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ETIOLOGY OF DENTAL IRREGULARITIES. 693

jaw by interstitial growth will be interrupted. When a deciduous tooth
is extracted, the contiguous teeth, whether deciduous or permanent,
tend to move toward eacli other and occupy the space which should be
preserved for the succeeding tooth.
But one rule is needed, as follows : Extract a deciduous tooth only
when it defects its successor.
Rules against Extraction of Deciduous Teeth.— 1. Do not
extract a deciduous latercd to make room for a permanent central incisor.
2. Do not extract a deciduous canine to make room for a permanent
lateral incisor.
Requests for such extraction will often be made by the parent, to
whom the explanation should be made that such extraction is liable to
prevent the natural growth of the jaw for the accommodation of the
permanent teeth ; also that, while the six anterior deciduous teeth are
replaced by larger permanent ones, the four temporary molars in each
jaw are replaced by the smaller bicuspids, and that when this takes
place, irregularities of the incisors, especially the lower ones, will cor-
rect themselves, unless the teeth are too large for the jaw, which cannot
be foretold with certainty at this age. Even if it could, no extraction
of deciduous teeth would be of benefit, but rather positive harm.
3. Do not extract a deciduous second molar till the first permanent
molar is firmly fixed in place, and not then unless the second bicuspid
has erupted or is about to erupt out of position.
Requests for extraction of deciduous molars are made on account
of cavities of decay, which should be filled and the teeth preserved for
service in mastication.
The only exception to these rules is in cases of incurable alveolar
abscess, which may endanger the alveolar border and the tooth forming
beneath.
(e) Early Loss of Permanent Teeth.—Irregularities may be due
also to early loss or injudicious extraction of permanent teeth.
An early loss of first permanent molars may cause upper or lower
protrusion.
An early loss of lateral incisors causes a narrowing of the anterior
portion of the arch and deprives the angles of the mouth of their
proper contour.
A loss of canines causes a depression of the angles of the mouth and
wins: of the nose.
(d) Delayed Eruption of Permanent Teeth.—The delayed eruption
of anv permanent tooth, after the loss of its deciduous predecessor, will
allow the teeth on each side of the s})ace to move toward each other and
thus prevent eruption, or crowd the erupting tooth out of the line either
labiallv or linguallv.