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origin and insertion will approach towards one another
in which case it is evident, that the lower jaw will move
downwards and backwards. But if the lower jaw be fixed,
as in the case supposed, and the vertebrae be also fixed, the
condyle will move upwards and forwards upon the eminence
in the joint, the fore part of the head will be pushed up-
wards and backwards by the condyle, and the hind part of
the head will be drawn down, so that the whole shall make a
kind of circular motion upon the upper vertebras ; and the
digastric muscle, pulling the hind part of the head towards
the lower jaw, and at the same time pushing up the condyles,
against the fore part of the head, acquires by this mechan-
ism, a very considerable additional power.
The action of the buccinator, and styloglossus, are of con-
siderable use in mastication. The former arises from the
back part of the upper and under jaw bones, and comes for-
ward within the cheek ; a considerable portion of which is
formed by it. It is of a triangular figure, is broadest behind,
forwards it becomes narrower, and is inserted at the angle
of the mouth inside. By its action in mastication, it con-
tracts, and presses in the cheek, by which the food is kept
between the teeth, from the exterior.
The stylo-glossus derives its name from its origin, which is
on a process of. the temporal bone called the styloid process,
from which, extending forwards and downwards, in an
oblique direction, it is inserted into the root of the tongue,
and running along its sides forms a portion of it. The ac-
tion of the stylo-glossus in mastication is opposed to that of
it draws the tongue backwards, and gives it
the buccinator :
a lateral motion, by which the food is kept between the teeth
internallv.