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7<)2 ORTHODONTIA AS AN OPERATIVE PROCEDURE. —
(A) It may be due to extraction of tlio lower Hrst molars at an age
when they were the only masticating teeth, as might be the ease during
an interval between the loss of the deciduous molars and the eruption
of the bicuspids. The impaction of the lower incisors upon the inclined
lingual surfaces of the upper incisors might move them forward, thus
causing U])per protrusion. (See Fig. 756.)
Thinnb-xuckiiKj.—To this practice were formerly ascribed all cases of
upper ])n)trusion, until inquiries developed the knowledge that in a

Fig. 757.

Upper protrusion—cause (d), from thumb-sucking. (Talbot.)

majority of cases no such habit had existed, or, if so, had been aban-
doned before the eruption of the permanent teeth. The fact that the
habit of thumb-sucking, which usually begins before the temporary
teeth are erupted, is indulged in during the years when the bony parts
are especially soft and yielding and is discontinued before the eruption
of the permanent teeth, and that nevertheless upper protrusion rarely
occurs with the deciduous teeth, has completely overthrown the old
theory. Yet thumb-sucking is occasionally persisted in till twenty-
eight permanent teeth are erupted, and occasionally causes protrusion.
Dr. Ottolengui says : ^ "It seems to me that if it is ever true at all
that thumb-sucking can cause a protrusion of the jaw, we have it within
our means to determine when such a condition has so resulted. If a
given case of protrusion is attributable to thumb-sucking, it must of
necessity follow that had the child not practiced the habit the jaw
would not have protruded. Admitting this, then, we come to this
that the protrusion has occurred in one of two ways : First, the length
of the arch around the circle has not been enlarged, but the projection
has been produced by a flattening of the sides — p;)inted arch. The teeth
^ Dental Cosmos, 1892, vol. xxxiv. p. 447.