Chapter 15
The Special Senses
573
15
canals posteriorly. Te saccule and utricle house equilibrium
receptor regions called
maculae
that respond to the pull of grav-
ity and report on changes of head position.
Semicircular Canals
Te
semicircular canals
lie posterior and
lateral to the vestibule, and each of these canals defines about
two-thirds of a circle. Te cavities of the bony semicircular ca-
nals project from the posterior aspect of the vestibule, each ori-
ented in one of the three planes of space. Accordingly, there is
an
anterior
, a
posterior
, and a
lateral
semicircular canal in each
internal ear. Te anterior and posterior canals are oriented at
right angles to each other in the vertical plane, whereas the lat-
eral canal lies horizontally (Figure 15.26).
Snaking through each semicircular canal is a correspond-
ing membranous
semicircular duct
, which communicates
with the utricle anteriorly. Each of these ducts has an enlarged
swelling at one end called an
ampulla
, which houses an equi-
librium receptor region called a
crista ampullaris
(literally, crest
of the ampulla). Tese receptors respond to angular (rotational)
movements of the head.
Cochlea
Te
cochlea
(kok
9
le-ah), from the Latin “snail,” is
a spiral, conical, bony chamber about the size of a split pea.
It extends from the anterior part of the vestibule and coils for
about 2½ turns around a bony pillar called the
modiolus
(mo-
di
9
o-lus)
(Figure 15.27a)
. Running through its center like a
wedge-shaped worm is the membranous
cochlear duct
, which
ends blindly at the cochlear apex. Te cochlear duct houses the
receptor organ of hearing, called the
spiral organ
or the
organ
of Corti
(Figures 15.26 and 15.27b).
Te cochlear duct and the
osseous spiral lamina
, a thin
shelflike extension of bone that spirals up the modiolus like the
thread on a screw, together divide the cavity of the bony co-
chlea into three separate chambers or
scalae
(
scala
5
ladder).
Te internal ear has two major divisions: the bony labyrinth
and the membranous labyrinth.
Te
bony labyrinth
is a system of tortuous channels worm-
ing through the bone. Te views of the bony labyrinth typi-
cally seen in textbooks, including this one, are somewhat
misleading because we are talking about a
cavity
here. Te
representation in Figure 15.24 can be compared to a plaster
of paris cast of the cavity or hollow space inside the bony
labyrinth.
Te
membranous labyrinth
is a continuous series of mem-
branous sacs and ducts contained within the bony labyrinth
and (more or less) following its contours
(Figure 15.26)
.
Te bony labyrinth is filled with
perilymph
, a fluid similar to
cerebrospinal fluid and continuous with it. Te membranous lab-
yrinth is suspended in the surrounding perilymph, and its inte-
rior contains
endolymph
, which is chemically similar to K
1
-rich
intracellular fluid. Tese two fluids conduct the sound vibrations
involved in hearing and respond to the mechanical forces occur-
ring during changes in body position and acceleration.
Te bony labyrinth has three regions: the
vestibule
, the
semi-
circular canals
, and the
cochlea
.
Vestibule
Te
vestibule
is the central egg-shaped cavity of the
bony labyrinth. It lies posterior to the cochlea, anterior to the
semicircular canals, and flanks the middle ear medially. In its
lateral wall is the oval window.
Suspended in the vestibular perilymph and united by a small
duct are two membranous labyrinth sacs, the
saccule
and
utri-
cle
(u
9
trĭ-kl) (Figure 15.26). Te smaller saccule is continuous
with the membranous labyrinth extending anteriorly into the
cochlea (the
cochlear duct
), whereas the utricle is continuous
with the semicircular ducts extending into the semicircular
Anterior
Semicircular ducts in
semicircular canals
Posterior
Lateral
Cristae ampullares
in the membranous
ampullae
Utricle in vestibule
Saccule in vestibule
Stapes in
oval window
Temporal
bone
Facial nerve
Vestibular nerve
Superior vestibular ganglion
Inferior vestibular ganglion
Cochlear nerve
Maculae
Spiral organ
Cochlear duct in cochlea
Round window
Figure 15.26
Membranous labyrinth of the internal ear.
The membranous labyrinth
(blue) lies within the chambers of the bony labyrinth (tan). The locations of the sensory organs
for hearing (spiral organ) and equilibrium (maculae and cristae ampullares) are shown in purple.
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