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BONES. 75
The Longitudinal Sinus commences at the foramen csecum, passes
upward along the frontal crest to the groove, and thence backward over
the internal surface of the dome of the brain-case to the internal occip-
ital protuberance.
7 he Horizontal Surface is divided by the ethmoidal not(;h into two
portions, the orbital plates. The orbital plates are two concavo-
convex, triangular surfaces of bone separated by the ethmoidal notch.
They are each composed of two thin plates of compact tissue, the space
between them being largely occupied by the frontal sinuses.
The Inferior Surfaces of the orbital plates are concave and form the
roofs of the orbits.
The Lachrymal Fossce are slight depressions just internal to the
external angular process within the orbits; they lodge the lachrymal
glands.
TJie Trochlear Fossce are small concavities, sometimes tubercles, situ-
ated immediately behind the internal angular processes, within the
orbits ; they afford attachments to the pulleys of the superior oblique
muscles.
The Superior or Encranial Surfaces of the orbital plates are convex
in form, deeply marked by eminences .and depressions for the convolu-
tions of the brain : they form the greater portion of the floors of the
anterior fossae of the brain-case. The " digital depressions " are so
called because of a fancied resemblance to markings made by pressing
the ends of the fingers upon some soft substance.
The Ethmoidal Notch, between the two orbital plates, is a quadri-
lateral opening. In the articulated skull this notch is filled by the
cribriform plate of the ethmoid bone. On each side of the nasal spine,
running its entire length, is a grooved surface which enters into the
formation of the roofs of the nasal chambers.
The Fronto-ethvioidcd Celh.—The borders of the ethmoidal notch
are marked by numerous depressions, which form half cells of irregular
shape. The ethmoid bone contains depressions of similar form in the
superior surface of its lateral masses, and when articulated with the
frontal bone forms the frontf)-ethmoidal cells.
The Anterior and the Posterior Ethmoidcd Foramina.—The borders
of the ethmoidal notch are each traversed at various angles by two
grooves. In the articulated skull these grooves, in connection with
similar ones in the ethmoid bone, form the anterior and posterior
ethmoidal foramina. The anterior foramina transmit the nasal nerves
and the anterior ethmt)idal blood-vessels, while the posterior foramina
transmit the posterior ethmoidal blood-vessels.
The Borders of the Frontal Bone.—The npper half of the ascending
border is thick, serrated, and slightly bevelled by the prolongation of
the upper table. The lower half oiP this border is thinner than the
upper; it is serrated and bevelled by extension of the inner plate. The
upper seven-eighths of the border of the bone on either side articulate
with the parietal bones, forining^the fronto-parietal (coronal) suture.
The lo^ver eighth is rough and triangular, and articulates with the great
wing of the Sphenoid bone, forming the spheno-frontal suture.
The inner border of the horizontal portion of the bone, just external