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BONES. 101

The Body or Horizontal Portion of the bone is parabolic in form,
the anterior portion presenting a slight vertical ridge, the symphysis.
This symphysis indicates the point of union between the primitive
halves of the bone, which unite shortly after birth. The body is
divided into two surfaces, external and internal ; and two borders,
superior and inferior.
The External or Facial Surface (Fig. 48). The vertical ridge in the
median line of the external surface extends outward and forward about
halfway between the upper and lower borders of the bone. It divides
to the right and left and forms a triangular process, the mental process
or chin, a feature exclusively human.
The Incisor Fossa.—Above the mental process and below the incisor
teetli is a shallow depression, the incisor fossa. This fossa gives origin
to the elevator muscle of the lower lip (levator labii inferioris). At the
side, a little below the incisor fossa, beneath the cuspid (canine) tooth, is
a depression for the origin of the depressor muscle of the lower lip
(depressor labii inferioris).
The Mental or Anterior Dental Foramen is not constant in its position.
When the teeth are imbedded in the bone it is generally placed midway
between the superior and lower borders of the bone, below the root of
the second bicuspid tooth, though it may appear as far back as the first
molar or as far forwaitl as the first bicuspid. This foramen transmits
the mental branches of the inferior dental nerve and vessels.
The Exicrnal Oblique Line commences at the lateral portion of the men-
tal process, passes backward beneath the mental foramen, and extends
slightly upward and backward to the anterior margin of the ramus of
the jaw. That portion of this line below the mental foramen gives origin
to the depressor muscle of the angle of the mouth (depressor anguli oris).
Between the line of origin of this muscle and the inferior border of
the bone is a roughened surface for the attachment of the platysma
myoides muscle. This roughened surface divides the body of the bone
into two portions, a superior alveolar or mucous portion, and an inferior
basilar or non-mucous portion.
The Superior Alveolar or Mucous Portion is situated within the ves-
tibule of the mouth, and is covered by mucous membrane and muco-
periosteum. It gives origin to the buccinator muscle just below the
three molar teeth.
The Inferior Basilar or Non-Mucous Portion is outside and below
the vestibule of the mouth, and is covered with periosteum similar to
other bones.
The Internal Surface (Fig, 49) in the median line is marked by a
slight vertical depression corresponding to the symphysis externally.
The Mi/lo-ht/oid or Internal Oblique Ridge commences at the base of
the coronoid process and extends downward and forward to a point
just below the genial tubercles, where it joins the ridge of the ojipo-
site side. This ridge is but faintly marked as it reaches the median
line of the bone ; it divides the internal surfice into t^vo portions, a
superior and inferior, and gives origin throughout its whole extent to
the raylo-hyoideus muscle. This muscle forms the floor of the mouth.
Between the posterior portion of this ridge and the wisdom tooth the