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DYSTROPHIES OF THE TEETH. 39
teeth are growing. This must be differentiated sharply from
deformities that are acquired after the teeth have grown. The
tissues of the teeth are not changed in any wise by physiological
processes after they are once formed.
This has relation to the time in the life of the child in which
the enamel in different groups of teeth is growing, having ref-
erence now only to the permanent teeth. The deciduous teeth
are always normal in this respect. The permanent teeth are
naturally divisible into three groups. The first group includes
the first molars, the incisors and the cuspids. The enamel of
this group is growing during the first five year's of the child's
life, excepting that the cuspids frequently continue to the sev-
enth year. The second group includes the bicuspids and second
molars. The enamel of this group begins growing at from five
to six years, and is completed at from nine to eleven. The third
group includes the third molars only. The enamel of these is
growing ordinarily from the tenth to the fourteenth or fifteenth
year, but presenting considerable variation of the time of com-
pletion of the growth. This is an approximate statement.
It occurs, therefore, that if the child is not in the locality
of endemic mottled enamel during the time of the growth of
the enamel of any one of these groups of teeth, that group will
not be marked. Or if a child is in the locality only during the
time of the growth of one, and elsewhere the rest of the time,
only that one group of teeth will be marked. It follows that if
a child born in the locality is removed and lives elsewhere for
the first five years, the first molars, incisors and cuspids will be
normal. If the child returns to the locality at the end of the
five years and continues to live there, the other two groups of
teeth will be mottled. Having lived a part of the time in this
area does not seem to carry with it a continuance of the injury
after removal. Neither does living elsewhere during the growth
of the enamel of the incisors, and then coming into the endemic
area, prevent the injury to the teeth which have yet to grow
their enamel.
Among the children examined there was every opportunity
for the study of this feature of the difficulty, for among them
there were many who had come into the territory at any and all
periods of the growth of the enamel. By examining the teeth
one could tell pretty closely the age at which they had come
1
into the locality. Lines can not be drawn too sharply, however,
for we must remember that among those born in that region,
about one in every ten persons is immune — that is, has nor-
mal teeth. This presents some very curious features. Some