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180 ■ ANATOMY OF THE TEMPORAL BONE WITH SURGICAL IMPLICATIONS
Figure 13
The most common area for dehiscence
of the facial nerve is in the region of the
oval window. In this case, the facial
nerve protrudes from the fallopian
canal and overlies part of the stapes
footplate (male, age 51 yr).
Figure 14
The facial nerve in its tympanic
segment is seen protruding from its
canal and encroaching on the oval window. This ear also shows the pathologic condition of endolymphatic
hydrops (female, age 68 yr).
CHAPTER 6: NEUROANATOMY ■ 181
Figure 16
In this ear there is a bony dehiscence of
the fallopian canal in the medial wall of
the facial recess (female, age 50 yr).
Figure 15
The facial nerve protrudes from its
canal to overlie the footplate partially
and nearly abut a posteriorly located
cochleariform process (male, age 82 yr).
histologic studies of the temporal bone (221, 222) and other observations
(213, 223–226) also confirm that the oval window area of the canal is the most
common site for dehiscence. Dehiscences of the facial nerve may also be
found adjacent to the tensor tympani tendon, in the facial recess (Fig. 16), and
in the medial wall of the anterior epitympanic recess (Figs. 17 and 18).
Dehiscences in the bony covering of the facial nerve provide areas of
vulnerability to surgical injury. This risk is increased when the nerve bulges
out of the dehiscence, as sometimes occurs in the oval window area (Figs. 19
and 20) (227). It is possible that dehiscences predispose the facial nerve
to inflammatory disease of the middle ear. Facial palsy may occur as a
complication of acute otitis media.
182 ■ ANATOMY OF THE TEMPORAL BONE WITH SURGICAL IMPLICATIONS
Figure 18
There is a bony dehiscence of the facial
nerve canal in the medial wall of the
anterior epitympanic recess. The nerve
bulges slightly into the recess (female,
age 72 yr).
Figure 17
The anterior epitympanic recess
appears as a single large cell with the
facial nerve dehiscent of bone in its
medial wall. In many cases, as shown
here, the facial nerve trunk in both its
tympanic and labyrinthine segments is
composed of several bundles (female,
age 65 yr).
CHAPTER 6: NEUROANATOMY ■ 183
Figure 19
This photograph of a partially
dissected temporal bone shows a
tumor-like herniation of the facial
nerve from the fallopian canal just
superior to the oval window (see
Fig. 20). Source: Courtesy of Johnsson
and Kingsley (227).
Figure 20
Same specimen as Figure 19, showing a
histologic cross section of the nerve
after it has been removed from the
facial canal. The entire nerve trunk
takes an omega-shaped course out of
its canal. Source: Courtesy of Johnsson
and Kingsley (227).
184 ■ ANATOMY OF THE TEMPORAL BONE WITH SURGICAL IMPLICATIONS
Figure 21
The genu of the facial nerve lies under
the dura of the middle cranial fossa.
This ear does not have an anterior
epitympanic recess (female, age 79 yr).
THE FACIAL HIATUS
The facial hiatus (hiatus canalis nervi petrosi majoris) is a dehiscence of
variable size present in the petrous part of the temporal bone in the floor of
the middle cranial fossa; it marks the entrance of the greater superficial
petrosal nerve into the middle cranial fossa. This nerve originates from the
geniculate ganglion, located on the anterior aspect of the genu of the facial
nerve. Usually the geniculate ganglion lies deep to the hiatus, in which case
the greater superficial petrosal nerve passes through a bony canal to reach
the hiatus. In some cases the geniculate ganglion lies under the dura (Fig. 21)
within the hiatus (228). According to Ge and Spector (229), at 15 weeks’
gestation the geniculate ganglion lies in a dural condensation superior to the
anterior part of the epitympanum, and the “primitive facial hiatus” provides
a route of communication between the middle cranial fossa and the middle
ear cavity. As the squamous part of the temporal bone develops, it separates
the geniculate ganglion from the epitympanic space. The superior surface of
the geniculate ganglion, however, is still dehiscent in the 35-week-old
embryo, and its perineural tissues are directly attached to the dura and
middle cranial fossa. This dehiscence may persist to a variable extent even
into adulthood. House and Crabtree (230) found that the geniculate ganglion
was exposed to the middle cranial fossa without a bony covering in 5% of
cases. Hall et al. (228) found a 15% incidence of partial or total exposure of
the geniculate ganglion to the middle cranial fossa in a study of 100 adult
temporal bones. Saito et al. (231) reviewed 400 temporal bones in the collection of the Massachusetts Eye and Ear Infirmary and found that in 9% of the
cases the facial hiatus measured more than 1.5mm in its greatest dimension,
leaving the geniculate ganglion open to the middle cranial fossa.
CHAPTER 6: NEUROANATOMY ■ 185
Figure 22
In the mastoid segment the sensory
component of the facial nerve is located
in the posterolateral part of the nerve
trunk (male, age 62 yr).
Variations in the anatomy of the facial hiatus are of significance for two
reasons: (1) when the geniculate ganglion and facial nerve lie within the hiatus,
they are vulnerable to injury during surgical procedures involving the floor of
the middle cranial fossa, and (2) the facial hiatus is used as an anatomic landmark in the middle fossa approach to the internal auditory canal.
BRANCHES OF THE FACIAL NERVE
The facial nerve gives off three major branches in its course through the
temporal bone: (1) The first is the greater superficial petrosal nerve (Fig. 4),
which originates from the anterior aspect of the geniculate ganglion. It surfaces at the facial hiatus and enters the middle cranial fossa, from which this
mixed nerve of parasympathetic and sensory fibers courses anteriorly
toward the foramen lacerum. It unites with the sympathetic fibers of the
deep petrosal nerve to form the vidian nerve (nerve of the pterygoid canal).
Leaving its canal anteriorly, the vidian nerve passes through the pterygopalatine fossa to enter the sphenopalatine ganglion. (2) The second branch
is the nerve to the stapedius muscle. It arises from the mastoid segment of the
facial nerve in the region of the pyramidal eminence. (3) The fibers of the
third branch, the chorda tympani nerve, are located in the sensory bundle of
the facial nerve trunk which occupies approximately 10% of its total crosssectional area (Fig. 22) (231). The sensory bundle (chorda tympani fibers)
occupies an anterolateral position in the tympanic segment and posterolateral position in the mastoid segment of the facial nerve before separating
from the facial nerve trunk. The nerve usually arises about 4mm superior to
the stylomastoid foramen, although it may arise distal to the stylomastoid
foramen. The nerve follows a course which is phylogenetically determined;
it is the pretrematic branch of the second branchial arch, and connects the
186 ■ ANATOMY OF THE TEMPORAL BONE WITH SURGICAL IMPLICATIONS
Vth cranial nerve (the nerve of the first branchial arch) with the nerve of the
second branchial arch (the facial nerve). Thus it runs with the facial nerve in
the area derived from the second branchial arch and with the trigeminal
nerve (mandibular division) in the region derived from the first arch.
It follows a recurrent route superiorly in its own canal (the canaliculus
chordae tympani), and enters the tympanic cavity through an opening in the
posterior wall, the iter chordae posterius, at the horizontal level of the round
window and the cochlear aqueduct. Here it is accompanied by the posterior
tympanic artery. The iter chordae posterius generally lies between the pyramidal eminence and the tympanic annulus in the vertical plane. In its transtympanic course the chorda tympani nerve is housed in a fibrous sheath and
shrouded in a layer of mucous membrane. As it heads anteriorly, it lies
medial to the posterior malleal ligament (Fig. 23) and then passes lateral to
the long process of the incus and medial to the neck of the malleus,
suspended between these two ossicles. The chorda tympani nerve passes
from the neck of the malleus, in which it may occupy a groove (Fig. 24), thus
paralleling the anterior process of the malleus. The chorda tympani
nerve takes a direct route from the iter posterius to the iter anterius; stretching of this nerve by disease processes or surgical manipulation causes temporary loss of secretion of the submandibular gland and loss of taste on the
ipsilateral anterior two-thirds of the tongue (232, 233). The iter chordae
anterius (canal of Huguier) marks the exit of the chorda tympani nerve from
the tympanic cavity as it enters the petrotympanic (Glaserian) fissure; here
the nerve is accompanied by the anterior tympanic artery. The nerve then
exits the skull at the medial surface of the spina angularis of the sphenoid
bone; occasionally it occupies a groove in this spine—the groove of Lucas—
as it travels anteriorly to join the lingual nerve.
Figure 23
This photomicrograph illustrates the pretympanic spine and its relationship to
the chorda tympani nerve. The posterior
malleal ligament is the thickened inferior
margin of the posterior malleal (mucosal)
fold. The posterior pouch of von Tröltsch
is located between the posterior
tympanic stria and the posterior malleal
ligament (female, age 72 yr).
CHAPTER 6: NEUROANATOMY ■ 187
Figure 24
The chorda tympani nerve frequently
passes in a sulcus on the medial surface
of the neck of the malleus at the base of
the anterior process. Prussak’s space is
located medial to Shrapnell’s membrane (female, age 55 yr).
Variations in the anatomy of the chorda tympani nerve may also occur.
The chorda of the infant normally separates from the facial nerve beyond the
skull which it re-enters by its own canal, anterior to the stylomastoid
foramen. This separate canal may persist into adulthood (189). At the other
extreme, the chorda tympani nerve may exit from the facial nerve at the level
of the lateral canal (234). The chorda tympani nerve varies in size and, like
the facial nerve, it may be bipartite (206, 213). The chorda tympani nerve’s
point of entry into the middle ear cavity may be as much as 1 to 2mm lateral
to the rim of the external auditory canal, and the nerve may pass laterally,
instead of medially, to the neck of the malleus (206, 213).
Variations in the location of the chorda tympani nerve are of significance in transcanal surgery. In these procedures the posterior part of the
tympanic membrane is elevated and the adjacent bony tympanic annulus is
removed to expose the posterior mesotympanum. Section of the chorda
tympani nerve in some patients may cause symptoms of partial ageusia or
dysgeusia and dry mouth. Stretching of the nerve may also cause these
symptoms with partial recovery after some months (233).
THE NERVUS INTERMEDIUS
The sensory component of the facial nerve, known variously as the nervus
intermedius, the nerve of Wrisberg, or the glossopalatine nerve (189), contains visceral afferent (taste) fibers and general visceral efferent (secretory)
fibers (Figs. 1 and 25).
Its efferent neurons lie in the superior salivatory nucleus, located
dorsomedial to the motor nucleus of the facial nerve. The chorda tympani
nerve and the greater superficial petrosal nerve, both branches of the nervus
188 ■ ANATOMY OF THE TEMPORAL BONE WITH SURGICAL IMPLICATIONS
intermedius, carry these secretory fibers to the submaxillary and sphenopalatine ganglia, respectively. The fibers from the submaxillary ganglion
innervate the submaxillary and sublingual glands, while those fibers from
the sphenopalatine ganglion supply the lacrimal gland and mucosal glands
of the nose and palate.
The neurons serving the sensory function of taste lie in the geniculate
ganglion; their fibers, which travel with the fibers of the tractus solitarius,
end in the nucleus of this tract. These fibers from the ipsilateral palatal and
pharyngeal mucosa travel in the greater superficial petrosal nerve, while
those from the ipsilateral anterior two-thirds of the tongue travel in the
chorda tympani nerve.
There also appears to be a somatic sensory component, serving the skin
of the external auditory canal by fibers which travel with the auricular
branch of the vagus (Arnold’s nerve); the facial nerve is linked to Arnold’s
nerve via branches which pass between the two nerves just before the facial
nerve leaves the stylomastoid foramen. The cell bodies are located in the
geniculate ganglion, and centrally their fibers end in the spinal tract of the
Vth cranial nerve (189).
Through histologic study of the temporal bones of two patients with
facial nerve lesions, Saito et al. (231) traced the course and the position of the
sensory nerve bundle within the facial nerve trunk. In the internal auditory
canal the nervus intermedius courses between the superior division of the
vestibular nerve and the facial nerve (Fig. 25). In the tympanic segment of
the facial nerve the sensory bundle is located dorsally, while in the vertical
segment it assumes a more lateral and posterior position. It finally exits anteriorly as the chorda tympani nerve.
Figure 25
The nervus intermedius is seen in the
posterior portion of the facial nerve
trunk in the internal auditory canal.
Part of this nerve continues beyond the
genu as the sensory bundle where it
assumes a lateral position within the
tympanic segment of the nerve trunk
(female, age 5 yr).
CHAPTER 6: NEUROANATOMY ■ 189
Figure 26
Occasionally a large vein accompanies
the facial nerve in the fallopian canal.
This vein may cause troublesome
bleeding during surgical procedures on
the facial nerve. Afibrous partition separates the fallopian canal from the compartment for the stapedius muscle
(male, age 40 yr).
THE VASCULAR SUPPLY OF THE FACIAL NERVE
The arterial supply of the facial nerve is derived from a variety of vessels as
it courses from the pons to the stylomastoid foramen. In its intracranial
segment it is supplied by the anterior inferior cerebellar artery and in its
internal auditory canal segment by the labyrinthine artery. The geniculate
ganglion is richly supplied by the superficial petrosal artery, a branch of the
middle meningeal artery. For the remainder of its course in the fallopian
canal, the nerve is supplied by the anastomosing branches of the superficial
petrosal and stylomastoid arteries (235).
The vascular supply of the facial nerve is not uniform throughout the
course of the nerve, nor does it occupy a constant proportion of the fallopian
canal. Ogawa and Sando (203) found that in the labyrinthine segment of the
facial nerve canal the vascular channels occupied 12% of the cross-sectional
area, while in the tympanic segment the figure was 63% and in the mastoid
segment 54%. Alarge vein is frequently present in the fallopian canal (Fig. 26).
The Sensory Nerves of the Middle Ear
Jacobson’s nerve is the eponymic name for the inferior tympanic nerve; it
arises from the inferior ganglion of the IX nerve which is located in the
petrosal fossula at the caroticotympanic spine. Referred pain in the ear
caused by pharyngeal disorders is mediated by this nerve. Like the chorda
tympani nerve, this is a pretrematic nerve, and serves to interconnect the
VIIth (facial) and IXth (glossopharyngeal) nerves of the second and third
branchial arches, respectively. Having gained access into the middle ear via
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