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14 PATHOLOGY OF THE HARD TISSUES OF THE TEETH.
apparently immediately the condition of malnutrition has
passed. But the parts of the tooth which should have been
formed during that period are not formed at all. A certain part
of the tissue which should have constituted the perfect tooth has
been left out, and the distortion of form which we so often see
results from patching the second growth onto the first and the
total failure of particular portions of the enamel. This total
failure of the enamel is not in the direction of the thickness, but
is always on the lines of Retzius. Therefore, as we shall see later,
there is not a failure of the total thickness of the enamel at any
point, except in a few cases in which the injury occurs at a time
when calcification was just about to begin, as sometimes occurs
in the so-called Hutchinson tooth, and may, rarely, occur in
others. For instance, in the diagram, Figure 10, there are four
layers of enamel represented over the incisal edge. If total
arrest of growth should occur at the time the first two layers are
completed, the third and fourth layers will never be formed. The
enamel will remain over the incisal edge with only these two
layers. Then perhaps the fifth and sixth layers shown, more or
less, will also fail, and the seventh and eighth layers will overlap
the first and second somewhere near half their length, because
the formed part of the incisal edge sinks into the dentin pulp.
The dentin pulp has also stopped its growth at the same time
and the part that failed of growth is left out of the final tooth
form. These are the fundamental propositions presented in the
explanation of the histological groupings of tissue and the short-
ening of the tooth crown found in these cases.
Figure 11 is a photomicrograph* of a little more than the
incisal half of a crown of a central incisor, showing two zones
of injury. Figure 12 shows an entire crown with a single zone
of severe injury. In each of these the malnutrition was so severe
as to arrest the growth of both enamel and dentin. In each an
injury has occurred, affecting the incisal edge of the tooth. By
comparing these with the diagram it is easily seen that when a
certain thickness of growth of enamel had formed over this part,
development was arrested and no more enamel was formed. In
each case the enamel is thickest at the incisal edge and thins
away to the groove which encircles the tooth crown, which is
here presented in section. A band of very dark growth is seen
under the new after-growth of enamel following the lines of
Retzius on down to the dento-enamel junction. A comparison
* Note.— In this work any photographic illustration made by reflected light will
be called a photograph, even when moderately magnified. But when transmitted
light through a thin section has been used, it will be called a photomicrograph. It
has not been thought necessary to mention these terms in every instance.