414
UNIT 3
Organ Systems
Figure 21-3A.
Membranous labyrinth.
The endolymph-f lled
membranous
labyrinth
fl oats within the perilymph-f lled
osseous
(
bony
)
labyrinth
(Fig. 21-2). This anatomically
correct drawing shows the position o± the cochlea and semicircular canals within the head. The labyrinth is viewed ±rom the direc-
tion indicated by the
pointer
in the
small inset
o± the head. The position o± the nerves that innervate the
cochlea
,
cristae
of the
ampul-
lae
, and
maculae
of the utricle
and
saccule
are illustrated. The positions and planes o± section o± the photomicrographs that ±ollow in
this chapter are indicated by
blue lines, arrows
, or
boxes
. The
dashed oval
indicates the position o± the oval window in the osseous
labyrinth (Fig. 21-2).
Saccule
Facial nerve
(CN VII)
Cochlear nerve
(CN VIII)
Fig. 21-4B
Fig. 21-6
Fig. 21-9B
Ampulla
Fig. 21-3B
Fig. 21-4A
Fig. 21-8B
Fig. 21-10B,C
Vestibular nerve
(CN VIII)
Vestibular
ganglia
(of Scarpa)
Utricle
Endolymphatic sac
Semicircular
ducts
Cochlear
duct
Apex
of cochlea
Base
of cochlea
Position of
oval window
in osseous
labyrinth
Ductus reuniens
A
Cochlear
duct
Apex
of cochlea
Modiolus
Base
of cochlea
Bone
Cochlear
nerve
Osseous
spiral lamina
Spiral
ganglion
Spiral
ligament
Scala
tympani
B
Figure 21-3B.
Cochlear duct and modiolus.
The membranous
cochlear
duct
lies within a spiral-shaped tunnel
in the temporal bone. In this illustration, the cochlea has been cut
in hal± along the plane indicated by the
single blue line
in Figure
21-3A and in the
inset
. The spiral o± the cochlear duct makes
about two and one-hal± turns ±rom the base o± the cochlea to its
apex. The outer bony sur±ace o± the cochlea is indicated in
light
gray
. A spiral, screw-shaped bony structure, the
modiolus
±orms
the central core o± the cochlea and is indicated in
darker gray
. A
spiral cavity within the modiolus contains the cell bodies o± the
spi-
ral ganglion
(
green
) and the proximal axons o± the
cochlear nerve
(
CN VIII
). The
osseous
(
bony
)
spiral lamina
curves around the
modiolus like the threads o± a screw (
red dashed line
and
inset
).
The central edge o± the cochlear duct is attached to the spiral
lamina; the outer wall o± the cochlear duct is attached to the spiral
ligament. Histological sections o± these structures are shown in
Figures 21-4A and 21-4B.
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