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mastication, we may observe once for all, that their action
and re-action must be always equal, and that the teeth of the
upper and lower jaw are complete and equal antagonists
both in cutting and grinding.
When the lower jaw is depressed, the condyles slide for-
wards on the eminences ; and they return back again into
the cavities, when the jaw is completely raised. This sim-
ple action produces a grinding motion of the lower jaw
backwards on the upper, and is used when we divide any
thing with our fore teeth or incisores. For this purpose the
incisores are well formed ; as they are higher than the oth-
ers, their edges must come in contact sooner ; and as the up-
per project over the under, we find in dividing any sub-
stance with them, that we first bring them opposite to one
another, and as they pass through the part to be divided, the
lower jaw is brought back, while the incisors of that jaw
slide up behind those of the upper jaw, and of course pass
by one another. In this way they complete the division like
a pair of scissors ; and at the same time they sharpen one
another. There arc exceptions to this, for the teeth in some
people meet equally, viz. in those people whose fore teeth do
not project further than the gum, or socket, than the back
teeth ; and such teeth are not so fit for dividing : and in
some people the teeth of the lower jaw are so placed, as to
come before those of the upper jaw ; this situation is as fa-
vorable for cutting as when the overlapping of the teeth
is
the reverse, except for this circumstance, that the lower jaw
must be longer, and therefore its action weaker.
The other motion of the lower jaw, viz. when the lateral
teeth are used, is somewhat different from the former. In
opening the mouth, one condyle slides a little forwards, and
the other slides a little further back into its cavity; this
throws the jaw a little to that side, just enough to brino- the