Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4399_Библиотеки_им_академика_М_И_Перельмана
.pdf
Temporal Bone Histology and Radiology Atlas36
https://t.me/med1917
Figure 3–8. Coronal CT image shows: 1, EAC; 2, mastoid air cells;3, tegmen mastoideum;4, tegmen tympani; 5, tympanic seg-
ment of the facial nerve;6, labyrinthine segment of the facial nerve;7, petrous apex;8, basal turn of the cochlea;9, interscalar
septum;10, middle turn of the cochlea;11 , carotid canal;12, tendon of the tensor tympani;13, lateral process of the malleus;
14, lateral malleal ligament;15, malleus (head). (Reprinted, with permission, from Juliano AF, Ginat DT, Moonis G. Imaging
review of the temporal bone: Part I. Anatomy and inammatory and neoplastic processes. Radiology2013;269:1,17–33.)
the radiographic image is frequently 1 to 2 mm thick
while the histologic section is 20 microns (0.02 mm)
thick. Even a 0.3-mm-thick section from a CBCT is
10 times thicker than a histologic section. Therefore,
although the level of detail seen with current CT imaging is remarkable, it cannot show the detail that is seen
in the histologic section.
Figures 3–5, 3–6 and 3–7 are axial CT images of a
right temporal showing the structures of the ear from
a more superior cut (Figure 3–5), midlevel cut (Fig ure 3–6), and a more inferior cut (Figure 3–7). Fig ures 3–8 and 3–9 are coronal CT images demonstrating
that which is visible radiographically more anteriorly
(Figure 3–8) and more posteriorly (Figure 3–9).
It is important that the surgeon and radiologist
train their eyes to correlate the axial, coronal, and sagittal radiographic images with previously studied horizontal and vertical histologic sections.
3. Radiology Techniques for Optimal Computed Tomography and Magnetic Resonance Images 37
https://t.me/med1917
Figure 3–9. Coronal CT image shows:1, mastoid air cells; 2, tegmen mastoideum;3, tegmen tympani; 4, IAC; 5, vestibule;
6, hypotympanum;7, mesotympanum; 8, epitympanum; 9, cochlear promontory; 10, tympanic membrane; 11 , scutum; 12,Prus-
sak space; 13, malleus (head); 14, stapes (crus); 15, superior semicircular canal; 16, tympanic segment of the facial nerve; 17, oval
window; 18, crista falciformis; 19, EAC. (Reprinted, with permission, from Juliano AF, Ginat DT, Moonis G. Imaging review
of the temporal bone: Part I. Anatomy and inammatory and neoplastic processes. Radiology 2013;269:1, 17–33.)
within tissue types and the difference in relaxation times
MAGNETIC RESONANCE IMAGING
is used to distinguish normal from pathological tissues.
Each tissue has two relaxations—T1 or longitudinal reMagnetic resonance imaging (MRI) uses a magnet and
a radiofrequency pulse. In the magnet, the hydrogen atoms in the water of the individual’s body tissues line
up along the magnetic field. When RF pulses are sent
in, the atoms “flip” into another plane and then “relax”
when the pulse is turned off. This relaxation time varies
laxation time, and T2 or transverse relaxation time.
MRI resolution is measured by the number of
pixels in a specified field of view.
6
The higher the
resolution (and the higher the number of pixels), the
smaller are the structures that can be seen. The two
resolution parameters used in MRI for production of a
Temporal Bone Histology and Radiology Atlas38
https://t.me/med1917
two-dimensional image are basic and phase resolution. Basic resolution is the number of pixels in the
readout direction and is inversely proportional to the
size of the pixel. Therefore, the lower the resolution,
the higher will be the pixel size and the smaller will
be the number of pixels. Signal-to-noise ratio (SNR) is
directly proportional to the pixel size. As such, higher
resolution and therefore smaller pixels will reduce the
image’s SNR. Phase resolution is the number of pixels
in the phase direction. Decreasing phase resolution will
reduce image quality and scan time, and will increase
pixel size and therefore increase SNR considerably.
IAC/temporal bone MRI scans are performed as
follows. Initially, a three-plane localizer T1 weighted
low resolution scan is taken to localize and plan the
sequences. Then the actual scans are planned and are
obtained in axial, coronal, and sagittal planes. Various
sequence protocols are used depending on the clinical concern. Slice thicknesses by MR sequence are: T2
TSE axial—5 mm, T2 TSE coronal—3 mm, 3D CISS—
0.8 mm, T1 TSE axial and coronal—1 to 3 mm, and, when
imaging the inner ear in particular, axial high-resolution
T2-weighted images can be obtained in thickness as
little as 0.4 to 0.7 mm. The 3D CISS method offers high
resolution and contrast and can be extremely useful in
showing small structures surrounded by fluid, including detailed delineation of the seventh-eighth nerve
complex in the temporal bone as well as the membranous labyrinth (Figure 3–10). 3D CISS and 3D FSE offer
submillimeter slices, which provide much higher resolution than conventional techniques.
7
Routine Magnetic Resonance Imaging
Technique for the Temporal Bone
The standard MRI protocol for evaluation of the temporal bone in adults is detailed below for a 1.5T (Tesla)
magnet. This section is adapted from work by Hugh
Curtin, MD, et al.,2 and is limited to the temporal bone.
Brain image acquisition is not described.
The patient is placed in the supine position in
the head coil. Axial T1-weighted images are obtained
through the temporal bone from the arcuate eminence
through the mastoid tip using the following parameters: TR (time to repetition) = 300 ms; TE (echo time) =
12 ms; flip angle = 90 degrees; slice thickness = 3 mm;
distance factor = 0.10; matrix = 192 × 256 (phase to frequency encoding steps); FOV = 180 mm; two acquisitions, one saturation, time = 3 minutes, 15 seconds.
Axial CISS (constructive interference in steady
state; Siemens AG, Berlin/Munich, Germany) or 3D
(three-dimensional) FIESTA (fast imaging employing
steady-state acquisition; General Electric Healthcare,
Waukesha, WI) images are obtained through the internal auditory canals and pons using the following parameters: TR = 12.25; TE = 5.9; flip angle = 70 degrees;
one slab, slab thickness = 32 mm; effective thickness =
0.7 mm; number of partitions = 46; matrix = 230 ×
512; FOV = 200; swap left (L) to right (R), no saturation, time = 4 minutes, 20 seconds. Gadolinium is then
administered.
Thin section axial T1-weighted images of the
temporal bone are performed in two interleaved sets,
using the following parameters: TR = 450 ms; TE =
15 ms; flip angle = 90 degrees; slice thickness = 2 mm;
distance factor = 0.10; matrix = 192 × 256 (phase to frequency encoding steps); FOV = 170 mm; two acquisitions, swap L to R, one saturation, time = 4 minutes,
20 seconds for each set; total time = 8 minutes, 40 seconds.
Coronal T1-weighted images are obtained through
the internal auditory canals using the following parameters: TR = 450 ms, TE = 15 ms, flip angle = 90 degrees,
slice thickness = 3 mm, no gap, matrix = 192 × 256
(phase to frequency encoding steps), FOV = 170 mm,
three acquisitions, swap L to R, one saturation, time =
4 minutes, 22 seconds. Coronal and sagittal high-
resolution T1-weighted images may be useful for more
detailed imaging of the temporal bone (Figure 3–10).
The technical parameters are as follows: TR = 528 ms;
TE = 12 ms; flip angle = 90 degrees; slice thickness =
3 mm; distance factor = 0.20; matrix = 338 × 512 (phase
to frequency encoding steps); FOV = 200 mm; two acquisitions, swap L to R, no saturation, time = 5 minutes, 7 seconds.
MR angiography may be performed, most commonly for the indication of tinnitus. In those cases,
MRA is used to evaluate for dural arteriovenous fistulas, aneurysms, vasculopathies such as fibromuscular
dysplasia, or arteriovenous malformations (AVMs).
But this is outside the scope of the current Atlas.
Specic Ear Anatomy and Abnormalities
Identiable on Magnetic Resonance Imaging
In the temporal bone, MR imaging does not delineate
bony anatomy as well as CT scanning; MR imaging is
most useful in delineating soft tissue relationships in
this area. It is the “gold standard” in evaluation of cerebellopontine angle tumors, which represent up to 10%
3. Radiology Techniques for Optimal Computed Tomography and Magnetic Resonance Images 39
https://t.me/med1917
Figure 3-10. Axial CISS MRI sequence of the left cerebellopontine angle and IAC demonstrates
the normal cisternal and intracanalicular segments of the left CN VII (single black arrow) anterior
to CN VIII (double lined white arrow). Normal cerebrospinal uid (CSF) signal is seen, as is the
normal uid signal in the vestibule and lateral semicircular canal. (Reproduced, with permission,
from Reference 4 in Chapter 5.)
of intracranial tumors. Vestibular schwannomas (VS)
account for approximately 75% of these lesions, with
the remainder including meningiomas, arachnoid cysts,
epidermoids, lipomas, metastatic tumors, or vascular
lesions. Advances in MR techniques allow diagnosis of
tumors as small as 2 mm in size. Pre-infusion images
can be used to diagnose lesions such as lipomas, which
are bright on both T1 and T2 weighting. For complete
MR evaluation, gadolinium must always be used to
evaluate for tumor enhancement. However, screening
protocols exist using surface magnetic coils and special
algorithms with a technique called T2-weighted T2 FSE
which does not require contrast and these limited scans
focus on the internal auditory canal and brain stem. Using this technique, the tumor is hypointense relative to
cerebral spinal fluid (CSF).
8
Cases of labyrinthitis can show faint enhancement
of the fluid-filled spaces of the membranous labyrinth
on contrast T1-weighted images. This is thought to be
due to gadolinium accumulation due to inflammationinduced vessel wall breakdown. Labyrinthitis can ap-
pear in several portions of the labyrinth (i.e., cochlea,
vestibule, semicircular canals). This enhancement typically subsides after 6 months or more.
Ossification of the labyrinth, which can occur following labyrinthitis, may impede the ability for ideal
cochlear implantation insertion and outcomes. CT may
miss early ossification, while MR may show it. In these
cases, registration and fusion of both CT and MR of the
temporal bone can enhance diagnostic capability.
9
Endolymphatic sac tumors and intralabyrinthine
schwannomas are seen on MR imaging. It is also being
increasingly used in the diagnostic workup of children
with congenital hearing disorders, looking in particular
for an enlarged vestibular aqueduct, which can occur
with other malformations. In this condition, CT imaging demonstrates a vestibular aqueduct that is greater
than 1.5 mm at the midway between the common crus
and the external aperture. An associated finding is observed on T2-weighted FSE: a dilated endolymphatic
sac and duct of variable size. Notably, a large endolymphatic sac/duct can cause sensorineural hearing loss
Temporal Bone Histology and Radiology Atlas40
https://t.me/med1917
without a large vestibular aqueduct. Dysplasia of the
membranous labyrinth can be delineated using highresolution MR imaging. Hearing loss typically varies
with the extent of the membranous abnormality. For example, lateral semicircular cell carcinoma dysplasia, an
asymptomatic condition, is a common incidental finding on MR imaging.
Cholesterol granulomas are bright on all spin-echo
sequences, which differentiates them from cholesteatoma. Special techniques can be used to distinguish fat
present in normal marrow due to unique signaling properties. A ring of low signal intensity may confirm the
presence of hemosiderin-laden macrophages. The center may produce a cystic appearance consistent with
lipid and cholesterol crystals. Non-echo planar diffusion weighted MR imaging is proving to be useful in
identifying recurrent or residual cholesteatoma, and
may help the surgeon avoid having do to an unnecessary “second-look” procedure on a subset of patients.
SIGNIFICANCE
When comparing MR and CT images, it is important
to keep in mind the thickness of each slice. Similarly,
when comparing MR images to histologic sections, the
relative thicknesses must be taken into consideration,
as MR image thickness can vary from 10 to 100 times
the histologic sections thickness. There will be a difference in the fidelity of images between those modalities. MR imaging does not delineate bony anatomy as
well as does CT scanning, so the comparison between
histology and radiographic images in this Atlas will be
limited to CT.
Clinical familiarity with CT and MR images of the
temporal bone is helpful when viewing histologic sections. The clinician should keep in mind that each histological section is 20 µm in thickness, which is sizably
thinner than any “corresponding” radiographic image. The level of detail, therefore, will be much higher
in the lab specimen than on the image; however, it is
helpful to keep the one in the mind’s eye when viewing the other.
As described in Chapter 2, it is also feasible to image temporal bones once fixated in celloidin, prior to
sectioning. Those images can have thicknesses as low
as 0.5 mm and therefore a similar caveat applies.
REFERENCES
1. Virapongse C, Stephen LG, Rothman SLG, Kier EL, Sar-
war M. Computed tomographic anatomy of the temporal
bone. AJNR 1982;3:379–389.
2. Caruso PA, Smullen JL, Liu R, Cunnane MB, Curtin HG.
Temporal bone imaging technique. http://radiologykey
.com/temporal-bone-imaging-technique/.
3. Juliano AF, Ginat DT, Moonis G. Imaging review of the
temporal bone: Part I. Anatomy and inflammatory and
neoplastic processes. Radiology
4. Jäger L, Bonell H, Liebl M, et al. CT of the normal temporal
bone: Comparison of multi– and single–detector row CT.
Radiology 2005;235:1, 133
5. Minor LB, Solomon D, Zinreich JS, Zee DS. Sound- and/
or pressure-induced vertigo due to bone dehiscence of
the superior semicircular canal. Arch Otolaryngol Head
Neck Surg 1998; 124:249–258.
6. Lemmerling M, De Foer B, Smet B. Temporal bone imaging
techniques. In: Lemmerling M, De Foer M, eds. Temporal
Bone Imaging, Medical Radiology. Diagnostic Imaging. Berlin:
Springer-Verlag; 2014.
7. George R, Singh R, Dela Cruz J, et al. MRImaster.com, Oct
2014.
8. Valvassori GE. The internal auditory canal revisited. The
high-definition approach. Otolaryngol Clin North Am. 1995
Jun. 28(3):431–451.
9. Bartling SH, Peldschus K, Rodt T, et al. Registration and
fusion of CT and MRI of the temporal bone. J Comp Asst
Tomography 2005;29(3):305–10.
2013;269:1, 17–33.
–141.
CHAPTER 4
https://t.me/med1917
Temporal Bone Osteology
Hosakere K. Chandrasekhar
LATERAL SKULL AND BASE OF SKULL
Knowledge of temporal bone osteology is foundational
before embarking on surgery of the temporal bone.
Parts of the temporal bone are visible on the lateral surface of the intact skull and skull base. Examination of
the lateral view of an articulated skull reveals the semicircular squamous part of the temporal bone above the
bony external auditory meatus. The squamous part
articulates above with the parietal bone, in front and
above with the frontal bone, and below and anteriorly
with the sphenoid bone. The suture between the frontal and parietal, the coronal suture, curves down and
forward to articulate with the greater wing of the sphenoid. The upper border of the greater wing articulates
with the inferior angle of the parietal bone and this suture meets the coronal suture at a point known as the
pterion. Thus, the pterion is at the region where the frontal, parietal, temporal, and sphenoid join on the side
of the skull. Here the bone is thin and weak; it overlies
the anterior division of the middle meningeal artery.
Similarly at the meeting point of parietal, occipital, and
mastoid temporal bones, behind the bony external auditory meatus, is a region called the asterion. Both the
pterion and asterion are thin areas of the cranium, and
are useful sites for neurosurgical access (Figure 4–1).
CLINICAL CAVEAT: A relatively low force blunt
injury to the skull at the pterion where the bone is
weak and overlies the middle meningeal artery may
cause subdural hematoma. The clinician’s index of
suspicion for such an intracranial complication with
trauma at this site should, therefore, be low.
The paired temporal bones contribute to both the
base and the lateral walls of the skull. They serve a
dual function: the rock-hard petrous temporal at the
base protects the otic capsule within it and the laterally situated squamous bone contributes to protect the
contents of the cranium. Together the superior surfaces
of the petrous bones constitute parts of the middle and
posterior cranial fossae. Inferiorly the irregular surface
of the petrous bone presents processes and foramina in
the mid-third of the base for passage of vital structures.
TEMPORAL BONE OVERVIEW
Developmentally the temporal bone is divisible into
four portions. These are: the squamous, petromastoid,
tympanic, and styloid process. These morphologically
distinct elements have fused in the course of evolution
of higher vertebrates.
1. The squamous part is a dermal bone evolved to help
enclosure of the brain.
2. The petromastoid part is pre-formed in cartilage;
as such, it preserves the precise orientation of the
membranous labyrinth.
3. The tympanic part formed in mesenchyme. It is ho-
mologous with the os angulare, part of the composite
lower jaw of many reptiles and bony fishes which
is integrated into the skull and adapted to form the
tympanic cavity and external auditory meatus and
support the tympanic membrane. This portion is
concerned with sound transmission.
4. The styloid process—which is the dorsal element of
the hyoid arch seen projecting down between the
41
Figure 4–1. Lateral skull view showing the relationship of the frontal, parietal, and squamosa portion of the temporal bones.
https://t.me/med1917
The circle is on the pterion; the triangle is on the asterion. The relationship of the anterior middle meningeal artery (A) on the
medial surface of the skull to the pterion is seen. The posterior middle meningeal artery is marked as P.
42
4. Temporal Bone Osteology 43
https://t.me/med1917
tympanic bone and jugular fossa—is mentioned here
because it contributes parts of the ossicles.
The Squamous Temporal Bone
The squamous part of the temporal bone is located anterosuperiorly and is thin and partly translucent. Superiorly, it articulates with the parietal bone and anteroinferiorly, with the greater wing of the sphenoid.
Posteriorly, it merges with the mastoid process. Its external surface is smooth and slightly convex and forms
part of the temporal fossa. This area gives attachment
to the temporalis muscle. Medially and above the external auditory meatus it is grooved vertically by the
middle temporal artery (Figure 4–2A).
The supramastoid crest curves backward and up-
ward across the posterior part of the squamous bone.
The crest traced forward joins the superior border of
the zygomatic process. The temporal muscle and fascia are attached to the crest. Between the anterior end
of the crest and the posterosuperior quadrant of the
external auditory meatus is the suprameatal triangle,
also referred to as MacEwen’s triangle or the cribrose area
for the punctations seen in the bone. The suprameatal
triangle is an important surgical landmark during mastoidectomy: the mastoid antrum is approximately 1.25 cm
deep to it. Anteriorly the triangle presents the supra-
meatal spine of Henle.
The cerebral surface of the squamous bone is concave and presents grooves for temporal lobe convolutions. Additionally, the middle meningeal artery grooves
this surface.
The zygomatic process of the squamous bone juts for-
ward from its lower region. The process twists forward
from a broad base. Posteriorly it is seen to have two
roots that join to form the tubercle of the zygomatic root,
providing attachment to the lateral temporomandibular
ligament. Going posteriorly, the posterior root contin-
ues as the supramastoid crest. The anterior root turns
medially to form the articular tubercle for the temporomandibular joint. Behind this area the posterior root
ends sharply at the tympanic bone to form the postglenoid tubercle. The superior border of the zygomatic
process is sharp and gives attachment to the temporal fascia. The masseter muscle arises from its medial
surface and inferior border. The lateral surface is subcutaneous. The anterior end of the zygomatic process
articulates with the zygomatic bone. There is a small
area anterior to the articular tubercle on the inferior aspect and this forms the roof of the infratemporal fossa.
The mandibular fossa, limited anteriorly by the ar-
ticular tubercle, has an anterior articular area formed
by the temporal squama and a posterior non-articular
area by the anterior surface of the tympanic bone. In life
this area has a small extension of the parotid gland. The
post-glenoid tubercle seen behind this area prevents
posterior displacement of the temporomandibular joint.
The mandibular fossa presents the squamotympanic fissure that leads into the tympanic cavity and generally
carries the anterior ligament of the malleus and the anterior tympanic branch of the maxillary artery.
The Petromastoid Temporal Bone
The petromastoid portion of the temporal bone can be
subdivided into the mastoid part and the petrous part.
The mastoid bone is a downward projecting pro-
cess immediately behind the external auditory meatus.
Above, it fuses with the squamosa and behind the squamosa it articulates with the parietal as well as occipital
bones. Medially the mastoid process fuses with the petrous bone. It is of dual origin: the squamous part overlies the petrous part and the resulting petrosquamous
septum fuses completely or partially.
The outer surface quite often presents a foramen
for transmission of the mastoid emissary vein which
connects with the sigmoid sinus.
CLINICAL CAVEAT: When initiating mastoidectomy, it is important to fully identify the cribrose
area and begin drilling anterosuperiorly at the suprameatal triangle so that the antrum is encountered deep. A common error made by novice otologic surgeons is beginning the drilling inferiorly.
This misorientation places the second genu of the
facial nerve at increased risk of surgical trauma.
CLINICAL CAVEAT: The mastoid emissary vein can
be encountered, with resultant venous bleeding, as
the soft tissues are reflected off of the mastoid bone
in preparation for mastoidectomy. This bleeding is
easily controlled with application of pressure and
bone wax.
Figure 4–2. Temporal bone shown in three projections: lateral, medial, and inferior. (From F. Paulsen, J. Waschke, Sobotta Atlas
https://t.me/med1917
of Human Anatomy. 15thed. Munich: ©Elsevier GmbH, Urban & Fischer; 2013. With permission.)
44
4. Temporal Bone Osteology 45
https://t.me/med1917
Two superficial muscles—the occipitofrontalis and
auricularis posterior—arise from its surface above, and
below the process gives attachment to the splenius capitus and longissimus capitis overlain by the sternomastoid muscle. On its medial surface is a deep notch
for origin of the posterior belly of the digastric muscle.
Medial to the notch is a shallow groove for the occipital
artery. The internal surface presents the deep vertical
notch for the sigmoid sinus.
At birth, the mastoid process is absent or rudimentary. It forms postnatally, as the sternocleidomastoid muscle develops and pulls on the bone.
CLINICAL CAVEAT: Because the extratemporal facial nerve is not protected by the mastoid bone at
birth or in early childhood, it is susceptible to blunt
trauma such as forceps for delivery or minor head
trauma from falling off of a tricycle, and is at risk for
laceration injury from a postauricular incision that
extends below the nascent mastoid tip (Figure 4–3).
The petrous bone is pyramidal in shape and is diagonally placed between the sphenoid and occipital
bones in the cranial fossa. The petrous part has an apex,
a base, three surfaces, and two margins or borders.
The acoustic labyrinth lies within it. The apex is anteromedial and irregular and angled between the posterior border of the greater wing of the sphenoid and
basi occiput. It forms the posterolateral boundary of the
foramen lacerum, above which travels the lacerum
segment of the internal carotid artery. The base of the
pyramid is posterolateral and fused with the squama
(Figure 4–2B).
i. Anterior Surface
This surface (anatomically called the anterior surface
and surgically referred to as the superior surface) forms
the posterior part of the middle cranial fossa and is continuous with the cerebral surface of the squama. The entire anterior surface is adapted to the inferior tem poral
gyri.
Behind the apex is the impression for the trigeminal ganglion. Anterolateral to this the bony scale roofs
the carotid canal. A ridge separates this from the roof of
the internal auditory meatus and cochlea. Posterior to
this area is the arcuate eminence overlying the loop of
CLINICAL CAVEAT: The middle cranial fossa approach to the internal auditory canal (IAC) depends
on the relationship of the arcuate eminence and the
facial canal with the greater petrosal nerve in it.
When observing the superior or anterior surface of
the petrous bone, identify the angle created by the
arcuate eminence and the greater petrosal nerve.
Mentally bisecting that angle will give you the lie
of the IAC. Remember that there is more room for
drilling around the canal medially than there is laterally, where the cochlea lies just anterior and the
vestibule just posterior to the IAC.
the superior semicircular canal. The arcuate eminence
ranges in presentation from obvious and prominent to
nearly flat. Lateral to this area is the tegmen tympani—
the roof of the tympanum or middle ear space. The anterior margin toward the apex, also called the anterior
angle, is free, and with the large wing of the sphenoid
bone, it bounds an opening—the muscular tubal canal—which is subdivided by a flange of bone into a
small upper canal for the tensor tympani muscle and a
larger lower canal for the Eustachian tube. Further in
an anterior direction toward the apex, two small openings can be seen, each with a groove running forward
from it. The larger, medial one is the groove for the facial canal which transmits the greater petrosal nerve
and the petrosal branch of the middle meningeal artery. The smaller lateral groove is for the lesser petrosal
nerve and the superior tympanic artery.
ii. Posterior Surface
The posterior surface lies in an almost vertical plane as
it faces the posterior cranial fossa. About midway between the base and apex of the pyramid is situated the
circular opening of the internal acoustic meatus (IAM),
which gives passage to the seventh and eighth cranial
nerves and vessels of the IAC.
Behind the opening of the meatus and somewhat
above is a depression representing the subarcuate
fossa. In the fetus it is termed the petromastoid canal. It
runs through the arc of the superior semicircular canal,
and in the fetus it is capacious, carrying blood vessels
to the developing otic capsule. During post-infancy
life, the contents of the fossa eventually fibrose or, at
most, carry a small subarcuate artery.
Соседние файлы в папке Библиотека им академика М.И. Перельмана