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698 ORTHODONTIA AS AN OPERATIVE PROCEDURE.

peridental membrane behind the tooth. This is succeeded by resorp-
tion of the hard tissues in front by osteoclasts and aforiaation of new
hone by the osteoblasts behind the moving tooth.
This latter action is much slower than the former, and depends on
the tooth being held firmly in its advanced position. Any slight return
will interfere with the formation of new tissue, and a tooth repeatedly
moved forward and allowed repeatedly to recede will never become firm.
When a tooth is rotated in its socket, there must be a stretching
of the fibers of the peridental membrane. If the fibers had not con-
siderable elasticity, those opposing the rotation of the teeth would be
ruptured instead of stretched, and would not tend to twist the tooth
back to its old position. A tootli is sometimes forced back by the pres-
sure of adjoining teeth, but such contingencies are not here under con-
sideration. If the root is curved or is not round, there may be some
resorption and rebuilding of the walls of the alveolus.
2. Bending of the Alveolar Ridge.—When several teeth are
moved in the same direction at the same time there is a movement of
the alveolar ridge as if it were a semi-plastic mass. This movement is
easily proved by the following observations :
After a case of upper protrusion is reduced the labial portion of the
alveolar ridge appears no thicker than before. If the only movement
were of the roots through the ridge by resorption in advance of the
moving tooth and formation of new bone behind, the labial portion
would remain as prominent as before.
In spreading the arch rapidly, if movement took place only after
resorption, the teeth might be pushed out of the ridge, but the external
plates of the alveolar process will be found no thinner than before,
while the vault of the palate is perceptibly broadened.
Dr. C. S. Case says that when teeth and roots are moved forward,
sometimes ridges will appear on the outer surface of the alveolar
process, but that the spaces or hollows between will soon fill up so as to
present an even surface.
3. Separation of the Superior Maxill.e at the Symphysis.
—When strong pressure is applied upon molars and bicuspids to spread
the arch the superior maxillae may be separated at the symphysis. (See
Figs. 617 and 618.)
Such separation was first recorded by Dr. E. C. Angell of San Fran-
cisco^ in 1885, and has been noticed by Guilford, Black, Talbot, Farrar,
Ottolengui, and others since. Drs. Talbot" and Ottolengui^ regard it
as an advantage as giving room for re-arranging crowded incisors more
^ Dental Cosmos, vol. ii. p. 540.
-Discussion in World's Columbian Dental Congress, vol. ii. p. 722.
* Dental Practitioner, vol. xxxv., No. 4, October 1894.