210
UNIT 2
Covering, Support, and Movement of the Body
7
receptors in the nasal cavities to the brain. Projecting superiorly
between the cribriform plates is a triangular process called the
crista galli
(kris
9
tah gah
9
le; “rooster’s comb”). Te outermost
covering of the brain (the dura mater) attaches to the crista galli
and helps secure the brain in the cranial cavity.
Te
perpendicular plate
of the ethmoid bone projects in-
feriorly in the median plane and forms the superior part of the
nasal septum, which divides the nasal cavity into right and leF
halves (±igure 7.5c and d). ±lanking the perpendicular plate on
each side is a
lateral mass
riddled with sinuses called
ethmoi-
dal air cells
(±igures 7.10 and 7.14), for which the bone itself is
named (
ethmos
5
sieve). Extending medially from the lateral
masses, the delicately coiled
superior
and
middle nasal con-
chae
(kong
9
ke;
concha
5
shell), named aFer the conch shells
found on warm ocean beaches, protrude into the nasal cavity
(±igures 7.10 and 7.13a). Te lateral surfaces of the ethmoid’s
lateral masses are called
orbital plates
because they contribute
to the medial walls of the orbits.
Sutural Bones
Sutural bones
are tiny, irregularly shaped bones or bone clus-
ters that occur within sutures, most oFen in the lambdoid su-
ture (±igure 7.4b). Structurally unimportant, their number
varies, and not all skulls exhibit them. Te significance of these
tiny bones is unknown.
Check Your Understanding
3.
Look at Figure 7.4. Which of the skull bones illustrated in
view (a) are cranial bones?
4.
Which bone forms the crista galli?
5.
Which skull bones house the external ear canals?
6.
What bones abut one another at the sagittal suture? At the
lambdoid suture?
For answers, see Appendix H.
from the junction of the body and greater wings (±igure 7.9b).
Tey anchor the pterygoid muscles, which are important in
chewing.
A number of openings in the sphenoid bone are visible in
±igures 7.7 and 7.9. Te
optic canals
lie anterior to the sella
turcica; they allow the optic nerves (cranial nerves II) to pass to
the eyes. On each side of the sphenoid body is a crescent-shaped
row of four openings. Te anteriormost of these, the
superior
orbital fissure
, is a long slit between the greater and lesser wings.
It allows cranial nerves that control eye movements (III, IV, VI)
to enter the orbit. Tis fissure is most obvious in an anterior
view of the skull (±igure 7.4; see also ±igure 7.9b). Te
foramen
rotundum
and
foramen ovale
(o-va
9
le) provide passageways
for branches of cranial nerve V (the maxillary and mandibu-
lar nerves, respectively) to reach the face (±igure 7.7). Te fo-
ramen rotundum is in the medial part of the greater wing and
is usually oval, despite its name meaning “round opening.” Te
foramen ovale, a large, oval foramen posterior to the foramen
rotundum, is also visible in an inferior view of the skull (±ig-
ure 7.6). Posterolateral to the foramen ovale is the small
foramen spinosum
(±igure 7.7); it transmits the
middle menin-
geal artery
, which serves the internal faces of some cranial bones.
Ethmoid Bone
Like the temporal and sphenoid bones, the delicate
ethmoid
bone
has a complex shape
(Figure 7.10)
. Lying between the
sphenoid and the nasal bones of the face, it is the most deeply
situated bone of the skull. It forms most of the bony area be-
tween the nasal cavity and the orbits.
Te superior surface of the ethmoid is formed by the paired
horizontal
cribriform plates
(krib
9
rĭ-form) (see also ±igure 7.7),
which help form the roof of the nasal cavities and the floor of
the anterior cranial fossa. Te cribriform plates are punctured
by tiny holes (
cribr
5
sieve) called
cribriform foramina
that al-
low the filaments of the olfactory nerves to pass from the smell
Orbital
plate
Ethmoidal
air cells
Perpendicular
plate
Middle
nasal concha
Cribriform plate
with cribriform
foramina
Crista galli
Left lateral
mass
Figure 7.10
The ethmoid bone.
Anterior view. (For related images, see
A Brief Atlas of the
Human Body
, Figures 3 and 10.)
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