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BONES. 63

of bone, the middle clinoid process. Occasionally this process is con-
nected by a spiculura of bone with the anterior clinoid process, forming a
foramen ; more rarely to both the anterior and posterior processes, form-
ing two foramina and a continuous and uninterrupted border from the
superior anterior angle of the lesser wing to the superior angle of the
dorsum sellte.
The lateral surfaces of the body of the sphenoid bone are almost
entirely within the brain-case. A small portion, however, of this
surface runs forward and forms the proximal border of the foramen
rotundum, the base and proximal boundary of the sphenoidal fissure,
and continues forward until it meets the sphenoidal turbinated bones.
Fig. 21.
Jf^dJl. Cli^md procc^^
J,7.„,^rf^Z Smvc
_Fustcrior CUncid jproccss
\ f^^^^^^p,. i
Wiut/rnf tu: PTC I

Sphenoid Bone, superior surface.
On that portion of the lateral surface within the brain-case is a marked
depression, the sigmoid groove, for the accommodation of the internal
carotid artery and the cavernous sinus.
The posterior surface is quadrilateral in shape. Until about the
fifteenth year this surface is separated from the occipital bone, \\\i\\
which it articulates, by a layer of cartilage. At this time ossification
commences between the two bones, and is completed about the twentieth
year.
In the middle of the anterior (nasal) surface is a thin vertical lamina
of bone which forms part of the septum of the nose and articulates with
the perpendicular plate of the ethmoid bone. On each side of this lam-
ina are irregular openings, varying in size in different bones, and often
in the same bone ; they lead into the sphenoidal sinuses situated in the
body of the bone. The body of the bone is completely hollowed by
these sinuses, which accounts for the complete thinness of its walls.
The septum of bone between these cavities is generally deflected to