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406 DENTAL ANATOMY.
cess, which rises high above the level of the surrounding parts. The
posterior border is interrupted by two notches, between which is situ-
ated the condyle. Immediately in front of the condyle is a wide and
deep depression, the masseteric fossa, for the insertion of the powerful
masseter muscle. In front of and below the condyle, on the inner side,
is a conspicuous opening, the inferior dental canal, which gives passage
to the inferior dental artery and nerve. On the external surface, behind
and below the root of the canine, is another opening, the mental foramen,
through which a part of the nerve makes its exit to be distributed to the
lower lip.
The Muscles.—The principal muscles concerned in the movement of
the lower jaw are the temporal, masseter, external, and internal ptery-
goids, the digastric, genio-hyoid, and mylo-hyoid.
The Temporal is a broad, thick, fleshy muscle which covers the side
wall of the brain-case from the post-orbital process in front to the lamb-
doidal or occipital crest behind, reaching as high up as the sagittal crest
above, and completely filling up the temporal fossa, to which it gives its
name. Its fibres converge fan-wise to be inserted into tlie summit of
the coronoid process of the ramus of the mandible. Its principal action
is to elevate the lower jaw. By its leverage and great strength the ani-
mal is enabled to take a firm grip upon its prey.
The 3Iasseter is a short, thick muscle arising from the under and a
part of the outer surface of the malar bone^ as well as the posterior part
of the maxillary, and, passing downward and backward, is inserted into
the masseteric fossa of the ramus. . Its action is similar to that of the
preceding muscle.
The Interned Pterygoid muscle consists of a strong bundle of muscu-
lar fibres which takes its origin from the pterygoid fossa in the base of
the skull, and passes doAvnward and outward to its insertion in the lower
part of the angular process. By its contraction the lower jaAV is drawn
upward and inward, but owing to the manner in which the teeth inter-
lock no extensive lateral movement is possible. The most reasonable
view of the action of this muscle, as well as the succeeding one, is,
that by the contraction of those of one side the sectorial apparatus of the
side op])osite is enabled to perform a more perfect shearing movement,
just as the blades of a pair of scissors nuist be pressed closely togetlier
in order to make them cut. From the direction of its fibres it likewise
assists in elevating the jaw.
The External Pterygoid arises from the pterygoid plate of the sphe-
noid bone, and is inserted into the base of the condyle, and as far
forward as the inferior dental canal. Its action has already been
alluded to.
The Digastric is a large muscle M-hich arises from the skull behind
the auditory bulla in a strong bony prominence, the paramastoid pro-
cess, and passes forward to its attachment on the inferior margin of the
ramus in front of the angular process. Its action is to depress the jaw
and open the mouth.
The Genio- and 3Iylo-hyoid muscles are broad muscular sheets which
lie between the rami forming the floor of the mouth in the recent state,
being attached to the hyoid bones and the " fork " of the jaw. They