212
UNIT TWO
warming, and f ltering air as it enters the respiratory
tract. The lateral portions oF the ethmoid bone contain
many small air spaces, the
ethmoidal sinuses.
Figure 7.23
shows various structures in the nasal cavity.
Projecting upward into the cranial cavity between the
cribriForm plates is a triangular process oF the ethmoid
bone called the
crista galli
(kris
ta
˘ gal
li) (cock’s comb).
Membranes that enclose the brain attach to this process.
Figure 7.24
shows a view oF the fl oor oF the cranial cavity.
Facial Skeleton
The
facial skeleton
consists oF thirteen immovable bones
and a movable lower jawbone. In addition to Forming the
basic shape oF the Face, these bones provide attachments For
muscles that move the jaw and control Facial expressions.
The bones oF the Facial skeleton are as Follows:
1.
Maxillary bones.
The maxillary (mak
sı˘-ler
e) bones
(sing., maxilla, mak-sil
ah; pl., maxillae, mak-sı˘l
e)
Form the upper jaw; together they Form the keystone
oF the Face, because the other immovable Facial bones
articulate with them.
Portions oF these bones comprise the anterior rooF oF
the mouth (
hard palate
), the fl
oors oF the orbits, and the
sides and fl
oor oF the nasal cavity. They also contain the
sockets oF the upper teeth. Inside the maxillae, lateral to
the nasal cavity, are
maxillary sinuses.
These spaces are
the largest oF the sinuses, and they extend From the fl
oor
oF the orbits to the roots oF the upper teeth.
Figure 7.25
shows the locations oF the maxillary and other paranasal
sinuses.
During development, portions oF the maxillary
bones called
palatine processes
grow together and Fuse
along the midline, or median palatine suture. This Forms
the anterior section oF the hard palate (see f
g. 7.20).
The inFerior border oF each maxillary bone projects
downward, Forming an
alveolar
(al-ve
o-lar)
process.
A
zygomatic process
projects anteriorly From the
temporal bone in the region oF the external acoustic
meatus. It joins the temporal process oF the zygomatic
bone and helps Form the prominence oF the cheek, the
zygomatic arch
(f
g. 7.20).
5.
Sphenoid bone.
The sphenoid (sFe
noid) bone
(f g. 7.21)
is wedged between several other bones in the anterior
portion oF the cranium. It consists oF a central part and
two winglike structures that extend laterally toward each
side oF the skull. This bone helps Form the base oF the
cranium, the sides oF the skull, and the fl oors and sides oF
the orbits. Along the midline within the cranial cavity, a
portion oF the sphenoid bone indents to Form the saddle-
shaped
sella turcica
(sel
ah tur
si-ka) (Turk’s saddle). In
this depression lies the pituitary gland, which hangs From
the base oF the brain by a stalk.
The sphenoid bone also contains two
sphenoidal
sinuses.
These lie side by side and are separated by a bony
septum that projects downward into the nasal cavity.
6.
Ethmoid bone.
The ethmoid (eth
moid) bone
(f
g. 7.22)
is located in Front oF the sphenoid bone. It consists oF
two masses, one on each side oF the nasal cavity, joined
horizontally by thin
cribriform
(krib
rı˘-Form)
plates.
These plates Form part oF the rooF oF the nasal cavity,
and nerves associated with the sense oF smell pass
through tiny openings (
olfactory foramina
) in them.
Portions oF the ethmoid bone also Form sections oF the
cranial fl
oor, orbital walls, and nasal cavity walls. A
perpendicular plate
projects downward in the midline
From the cribriForm plates to Form most oF the nasal
septum.
Delicate, scroll-shaped plates called the
superior
nasal concha
(kong
kah) and the
middle nasal concha
project inward From the lateral portions oF the ethmoid
bone toward the perpendicular plate. These bony plates
support mucous membranes that line the nasal cavity.
The mucous membranes, in turn, begin moistening,
Transverse section
Crista galli
Orbital
surface
Ethmoidal
sinuses
Cribriform
plate
Crista galli
Perpendicular
plate
Superior
nasal concha
Middle nasal
concha
Perpendicular
plate
(a)
(b)
FIGURE 7.22
The ethmoid bone. (
a
) Superior view and (
b
) Posterior view.
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