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dystrophies of the teeth. 13

Histological Chaeacteeistics.
In presenting the histological characteristics in this form
of dystrophy, it may he stated that all of the cases thus far
examined by myself, no matter how different their outward
appearance, present one plan of departure from the normal
arrangement of tissues. The differences are due only to posi-
tion, the number of zones of injury and in the details of severity.
This plan is inseparably linked with the plan of development
of the dental tissues.
Except in the pits that often accompany it, the zones of
injury always follow the lines of Eetzius very rigidly. In the dia-
gram, Figure 10, the lines of Retzius are made especially prom-
inent to recall distinctly their direction on different parts of the
enamel cap of the crown of the tooth. In microscopic observa-
tion these are usually clearly seen in some parts of the enamel
cap, particularly in central labio-lingual sections. They vary,
however, indefinitely in prominence in different sections, and in
different parts of the same section. Generally, they do not show
clearly in all parts of a section, and those who have not studied
them carefully should refresh their memory as to the course of
these lines in different positions on the crown of the tooth. These
lines are the index to the growth of the enamel cap. They are
the real lines of accretion and show distinctly the order in which
the enamel cap is built up, layer after layer, in its growth. This
growth begins at the dento-enamel junction, in positions which
represent the cutting edge (or points of the cusps in molars and
bicuspids) and grows from within outward, while the dentin
begins its growth at the same point and progresses from without
inward. The growth of dentin is always a little in advance of
the enamel as it grows from the incisal edge of the front teeth
(or the points of the cusps of other teeth) toward the gingival
line.
This contemporaneous accretional deformity, in all cases,
consists of an arrest, or partial arrest, of growth of both enamel
and dentin in the particular zone being developed at the particu-
lar time. In the milder cases growth is imperfect, leaving certain
definite markings outlining the particular parts of the tissue
then being formed. In all the severe cases the growth of both
enamel and dentin is arrested. There seems to be no recovery
of the part of the enamel organ that was at the time in active
function. No more enamel whatever is formed over the area
affected after recovery from the condition of malnutrition,
except as the new formation is telescoped over the area of the
old. The dentin pulp, however, rebegins its growth function