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170 ■ ANATOMY OF THE TEMPORAL BONE WITH SURGICAL IMPLICATIONS
enchondral bone (for instance, the cochlea, superior canal, and facial nerve
at the area nervi facialis) demonstrated relatively little variability in
anatomic relationships. In contrast, neural and vascular structures showed a
considerable degree of variability, both with respect to each other as well as
to inner ear landmarks.
Fisch (185) has modified the House (182) approach to the internal auditory canal, using the arcuate eminence as his primary reference point; however, as noted by Parisier (184), the prominence as well as its relationship to
the superior canal is quite variable, depending upon the pattern and degree
of temporal bone pneumatization. According to Fisch, the superior canal
forms a 60° angle with respect to the superior vestibular nerve; he uses this
fact to locate the posterior margin of the internal auditory canal. Parisier
noted considerable variability in this relationship.
THE FACIAL NERVE
Chapter 6
Neuroanatomy
The Functional Components
The facial nerve is the nerve of the second branchial arch, and as such
innervates structures derived from Reichert’s cartilage (see “Embryology,”
chap. 9). Five populations of fibers contribute to the facial nerve trunk (189,
190): (1) special visceral efferent fibers, which supply the striated muscles of
facial expression, the stapedius muscle, the stylohyoid muscle, and the
posterior belly of the digastric muscle; (2) general visceral efferent fibers (preganglionic secretory fibers) which are distributed to the lacrimal and seromucinous glands of the nasal cavity via the greater superficial petrosal nerve
(see also Nervus Intermedius, p. 187) and to the submaxillary and sublingual
glands through the chorda tympani nerve; (3) special sensory fibers for taste
from the anterior two-thirds of the tongue through the chorda tympani
nerve and from the tonsillar fossae and palate via the greater superficial
petrosal nerve; (4) somatic sensory fibers supplying the external auditory canal
and adjacent conchal region, as well as conveying proprioceptive information from the facial muscles; and (5) visceral afferent fibers serving the mucosa
of the nose, pharynx, and palate. Three nuclei supply the fibers to the facial
nerve: (1) The motor nucleus is located in the caudal aspect of the pons. Its
superior part, which supplies the frontal and orbicularis oculi muscles,
receives both crossed and uncrossed fibers from the precentral gyrus (motor
cortex). The inferior part of the facial motor nucleus receives only homolateral, uncrossed cortical information to innervate the remainder of the facial
musculature, save for the levator palpebrae superioris. The blink reflex and
stapedius reflex are mediated through internuclear connections in the
medulla oblongata (191). (2) The superior salivatory nucleus is situated dorsal
to the motor nucleus and carries parasympathetic secretory stimuli to the
submaxillary, sublingual, lacrimal, nasal, and palatine glands. (3) The
nucleus of the solitary tract which is located in the medulla oblongata receives
the taste, proprioceptive, and cutaneous sensory fibers of the facial nerve.
The origins of the motor roots and sensory roots (nervus intermedius)
are located at the inferior border of the pons; there, in a recess between the
inferior cerebellar penduncle and the olive, the motor root lies medially with
respect to the nervus intermedius and the acoustic nerve lies laterally. A
schema of the distribution of the motor, taste, and parasympathetic fibers is
seen in Figure 1.
171
172 ■ ANATOMY OF THE TEMPORAL BONE WITH SURGICAL IMPLICATIONS
Figure 1
Schematic diagram of the facial
nerve illustrating the distribution of motor ( ), taste
( ) and parasympathetic
( ) fibers (255).
THE NORMAL COURSE IN THE TEMPORAL BONE
The combined sensory and autonomic components of the facial nerve are
separated from the motor component distally as far as the geniculate ganglion into the nervus intermedius (see p. 187) and motor trunk. Distal to the
geniculate ganglion, the sensory component is segregated into a discrete
bundle within the trunks of the facial nerve. The question as to whether
there is a topographical organization in the facial nerve has been argued positively (192–194) and negatively (195, 196), but the issue appears finally to
have been resolved by Gacek and Radpour (197). Using an anterograde
degeneration technique in combination with selective lesions of the facial
nerve in cats, they could find no topographic segregation of the motor fibers.
The course of the facial nerve may be divided anatomically into five
segments:
(1) The first or intracranial segment of the facial nerve spans the 23 to 24mm
between its origin at the pons and the internal auditory canal, cradled in a
groove on the superior surface of the cochlear nerve. The nervus intermedius parallels the facial nerve and joins it in a spiraling fashion in the
fundus of the internal auditory canal (198).
(2) The second or internal auditory canal segment is 7 to 8mm in length. The
facial nerve maintains its superior position relative to the cochlear nerve and
passes above the transverse (falciform) crest to enter the fallopian canal at
the area nervi facialis.
(3) The third, labyrinthine (intratemporal) segment (Fig. 2), is the shortest at only
3 to 4mm. It begins at the area nervi facialis and heads anteriorly
and laterally, running superior to the cochlea and vestibule and nearly perpendicular to the petrous pyramid, until it reaches the geniculate ganglion. The
geniculate ganglion houses the cell bodies serving both the sensory (taste)
CHAPTER 6: NEUROANATOMY ■ 173
Figure 2
This vertical section shows the labyrinthine
segment of the facial nerve. In this region the
nerve is susceptible to injury by transverse
fractures of the temporal bone (male, age 71 yr).
fibers of the chorda tympani nerve and the greater superficial petrosal nerve,
and the preganglionic secretory fibers for the sphenopalatine ganglion.
Ganglion cells serving pain reception are also believed to be within the geniculate ganglion (199) and are held responsible for the pain of petrosal neuralgia.
In a topographic study, Dobozi (200) consistently found that the geniculate ganglion, which appears triangular in the horizontal plane of sectioning, averaged
1.09mm in length, 0.76 mm in width, and 0.6 to 0.8 mm in height, showing
little variability among the specimens examined. Ultrastructural studies of the
guinea pig geniculate ganglion (201) have demonstrated two types of ganglion
cells: light cells and smaller dark cells. However, the exact functional significance of these two types of cells remains obscure. The anterior limit of the
geniculate ganglion lies in close relation to the middle cranial fossa, from which
it is usually separated by a bony plate; however, it may lie free in a bony dehiscence in the floor of the middle cranial fossa (see the section on The Facial
Hiatus, p. 184). Distal to the geniculate ganglion, the facial nerve turns abruptly
posteriorly, forming the first genu of the facial nerve (Figs. 3–5). The bony
174 ■ ANATOMY OF THE TEMPORAL BONE WITH SURGICAL IMPLICATIONS
Figure 4
In this ear the germ of the facial nerve
lies deeply buried in the petrous bone
far from the dura of the middle cranial
fossa. The greater superficial petrosal
nerve channels anteriorly through bone
to emerge from a small facial hiatus
(female, age 53 yr).
Figure 3
In approximately 5% of cases (230), the
genu of the facial nerve lies dehiscent
in the middle cranial fossa (female, age
16 yr).
fallopian canal, dubbed with a misnomer of “aqueductus” by Gabriele
Falloppio [cited by Politzer (202)] because it reminded him of a water pipe,
courses about 30mm from the area nervi facialis to the stylomastoid foramen.
Not only is this bony canal riddled with multiple deficiencies in its walls (see
the section on Dehiscences of the Facial Nerve, p. 179), but also it can act as a
strangulating tunnel in the presence of facial nerve edema and facial palsy.
(4) The fourth or tympanic segment parallels the longitudinal axis of the
petrous pyramid, running posteriorly and laterally on the medial wall of the
CHAPTER 6: NEUROANATOMY ■ 175
Figure 5
The genu of the facial nerve normally
lies in the medial wall of the epitympanum and is protected by a bony covering. There is an otosclerotic focus
partly surrounding the labyrinthine
segment of the nerve (male, age 62 yr).
tympanic cavity between the lateral canal superiorly and the oval window
inferiorly for 12 to 13mm. At the sinus tympani the nerve turns inferiorly.
This second genu marks the beginning of the mastoid segment.
(5) The fifth or mastoid segment carries the nerve vertically downward in the
posterior wall of the tympanic cavity and the anterior wall of the mastoid,
a distance of 15 to 20mm, to the nerve’s egress from the skull at the stylomastoid foramen.
Ogawa and Sando (203) studied the relative cross-sectional area of the
facial nerve with respect to its canal in histologic preparations of 18 normal
temporal bone specimens. They found that in its labyrinthine and tympanic
segments the nerve occupied, on the average, somewhat more than 45% of
the canal, whereas in the mastoid segment the figure was 32%.
ABNORMAL COURSES IN THE TEMPORAL BONE
The otologic surgeon must be aware of the various anomalous courses of the
facial nerve. Working in bone with a high-speed cutting bur, the surgeon
must develop a technique which will allow for an unexpected encounter
with the nerve without injury to it.
There are numerous examples of cases in which the facial nerve
pursues an anomalous course through the petrous bone (191, 204–212). The
most common example is that in which the main nerve trunk runs anterior
and inferior to the oval window (213, 214). Rarely, the nerve pursues a
course anterior to both the oval window and the round window (215, 216).
The infant lacks a true mastoid process and possesses only a rudimentary tympanic ring, leaving the facial nerve vulnerable to injury at the stylomastoid foramen. In subsequent development, the nerve becomes more
secluded by the medial migration of the stylomastoid foramen dictated by
the growth of the tympanic ring and mastoid.
176 ■ ANATOMY OF THE TEMPORAL BONE WITH SURGICAL IMPLICATIONS
Figure 6
The sketch shows the second genu of
the facial nerve more posteriorly
located than usual. After Miehlke (256).
Figure 7
The mastoid segment of the facial
nerve in this ear has divided into three
separate bundles, each of which exits
separately from the temporal bone
(female, age 61 yr).
Dehiscences in the bony canal also render the facial nerve potentially
vulnerable to surgical injury. The mastoid segment of the nerve may be
displaced several millimeters posteriorly and/or laterally (Fig. 6). There are
cases of bi- or even tri-partition of the nerve in which the individual
branches course to their separate points of exit from the skull, each in its own
canal (Figs. 7–9). The nerve may pass anterosuperiorly to the cochlea rather
than posterosuperiorly as it normally does (Figs. 10 and 11). In its mastoid
segment it may swerve more posteriorly than normal (Fig. 12).
CHAPTER 6: NEUROANATOMY ■ 177
Figure 8
The sketch demonstrates tripartition of
the facial nerve distal to the second
genu. Compare to Figure 7. Source:
After Miehlke (256).
Figure 9
The sketch demonstrates bifurcation of
the facial nerve. Source: After Miehlke
(256).
178 ■ ANATOMY OF THE TEMPORAL BONE WITH SURGICAL IMPLICATIONS
Figure 11
In this ear the facial nerve in its
labyrinthine segment takes an aberrant
course anterosuperior to the cochlea
(male, age 56 yr).
Figure 10
The facial nerve may pass anterior to
the cochlea as shown here. The nerve
may also course between the oval and
round windows or over the promontory just anterior to both windows.
Source: After Miehlke (256).
CHAPTER 6: NEUROANATOMY ■ 179
Figure 12
The sketch demonstrates the facial
nerve coursing in the mastoid far
posterior to its normal location. Source:
After Miehlke (256).
Other anomalous courses are more generally associated with aural
malformations, such as an hypoplastic facial nerve passing through the
obturator foramen of the stapes (217).
Litton et al. (218) studied the anatomic relationship of the facial nerve
in adults with respect to the tympanic annulus and noted great variability in
the course of the facial nerve through its tympanic and mastoid segments. It
is to be remembered that the tympanic annulus in the adult is not in the
sagittal plane, but rather is directed anteriorly and inferiorly. The usual
location of the facial nerve is 1.4mm posterior and 2.3 mm medial to the
posterosuperior part of the tympanic annulus. The descending segment of
the facial nerve is consistently located posterior to the posteroinferior
margin of the tympanic annulus. The facial canal, as it heads laterally and
inferiorly, crosses the plane of the tympanic annulus in its lower one-half.
DEHISCENCES OF THE FACIAL NERVE
The facial nerve passes through the temporal bone, protected throughout
most of its course by the bony sheath of the fallopian canal. Politzer (219)
described “congenital gaps in the facial canal.” As documented by Baxter
(220), it is not unusual for gaps to exist in the continuity of this bony sheath;
he found dehiscences of the fallopian canal, defined as non-pathologic gaps
of 0.4mm or greater in diameter, in either the tympanic or mastoid segments
of the facial nerve in 55% of the temporal bones studied. Moreover, more than
one dehiscence was found in 22% of the ears examined. The most common
site of dehiscence of the bony canal involved the tympanic segment adjacent
to the oval window (Figs. 13–15), where the facial nerve normally overhangs
the oval window niche. The average width of the dehiscences was 0.92mm
in the oval window region and 0.73mm in the mastoid segment. Other
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