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SYSTEMIC CONDITIONS. 123
cusps of the molars and bicuspids. "We will find only a variation
from the straight line in these positions on many teeth without
intertwining of bundles of rods. Enamel with straight, or paral-
lel, rods may, when undermined, be split off about the margins
of a breach that has been made in it almost as easily as straight
grain pine, if it is touched just right with a sharp instrument.
If not approached in the right direction, it is very hard to cut.
On the other hand, many teeth have enamel over most of their
surfaces that is curled; that is, the enamel rods, instead of
pursuing a straight course, are much interwoven among each
other, usually in the form of small bundles of rods, twisting in
among other bundles of rods. This gives to the enamel, when
cut in sections and etched, a wavy, twisted appearance. This
is called curled enamel. See Figures 147, 148. This form of
enamel does not split nearly so readily as enamel with parallel
rods. In fact, it is much more difficult to cut it by ordinary
means. It should be noted, however, that in nearly all cases of
curled enamel, the rods straighten up and become parallel before
reaching quite to the outer surface. The checking of this straight
portion to the part that is curled in Figure 147 is suggestive.
Often the inner half will be curled and the outer half of the
thickness straight. There are all sorts of differences to cutting
instruments between the perfectly straight enamel, as shown
in Figure 146, and the abundant intertwining of bundles of rods
seen in Figure 147. The gingival portion of the enamel from the
labial surface of an incisor is represented by photomicrograph
in Figure 149, which presents much variation in the direction
of the rods in its different parts. Some places they are parallel
but much bent. Then, for another short space, they are much
interwoven. In this respect the specimen is rather remarkable.
In the cross section of the tooth at the point where the section
was taken, shown in the photomicrograph, Figure 150, the rods
are neither quite straight nor quite parallel, but do not depart
much from either. But it may be seen how the rods are split up
and clinging across the crack near the letter a, reminding one
of what occurs in splitting cross-grained wood. One would,
however, split off chips from the cut surface at a very easily.
But to split off chips from the cut surface at b would be very
difficult, not because the rods are more interwoven, but because
of the direction of rods toward the cut surface. These differ-
ences seem to have no reference to the calcification of the enamel,
nor the amount of calcium salts it may contain. In studying the
difference of structure in its relation to caries of the teeth, no