Page 34 - My FlipBook

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Rotation or a grinding motion and the extension and re-
traction of the jaw, are performed by some of the muscles
acting singly, and by a compound motion of others.
Those which elevate the jaw and keep it in oppo-
First.
sition to the other.
Masseter.

The masseter is placed between the chin and the ramus
of the lower jaw ; it is of an oblong shape and evidently
.
consists of two portions, an external and an internal, which
may be readily recognized by the course of their fibres as
they decussate.
As a whole it arises tendinous and fleshy from the malar
process of the upper maxilla ; from the inferior edge of the
malar bone between the maxillary and the zygomatic sutures,
and from the zygomatic process of the temporal bone.
Of its two portions the internal is the smaller, and is in-
serted tendinous into the outer part of the root of the coro-
noid process of the lower jaw ; while the external extends
from the malar bone to the angle of the lower jaw, where it.
is inserted, tendinous, and fleshy. A part of the internal por-
tion may be seen at the zygomatic suture, behind the exter-
nal, without the latter being raised up. When both portions
act together they close the jaws ; the external alone draws
the jaw forwards and the internal alone, will draw it back-
wards.

Temporalis)

The temporal muscle is placed on the side of the head and
occupies its middle inferior region. It is covered externally
by the fascia temporalis, a thick, dense, tendinous mem*