Page 17 - My FlipBook

EXAMINATION OF TIIK MOUTH. 5

swaged upon a model from an impression taken high over
the region of the cuspids (as should always be done) shows
at once the dei)ression of the left side, which occurs, to a
greater or less extent, in 95 per cent, of cases. The dif-
ference becomes apparent in arranging artificial teeth.
Every dentist of experience must have observed that greater
length of teeth and gums is required upon the left side
than upon the right. How often it is seen that the left
side of the lip rises higher in talking and laughing, than the
right side. The difference in the two sides of the lower
jaw' does not occur as often, but is apparent in the diverg-
ence of the left side from a line drawn through the center
of the model, so that the posterior teeth on that side must
be set farther in upon the plate.
For the purpose of restoring the contour of the face this
deformity, when present, should be recognized and studied
especially by those engaged in arranging artificial teeth
and waxing up plates. These peculiarities are cited simply
as two or three instances of the many which may arise in
dental practice, calling for good judgment upon the part
of the dentist, in deciding upon the best course to pursue.
Not only should the plaster models be studied but the mouth
should be carefully and critically examined before taking
the impression.

MATERIALvS EMPLOYED IN OBTAINING
IMPRESSIONS OF THE MOUTH.

Plaster-of-Paris.—Plaster-of-Paris, technically, calcium
sulphate (CaSO^), .has been employed for many years for
taking impressions of the mouth, and is in nearly all cases
the very best material known for that purpose. Plaster-
of-Paris occurs in nature, as a mineral called gypsum. This
differs, however, from the dried calcium sulphate of com-