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THE DECIDUOUS TEETH. 689
of the arch, but more often crowded out of Hue, being seen occasionally.
This is evidently a reversion to the six incisors of the normal mam-
malian formula
^3-3 1-1 ^ 4-4 ,^3-3
I3I3 ^1=1 P»^ 4-1 ^ 3-3 44.
Occasionally a bicuspid (premolar) is duplicated, a reversion to the
before-mentioued type, in which four premolars are normal.
The dilFerence in the order of eruption of the upper and lower canines
has an effect upon the position of those teeth. The upper canine erupts
out of line more often than the lower, while irregularity of the lower
bicuspids is more frequent than of the upper. In each case, being the
last of the successional teeth to erupt, there is often insufficient room to
enable them to assume their normal positions. (See Fig. 605.)
The Deciduous Teeth.
The position of the deciduous teeth is almost always normal. One
or two teeth may be misplaced, either by an inherited tendency or by
Fig. 60f).
Upper and lower jaws of a child aged about six and a half years, showinp; portions of the
developing permanent teeth and the roots of the deciduous teeth.
pernicious habits, such as sucking the thumb. The irregularity, how-
ever, is so slight and so infrequent, and the deciduous teeth are retained
in the mouth for so short a time, that there is no occasion for treatment.
A regular arch of the deciduous teeth does not, however, foretell a
regular arch of the permanent teeth. Fig. 606 will make evident that
the crowns of the permanent incisors, canines, and bicuspids are formed
44
THE DECIDUOUS TEETH. 689
of the arch, but more often crowded out of Hue, being seen occasionally.
This is evidently a reversion to the six incisors of the normal mam-
malian formula
^3-3 1-1 ^ 4-4 ,^3-3
I3I3 ^1=1 P»^ 4-1 ^ 3-3 44.
Occasionally a bicuspid (premolar) is duplicated, a reversion to the
before-mentioued type, in which four premolars are normal.
The dilFerence in the order of eruption of the upper and lower canines
has an effect upon the position of those teeth. The upper canine erupts
out of line more often than the lower, while irregularity of the lower
bicuspids is more frequent than of the upper. In each case, being the
last of the successional teeth to erupt, there is often insufficient room to
enable them to assume their normal positions. (See Fig. 605.)
The Deciduous Teeth.
The position of the deciduous teeth is almost always normal. One
or two teeth may be misplaced, either by an inherited tendency or by
Fig. 60f).
Upper and lower jaws of a child aged about six and a half years, showinp; portions of the
developing permanent teeth and the roots of the deciduous teeth.
pernicious habits, such as sucking the thumb. The irregularity, how-
ever, is so slight and so infrequent, and the deciduous teeth are retained
in the mouth for so short a time, that there is no occasion for treatment.
A regular arch of the deciduous teeth does not, however, foretell a
regular arch of the permanent teeth. Fig. 606 will make evident that
the crowns of the permanent incisors, canines, and bicuspids are formed
44
