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690 ORTHODONTIA AS AN OPERATIVE PROCEDURE. —
ill the jaw before the roots of the decicluoiis teeth are absorbed. From
want of space they are crowded together, and already present all phases
of irregularity.
The late Prof. J. H. M'Mullen said of such cases : " When we are
examining a series of jaws of different ages, arranged so as to show
deciduous and permanent teeth, it is not a surprisiug matter that there
should be irregularity in the permanent set; but when observing their
crowded and irregular arrangement in the jaw ])rior to eru])tion, it is
rather a matter of astonishment that they should ever assume a regular
symmetrical appearance."
As the permanent teeth are larger than the deciduous, the question
naturally arises, How is room provided for them ?
This may be answered, first, by dividing the permanent teeth into
two classes : those which replace the deciduous teeth and those which
do not. For the first class—that is, the incisors, canines, and bicuspids
room is made by interstitial growth and by the fact that the biscuspids
are smaller than the deciduous molars which they replace. For the
permanent molars room is made by growth of the posterior portion of
the jaw.

Etiology of Dental Irregularities.
^he causes of irregularities of the teeth may be divided into three
classes—hereditary, constitutional, and acquired.
As children inherit other peculiarities of structure from father,
mother, grandparent, or even from more remote ancestors, so may irreg-
ularities of the teeth be inherited. The causes are operative before the
birth of the child.
Hereditary causes may be divided into two : (a) Direct, in which a
child inherits some distinct irregularity just as he may inherit some
other distinctive feature. (6) Indirect, in which he inherits separate
peculiarities which combined will cause an irregularity. For example,
large teeth may be inherited from one parent and small jaws from the
other, and thus Avill be produced an irregularity of some kind, but not
inherited directly from either. A child may inherit tone of voice,
peculiar gait, or other habit ; so he may inherit a habit which will cause
an irregularity. The intermarriage of different races is a prolific cause
of irregularities of indirect heredity.
Dr. Talbot^ defines as constitutional irregularities "those that
develop w^ith the osseous system," such as those due to excessive devel-
opment or to lack of development of either the upper maxillary, inter-
maxillary, or lower maxillary bones, or of the ramus or body of the
latter ; too high vault, too narrow vault, etc.
1 Etiology af Osseous Deformities of Head, Jaus, and Face, 3d ed. p. 16.