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formly in all its parts, and at that time as far as the teeth ex-
tend, it approaches nearly to a semi-circle ; at about three
years of age, when all the temporary teeth have appeared, it
begins to lose its semi-circular form, and become somewhat
elongated ; an extension takes place between the last tem-
porary molaris and the coronoid process ; and in that part in
an advanced state of formation, the first permanent molaris
will be found.
At about seven or eight years of age, the jaw is more ex-
tended, the first permanent molaris has grown up, and the
second is advancing in formation. At about eleven or twelve
years of age, it will be found still longer ; the second mola-
ris is ready to come through the gum, and the third molaris
has begun to form.
The jaw acquires its full proportion, at about eighteen or
twenty years of age, when the third molaris makes its ap-
pearance, and the teeth are seen in the figure of their ar-
rangement to form part of an ellipsis.
The growth of the jaw being nearly confined to the part
situated behind the temporary teeth, where the permanent
^nolares are placed, the anterior part of the jaw undergoes
little more than an alteration in form ; it adapts itself to the
permanent teeth there situated, and scarcely receives any in-
crease of size.
The same comparison of jaws exhibits the cause of irreg-
larity in the permanent incisores and cuspidati.
When a
child is about to shed its teeth, the first permanent molares
come through the gums behind the temporary molares, and
therefore the teeth which are situated anteriorly to the per-
manent molares can obtain no additional space.
The permanent incisores occupy the space of the tempo-
rary incisores, and half of that of the cuspidati.
It com-
monly happens that the bicuspides are earlier in their appear-