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the four permanent incisores of the under jaw had appear-
ed, and also the two middle incisores of the upper, and were
perfectly regular, though the temporary cuspidati and grind-
ers had remained in the former, and the lateral incisores
cuspidati, and grinders in the latter. Surely then these
teeth could have gained no room from the difference in size
of the grinders and bicuspides ; it must therefore be owing
to an increase of the arches of the jaws in these parts ex-
actly in proportion to the difference in size of the temporary
and permanent incisores. There is still in this case a further
necessity for a considerable increase of the arches, on ac-
count of the irregular situation of the permanent teeth, as
well as because several of them had not as yet arrived at
their full size.
The pressure of the front teeth on one another as they
rise in the jaw, appeal's to have some effect in occasioning it
to extend at certain parts, or to make the grinders move back-
wards, to which indeed all the teeth have a tendency. I have
a preparation of the upper jaw of an adult, in which the
temporary cuspidatus of the right side remained in ; the per-
manent cuspidatus probably from the resistance of the for-
mer, penetrated at the internal part of the mouth : in the
left side the teeth are all perfectly regular : on comparing
the situation of the anterior permanent grinders, I found that
of the right side nearer the symphysis than that of the left
in proportion to the difference in size, of the temporary and
permanent cuspidati. I have met several instances of the
permanent cuspidati appearing at first irregular, as they most
commonly appear later than the bicuspides. but in ten or
twelve months after, the jaw increased sufficiently so as
to allow them to become regular. I have met many rases
where the arches of the jaws continued to increase, even
after the permanent teeth were complete : so that all the