The =frontal bones= (Ossa frontalia) are situated on the limits of the
cranium and face, between the parietals behind and the nasal bones in
front. Each is irregularly quadrilateral, and consists of frontal,
orbital, and temporal parts.
The =frontal part= (Pars naso-frontalis) forms the basis of the
forehead. Its =external surface= (Facies frontalis) is nearly flat, and
is smooth and subcutaneous; it is separated from the temporal part by
the external frontal crest (Crista frontalis externa). At the junction
with the orbital part the =supraorbital= or =zygomatic process= (Proc.
zygomaticus) curves outward and downward to join the zygomatic arch. The
process partially separates the orbit from the temporal fossa; its root
is perforated by the =supraorbital foramen=, or presents instead a notch
on its anterior border; its upper surface is convex, while the lower or
orbital surface is concave and smooth, forming a shallow fossa for the
lacrimal gland (Fossa glandulæ lacrimalis). The =internal surface=
enters into the formation of the cranial cavity and the frontal sinus.
The cranial surface presents digital impressions for the cerebral gyri.
The two plates of the bone separate and diverge in front, and thus
inclose a large air-space which is part of the =frontal sinus=. The
cranial plate curves downward and articulates with the cribriform plate
of the ethmoid bone; the facial plate extends forward and joins the
nasal and lacrimal bones.
The =orbital part= (Pars orbitalis) forms the major part of the inner
wall of the orbital cavity. It is separated from the frontal part by a
prominent ridge which is part of the orbital margin. Its =external= or
=orbital surface= is concave and smooth, and presents superiorly a small
depression (Fovea trochlearis), which is bridged by a small bar of
cartilage, around which the superior oblique muscle of the eye is
reflected. The lower border concurs with the orbital wing of the
sphenoid in the formation of the ethmoidal or internal orbital foramen.
The =internal surface= faces into the frontal sinus and gives attachment
to the lateral mass of the ethmoid.
The =temporal part= is separated from the orbital part by the deep
=sphenoidal notch= (Incisura sphenoidalis), which is occupied by the
orbital wing of the sphenoid. Its =external surface= forms part of the
inner wall of the temporal fossa. The =internal surface= is largely
covered by the orbital wing of the sphenoid in the young subject, but
later forms part of the wall of the frontal sinus.
The principal connections of the frontal bone are as follows: (1) The
inner border joins its fellow at the =frontal suture=. (2) The
anterior border meets the nasal and lacrimal at the =naso-frontal= and
=fronto-lacrimal sutures=. (3) Laterally it forms the =spheno-frontal=
suture with the orbital wing of the sphenoid, and also joins the
palate bone and maxilla. (4) Posteriorly it meets the parietal at the
=parieto-frontal= (or coronal) =suture=, and articulates below this
with the squamous temporal. (5) The extremity of the supraorbital
process unites with the zygomatic process of the temporal bone.
=Development.=—Each ossifies in membrane from one center which appears
in the root of the supraorbital process. In the new-born foal there is a
slit between the cranial plate and the orbital and temporal plates which
receives the cartilaginous margin of the orbital wing of the sphenoid.