Chapter 7
The Skeleton
209
7
central wedge that articulates with all other cranial bones. It is
a challenging bone to study because of its complex shape. As
shown in
Figure 7.9
, it consists of a central body and three
pairs of processes: the greater wings, lesser wings, and pterygoid
processes (ter
9
ĭ-goid). Within the
body
of the sphenoid are the
paired
sphenoidal sinuses
(see Figures 7.5c and d, and 7.14).
Te superior surface of the body bears a saddle-shaped prom-
inence, the
sella turcica
(sel
9
ah ter
9
sĭ-kah), meaning “±urk’s
saddle.” Te seat of this saddle, called the
hypophyseal fossa
,
forms a snug enclosure for the pituitary gland (hypophysis).
Te
greater wings
project laterally from the sphenoid body,
forming parts of (1) the middle cranial fossa (Figures 7.7 and
7.2b), (2) the posterior walls of the orbits (Figure 7.4a), and
(3) the external wall of the skull, where they are seen as flag-
shaped, bony areas medial to the zygomatic arch (Figure 7.5).
Te hornlike
lesser wings
form part of the floor of the anterior
cranial fossa (Figure 7.7) and part of the medial walls of the or-
bits. Te trough-shaped
pterygoid processes
project inferiorly
Homeostatic Imbalance
7.1
Te mastoid process is full of air cavities (sinuses) called
mas-
toid air cells
. Teir position adjacent to the middle ear cavity
(a high-risk area for infections spreading from the throat) puts
them at risk for infection themselves. A mastoid sinus infec-
tion, or
mastoiditis
, is notoriously difficult to treat. Because the
mastoid air cells are separated from the brain by only a very
thin bony plate, mastoid infections may spread to the brain as
well. Surgical removal of the mastoid process was once the best
way to prevent life-threatening brain inflammations in people
susceptible to repeated bouts of mastoiditis. ±oday, antibiotic
therapy is the treatment of choice.
Sphenoid Bone
Te bat-shaped
sphenoid bone
(sfe
9
noid;
sphen
5
wedge) spans
the width of the middle cranial fossa (Figure 7.7). Te sphenoid
is considered the keystone of the cranium because it forms a
Optic
canal
Greater
wing
Hypophyseal
fossa of
sella turcica
Lesser wing
Foramen
rotundum
Foramen
ovale
Foramen
spinosum
Body of sphenoid
Superior
orbital fissure
(a) Superior view
Body of sphenoid
Greater
wing
Superior
orbital
fissure
Lesser
wing
Pterygoid
process
(b) Posterior view
Figure 7.9
The sphenoid bone.
(For related images, see
A Brief Atlas of the Human Body
,
Figures 5 and 9).
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