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BONES. 73 ;
The Supraorbital Arches are prominent curved borders of bone,
forming the superior boundaries of the orbits. They are most promi-
nent toward their outer extremities, and form the dividing-line between
the horizontal and ascending portions of the bone.
The Supraorbital Notch or Foramen.—At the inner third of the
supraorbital arch is a well-defined notch, the supraorbital notch
sometimes this is converted into a foramen bv a spiculum of bone
thrown out from its lower margin. When this spiculum is not present
the foramen is formed by fibrous tissue. It transmits the frontal
branches of the ophthalmic nerve, artery, and vein. There is generally
a small opening in the base of the notch for the passage of an emissary
vein from the diploe to join the ophthalmic vein.
The Frontal Notch is not constant : when it is, it is situated to the
*
median side of the supraorbital notch.
The External Angular Process.—The outer extremity of the supra-
orbital arch termin£ites in the external angular process. This is
strong, and projects to articulate with the frontal process of the malar
bone ; the outer margin forms a sharp curved crest, which is the com-
mencement of the temporal ridge and aifords attachment to the tem-
poral fascia. Just posterior to this crest is a slight concavity which
forms the anterior boundary of the temporal fossa.
The Internal Angular Process.—The inner extremity of the supra-
orbital arch terminates in the internal angular process, which is less
marked than the external, and articulates with the lachrymal bone. It
also gives origin to part of the orbicularis palpebrarum muscle.
The Nasal Eminence is between and slightly above the two internal
angular processes ; it is a rounded elevation which forms a portion of
the anterior wall of the frontal sinuses.
The Frontal Sinuses are two irregular chambers situated above and
between the orbital plates, and separated by a thin lamina of bone.
They appear about the second year, and are formed by the dissolution
of tissue through the agency of osteoclasts. They continue to increase
in size until advanced age, at which time they often extend over the
orbits and occupy a larger or smaller portion of the bone above the
superciliary ridge : in the hollow-horned animals these cavities extend
into the bony base or cores of the horns. They are lined by mucous
membrane, and communicate with the nasal chambers through the
infundibidum of the ethmoid bone.
The Nasal Notch is situated below the na'^al eminence : it is a
semilunar serrated border of bone for articulation with the ascending-
process of the superior maxillae, the vertical plate of the ethmoid, and
the nasal bones.
The Nasal Spine is a prominent process of bone in the centre of tlie
nasal notch : it is firmly wedged between the nasal bones when they are
in position.
The Superciliary Ridges are above the supraorbital arches : they are
broad externally, where they are continuous with the nasal eminence.
They curve outwardly, becoming less prominent as they approach the
lateral surface of the bone. The internal portions of these ridges give
origin to the corrugator supercilii muscles.