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ATROPHY OF THE TEETH. 15

these lines m 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. Thi.s
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.
Atrophy, in all cases, consists of an arrest, or partial arrest,
of growth of both enamel and dentin in the particular zone or
part of enamel and dentin, being developed at the particular
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 recover}'
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
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