X.)

established it is comparatively easy to split off the tissue from the sides
of the opening when the rods lie parallel with each other. Fig. 84
.shows a field of enamel illustrating the way in which the tissue splits
or cleaves in the direction of the rods.
Upon the axial surfaces the enamel rods are usually straight and
parallel with each other, except where there has been some flaw or
disturbance in development ; but upon the occlusal surface, although
sometimes straight, they are very often much twisted and wound
round each other, especially at their inner ends. This difference in the
arrangement of the rods causes the greatest difference in the feeling
of the tissue under cutting instruments. Such a specimen of enamel as
shown in Fig. 85 can be cut away easily, the tissue breaking through
to the dentin and splitting off in chuid-:s ; while a specimen like Figs.
86 and 87 will not cleave if supported upon sound dentin. If tlie outer