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16 PATHOLOGY OF THE HARD TISSXTES OF THE TEETH.
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 i^ulp 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 foimd in these cases.
In Figures 11 and 13 I present photomicrographs* made with
a very low power, of labio-lingual sections from two atrophied
teeth, so that the entire form of the crown of each may be seen.
Figure 12 is another photomicrograph from the same section from
which Figure 11 was made, but with greater amplification. The
one presents two zones of injury, while the other has but one. 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 thick-
ness 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 Eetzius on down
to the dento-enamel junction. A comparison now with the dia-
gram shows that the growth has been arrested on the lines of
accretion or lines of Retzius, as you may please to call these
Imes, Figure 10, in both cases. Also, it is seen that the second
injury in Figure 11 is similar in plan to the first, differing in
detail only because of the changed direction of the lines of accre-
tion. In Figures 11 and 12 the incisal edge is broken, as usually
occurs in these thin edges, but Figure 13 is from a tooth extracted
soon after it came through the gums and all of the tissue formed
is present.
Figure 14 is an illustration with a much higher power of the
labial side of the first zone of injury shown in Figures 11 and 12.
Figure 15 is from the second zone of injury on the labial side,
and Figure 16 is from the second zone of injury on the lingual
side. In these, the tissues and the lines of Retzius are fairly well
• NoTB.— 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 boon thought necessary to mention these terms in every instance.