• Squamous: Forms lateral wall of middle cranial fossa
• Mastoid: Aerated posterolateral part and contains mastoid
• Petrous: Pyramidal shape forms the medial part containing
inner ear, internal auditory canal & petrous apex
• Tympanic: V-shaped bone forming bony EAC
• Styloid: Forms styloid process
Squama / Squamous Part
• The squama forms the anterior and upper part of the bone,
and is scale-like & thin.
• Surfaces — Outer surface (convex / Temporalis muscle /
• Groove for the middle temporal artery.
• Temporal line or supramastoid crest,
attachment of the temporal fascia
and limits the origin of the
• The boundary between the squama and the mastoid portion
of the bone, as indicated by traces of the original suture, lies
about 1 cm.
• Below this line is a long, arched process, the zygomatic
• Superior border of zygomatic procress: Attachment of the
• Inferior & medial surface border attaches fibers of the
• The anterior end is deeply serrated and articulates with the
• The posterior end is connected to the squama by two roots,
the anterior and posterior roots.
• The posterior root, a prolongation of the upper border, is
• It runs backward above the external acoustic meatus, and is
continuous with the temporal line.
• The anterior root, continuous with the lower border, is short
but broad and strong
• It is directed medialward and ends in a rounded eminence,
the articular tubercle (eminentia articularis).
• Suprameatal triangle (Macewen), or mastoid fossa, through
which an instrument may be pushed into the tympanic
• The mandibular fossa (glenoid fossa)
• Divided into two parts by a narrow slit, the petrotympanic fissure
• The internal surface of the
squama is concave; it presents
depressions corresponding to
the convolutions of the
temporal lobe of the brain, and
grooves for the branches of the
middle meningeal vessels.
• The superior border (beveled & overlaps the squamous border
of the parietal bone)
• Posteriorly, the superior border forms an angle, the parietal
notch, with mastoid portion.
• The antero-inferior border
is thick, serrated, and beveled
for articulation with the great
wing of the sphenoid.
• The posterior end of the superior temporal line continues
inferiorly as the supramastoid crest and blends into the upper
edge of the zygomatic arch.
The supramastoid crest is located at the level of the floor of the
• Mastoid Portion (pars mastoidea)—The mastoid portion
forms the posterior part of the bone.
• The mastoid part is trabeculated and pneumatized to a
variable degree and contains the mastoid antrum.
• Borders—The superior border of the mastoid
portion is broad and serrated, for articulation with
the mastoid angle of the parietal.
• The posterior border, also serrated, articulates with
the inferior border of the occipital
between the lateral angle
and jugular process.
• Anteriorly the mastoid portion is fused with the descending
process of the squama above
• Below it enters into the formation
of the external acoustic meatus
and the tympanic cavity.
• Mastoid Air cells – Irregular in shape
• At the apex of the process are frequently quite small and
• Occasionally they are
and the mastoid is then
• Tympanic antrum is situated at the upper and front part of
the bone & may communicate with the mastoid cells
• Filled with air and lined by a prolongation of the mucous
membrane of the tympanic cavity, with which it
• The tympanic antrum is bounded above by a thin plate of
bone, the tegmen tympani.
• Below - mastoid process
• Laterally - squama just below
the temporal line
• Medially - lateral semicircular
canal of the internal ear which
projects into its cavity.
• It opens in front into that portion of the tympanic cavity
which is known as the attic or epitympanic recess.
• The tympanic antrum is a cavity of some considerable size at
the time of birth;
• Mastoid air cells may be regarded as diverticula from the
antrum, and begin to appear at or before birth
• By the fifth year they are
well-marked, but their
development is not
completed until toward
Petrous Portion (pars petrosa [pyramis])
• The petrous portion or pyramid is pyramidal
and is wedged in at the base of the skull
between the sphenoid and occipital.
• Directed medially, forward, and a little
• Has a base, an apex, three surfaces, and
three angles, and contains, in its interior, the
essential parts of the organ of hearing.
• The base is fused with the
internal surfaces of the squama
and mastoid portion.
• Rough and uneven, is received into the angular interval
between the posterior border of the great wing of the
sphenoid and the basilar part of the occipital
• It has the anterior or internal orifice of the carotid canal, and
forms the postero-lateral boundary of the foramen lacerum.
• The anterior surface forms the posterior part of the middle
fossa of the base of the skull,
• Inner surface of the squamous portion, to which it is united by
It is marked by depressions for the convolutions of the brain
Points for examination:
An eminence (eminentia arcuata) at the centre which
indicates the situation of the superior semicircular canal
Little lateral to this eminence,
a depression indicating the position
of the tympanic cavity
Hiatus of the facial canal
Greater superficial petrosal nerve and
the petrosal branch of the middle
Lateral to which
Lesser superficial petrosal nerve -
carotid canal - trigeminal
impression for the reception of the
The posterior surface
• Near the center is a large orifice,
the internal acoustic meatus
• The lateral end of the canal is closed by a
vertical plate, which is divided by a
horizontal crest, the crista falciformis,
into two unequal portions
In the portion beneath the crista falciformis are three sets of
• Area cribrosa media, consists of several small openings for
the nerves to the saccule;
• Foramen singulare, or opening for the nerve to the posterior
• Tractus spiralis foraminosus, consisting of a number of small
spirally arranged openings
• Canalis centralis cochleae
• These openings together with this
central canal transmit the nerves to
• Area cribrosa superior, pierced by a series of small openings,
for the passage of the nerves to the utricle and the superior
and lateral semicircular ducts.
• Area facians, with one large opening, the commencement of
the canal for the facial nerve (aquaeductus Fallopii).
• Aquaeductus vestibuli, (behind internal acoustic meatus)
transmits the ductus endolymphaticus together with a small
artery and vein.
• The inferior surface is rough and irregular, and forms
part of the exterior of the base of the skull.
The superior angle - the longest / grooved for the superior
petrosal sinus, and gives attachment to the tentorium
cerebelli; at its medial extremity is a notch, in which the
trigeminal nerve lies.
The posterior angle is
marked by a sulcus,
which forms, with a
corresponding sulcus on
the occipital bone, the
channel for the inferior
Its lateral half presents an excavation—
the jugular fossa—which, with the jugular
notch on the occipital, forms the jugular
The anterior angle is divided into two parts—a lateral joined to
the squama by a suture (petrosquamous), a medial, free,
which articulates with the spinous process of the sphenoid.
• At the angle of junction of the
petrous part - two canals
• The upper one - semicanalis m.
tensoris tympani (Tensor tympani)
• The lower one - semicanalis tubae
auditivae forms the bony part of the
• Septum canalis musculotubarii
• The middle ear or tympanic
cavity is an irregular, laterally
compressed space within the
petrous temporal bone.
• The tympanic cavity consists
of two parts: the tympanic
cavity proper, opposite the
tympanic membrane, and
the attic or epitympanic
recess, above the level of the
• The tympanic cavity is bounded laterally by the
• Medially, by the lateral wall of the internal ear
• Behind, with the tympanic antrum and through it
with the mastoid air cells
• Infront with the auditory tube
• The Tegmental Wall or Roof (paries tegmentalis) - tegmen
• The Jugular Wall or Floor (paries jugularis) is narrow, and
consists of a thin plate of bone (fundus tympani) which
separates the tympanic cavity from the jugular fossa.
• The Labyrinthic or Medial Wall is vertical in direction, consists
of the fenestrae vestibuli and cochlea, the promontory, and
the prominence of the facial canal.
Fenestra vestibuli (fenestra
ovalis) is an opening leading from
the tympanic cavity into the
Fenestra cochlea is an opening
leading into the cochlea
The promontory is a rounded
hollow prominence, formed by the
projection outward by the first
turn of the cochlea
• The prominence of the facial canal indicates the position of
the bony canal in which the facial nerve is contained
• This canal traverses the labyrinthic wall of the tympanic cavity
above the fenestra vestibuli
Courtesy: Dr. Dilip Panikar – Translab
• The auditory tube is the
channel through which the
communicates with the nasal
part of the pharynx.
• Approximately - 36 mm
• Directed downward, forward,
• It is formed partly of bone,
partly of cartilage and fibrous
Drawing of the ear by Max Brodel
• The osseous labyrinth comprises a layer of dense bone (otic
capsule) in the petrous part of the temporal bone and the
enclosed perilymphatic space, which contains a fluid very
similar to extracellular fluid, the perilymph.
The perilymphatic space
consists of a series of
semicircular canals, vestibule,
The Superior, posterior, and lateral
semicircular canals are at right
angles one to another
The semicircular ducts
provide sensory input
for experiences of
Courtesy: Dr. Dilip Panikar – Translab Exposure Steps c
• The vestibule is the central part of the osseous
labyrinth, and is situated medial to the tympanic
cavity, behind the cochlea, and in front of the
• It is a spiral-shaped cavity in
the bony labyrinth, in humans
making 2.5 turns around its axis,
• A core component of the cochlea is
the Organ of Corti, the sensory
organ of hearing, which is
distributed along the partition
separating fluid chambers in the
coiled tapered tube of the cochlea.
• The perilymphatic duct, or aqueduct of the cochlea, is situated
in a bony channel, the cochlear canaliculus.
• It connects the perilymphatic space to the subarachanoid
• This works as a type of shunt to eliminate excess
perilymph fluid from the perilymphatic space around
Tympanic Part (pars tympanica)
• The tympanic part is a curved plate of bone
lying below the squama and in front of the
Postero-superior surface forms
the anterior wall, the floor, and
part of the posterior wall of
the bony external acoustic
constitutes the posterior
boundary of the mandibular
• Its lateral border gives
attachment to the cartilaginous
part of the external acoustic
• Internally, the tympanic part is
fused with the petrous portion.
• Posteriorly, it blends with the
squama and mastoid part.
• Its upper border fuses laterally
with the back of the postglenoid
process, while medially it bounds
the petrotympanic fissure.
• The medial part of the lower
border splits to enclose the root
of the styloid process, and is
therefore named the vaginal
• Directed downward and forward, from the
under surface of the temporal bone.
• Its distal part gives attachment to the
stylohyoid and stylomandibular ligaments,
and to the Styloglossus, Stylohyoideus,
and Stylopharyngeus muscles.
• The stylohyoid ligament extends from the
apex of the process to the lesser cornu of
the hyoid bone, and in some instances is
partially, in others completely, ossified.
• The meeting point of the
parietomastoid and squamous
sutures is located a few
millimeters below the lateral
end of the petrous ridge.
• The anterior edge of the
junction of the sigmoid and
transverse sinuses is located at
the junction of the squamous
and parietomastoid suture.