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40 PATHOLOGY OF THE HARD TISSUES OF THE TEETH.
one child in a family may have normal teeth, while the teeth of
the brothers and sisters are mottled. I saw a pair of twins, a
girl and a boy. The girl's teeth were horribly brown, while the
boy's teeth were normal. These two children ate at the same
table, slept in the same house, played together, and their habits
and environment had been the same since birth.
The rule is, other things being equal, that the younger the
child at the time of the occurrence of any injury to the develop-
ment of the enamel, the more grave the injury. According to
this rule the incisor teeth are more persistently and more
severely mottled than any other group of teeth. Curiously
enough, however, the first molars, which have generally just
begun formation of the enamel at birth, are notably less severely
mottled than the incisors, which begin the development of their
enamel a little later. In the contemporaneous accretional defor-
mities, the first molars are more severely injured than the inci-
sors, as a rule. For all the other teeth the general rule stated
seems to hold true.
SUSCEPTIBILITY TO CAEIES.
As to caries, the teeth of these children compare favorably
with those of other communities where endemic mottled enamel
is unknown. They have a mild climate and almost continuous
sunshine during the day. The children are out practically every
day the year round, and this in itself certainly has its effect in
limiting the amount of dental caries. But when the teeth do
decay, the frail condition of the enamel makes it extremely diffi-
cult to make good and effective fillings. For this reason many
individuals will lose their teeth because of caries, though the
number of carious cavities is fewer than elsewhere. Yet I was
of the opinion, at the end of several weeks' examination and
study of the conditions, that if the appearance of the teeth could
be endured, the injury in their development would, on the whole,
not reduce the general usefulness of the teeth.

Little is known, as yet of the etiology of this condition. Dr.
Frederick S. McKay,* of Colorado Springs, Colorado, conducted
a very extensive series of investigations during the years from
1908 to 1916, in which he visited many areas where this dys-
trophy was found to be endemic, and secured reports from many

*A series of four articles, with many splendid illustrations, by Dr. McKay, report-
ing his investigations, appears in the Dental Cosmos, Vol. LVIII, 1916, pp. 477, 627,
781, 894.