Anatomy and Physiology of Ear1
Look at the anvil of a blacksmith—how it is hammered and beaten; yet it moves not from its place. Let men learn patience and endurance from it.
—Sri Ramakrishna Dev
The temporal bone has an interesting multifaceted anatomy. The important structures present and their complicated anatomic interrelations make the temporal bone surgery a challenge.
- Relations: It articulates with five cranial bones: parietal, sphenoid, occipital, zygomatic and mandible. This pyramidal-shaped bone forms part of the base and lateral side of skull (Fig. 1). The petrous part separates middle cranial fossa from the posterior cranial fossa.
- Contents: It houses the hearing and vestibular organs. The important structures which pass through it include internal carotid artery, internal jugular vein, and facial nerve. So the temporal bone houses following structures:
- ▸ Bony portion of external auditory canal (EAC)
- ▸ Middle ear containing malleus, incus and stapes
- ▸ Internal ear containing peripheral portions of auditory and vestibular system
- ▸ Fallopian canal containing facial nerve
- ▸ Osseous canal for the internal carotid artery
- ▸ Bony covering for the sigmoid sinus and the jugular bulb.
- Parts: The five portions of temporal bones are referred to as separate bones and include:
- ▸ Squamous
- ▸ Petrous
- ▸ Tympanic
- ▸ Mastoid
- ▸ Styloid process.
It forms lateral wall of middle cranial fossa and roof of bony EAC. Its zygomatic process forms roof of the mandibular fossa (Fig. 2).
- Anterior surface (Fig. 3): Posterior boundary of middle cranial fossa
- ▸ Medially arcuate eminence of superior semicircular canal (SCC)
- ▸ Laterally tegmen tympani and tegmen antri
- ▸ Anteriorly canal for tensor tympani and eustachian tube
- ▸ At apex depression for trigeminal ganglion
- ▸ Foramina and sulci of greater and lesser superficial petrosal nerves.
- Posterior surface (Fig. 4): Anterior boundary of posterior cranial fossa
- ▸ Sulci for sigmoid sinus and superior petrosal and inferior petrosal sinuses
- ▸ Internal auditory meatus (IAM): Facial and auditory nerves and labyrinthine vessels
- ▸ Subarcuate artery depression lies superior and lateral to IAM
- ▸ Operculum opening for vestibular aqueduct lies inferolateral to IAM
- ▸ Jugular foramen is formed at the union of petrous and occipital bone.
- Inferior surface (Fig. 5):
- ▸ Opening for cochlear aqueduct at the superior-most limit of jugular foramen
- ▸ Inferior foramen of carotid canal for internal carotid artery is situated anterior to jugular fossa and separated from it by a wedge of bone called keel
- ▸ Tympanic canaliculus for tympanic branch (Jacobson's nerve) of glossopharyngeal nerve penetrates keel.
It forms anterior wall, floor and posterior wall of bony EAC, anterior wall and floor of middle ear and posterior part of mandibular fossa.
- Two sutures in bony EAC: Anteriorly tympanosquamous and posteriorly tympanomastoid
- Petrotympanic suture in middle ear through which chorda tympani nerve exits.
Sternocleidomastoid (SCM) muscle is inserted on mastoid tip. The posterior belly of digastric muscle is attached to the digastric sulcus that terminates anteriorly at the stylomastoid foramen.
Fig. 5: Inferior view of left temporal bone. Note that some adjacent part of greater wing of sphenoid is also seen
Emissary vein draining into the sigmoid sinus travels through mastoid foramen. Mastoid antrum is situated deeper to Macewen's triangle.
It is situated anterior to stylomastoid foramen and gives origin to three muscles (stylohyoid, styloglossus and stylopharyngeus) and attachment to two ligaments (stylohyoid and stylomandibular).
ANATOMY OF EAR
The ear is a complex organ. Its functional components, the hearing apparatus and balancing organ are situated in the temporal bone of skull. Ear has intimate relationship to the brain occupying middle and posterior cranial fossa, jugular bulb, sigmoid sinus, internal carotid artery and cranial nerve (CN) V, VI, VII, VIII, IX, X, XI and XII. For the sake of description, the ear is divided into three parts (Fig. 6):
- External ear
- Middle ear
- Internal ear.
The external ear is divided into auricle (pinna) and external acoustic or auditory canal (EAC). The TM separates external ear from the middle ear.
The auricle except its lobule is made up of (Figs 7 and 8) a framework of a single piece of yellow elastic cartilage (Fig. 8), which is covered with skin. The skin is adherent to the perichondrium on its lateral surface while it is comparatively loose on the medial surface. Epithelium is squamous keratinizing. Sebaceous glands and hair follicles are found in the subcutaneous tissue. Adipose tissue is present only in the lobule. There are various elevations and depressions, which can be seen on the lateral surface of pinna (Fig. 8).
- Incisura terminalis: This area is devoid of cartilage and lies between the tragus and crus of the helix.
- Nerve supply (Fig. 9):
- ▸ Auriculotemporal nerve (CN V3): It is a branch of mandibular division of trigeminal nerve and supplies anterosuperior part of lateral surface of pinna, including tragus and crus of helix.
- ▸ Cranial nerve VII (facial nerve): It innervates the skin of concha and antihelix, lobule and mastoid.
- ▸ Cranial nerve X (vagus nerve): Its auricular branch (Arnold's nerve) supplies to concha and postauricular skin.
- ▸ Greater auricular nerve (C2,3): This nerve of cervical plexus supplies most of the medial surface of auricle and posterior part of lateral surface and the postauricular region.
- ▸ Lesser occipital nerve (C2): This nerve of cervical plexus supplies upper part of medial surface of auricle and postauricular region.
External Auditory Canal
- Dimensions: EAC is 24 mm long and extends from the concha to the TM. Its anterior wall is 6 mm longer than the posterior wall. EAC is usually divided into two parts: (1) cartilaginous and (2) bony. Its outer one-third (8 mm) is cartilaginous and its inner two-thirds (16 mm) is bony.
- Direction: EAC is “S” shaped and not straight. Its outer one-third cartilaginous part is directed upward, backward and medially while its inner two-thirds bony part is directed downward, forward and medially.
- Cartilaginous EAC: It is a continuation of the cartilage that forms the framework of the pinna.
- ▸ Skin glands: The skin of the cartilaginous canal (Fig. 10) is thick and contains ceruminous and pilosebaceous glands that secrete wax. The hydrophobic, slightly acidic (pH 6.0–6.5) cerumen is formed in this part of EAC.
- Bony EAC: It is mainly formed by the tympanic portion of temporal bone but roof is formed by the squamous part of the temporal bone (Figs 2 and 11). In the anterosuperior region, squamous part articulates with tympanic bone (tympanosquamous suture). Inferiorly and medially squamous part joins with the lateral superior portion of the petrous bone (petrosquamous suture). Skin of the bony EAC is thin and continuous over the TM. Here the skin is devoid of subcutaneous layer, hair follicles, and ceruminous glands.
- ▸ Recess: Anteroinferior part of the deep bony meatus, medial to the isthmus has a recess, which is called the anterior recess.
- ▸ Foramen of Huschke: In children and occasionally in adults, anteroinferior bony EAC may have a deficiency that is called as foramen of Huschke.
- Relations of Bony EAC
- ▸ Superior: Middle cranial fossa
- ▸ Inferior: Parotid gland
- ▸ Posterior: Mastoid antrum and air cells and the facial nerve
- ▸ Anterior: Temporomandibular joint (TMJ)
- ▸ Medial: Tympanic membrane
- ▸ Lateral: Cartilaginous EAC.
- Epithelial migration: The skin of EAC has a unique self-cleansing mechanism. This migratory process continues from the medial to lateral side. The sloughed epithelium is extruded out as a component of cerumen.
- Nerve supply (Fig. 12):
- ▸ Auriculotemporal nerve (CN V3): It is a branch of mandibular division of trigeminal nerve and supplies anterosuperior wall of EAC.
- ▸ CN X (vagus nerve): Its auricular branch (Arnold's nerve) supplies to inferoposterior EAC.
- ▸ CN VII (facial nerve): It innervates the skin of the mastoid and posterior EAC.
Tympanic Membrane (Fig. 13)
- Dimensions: Its dimensions are: 9–10 mm height and 8–9 mm width. It is 0.1 mm thick.
- Position: Tympanic membrane is a partition wall between the EAC and the middle ear. It is positioned obliquely. It forms angle of 55° with deep EAC. Its posterosuperior part is more lateral than its anteroinferior part.
- Parts: Tympanic membrane consists of two parts: (1) pars tensa and (2) pars flaccida.
- ▸ Pars tensa: It forms most of TM.
- Annulus tympanicus: TM is thickened in the periphery and forms a fibrocartilaginous ring called the annulus tympanicus that fits in the tympanic sulcus.
- Umbo: The central part of TM near the tip of malleus is tented inwards and is called the umbo.
- Cone of light: A bright cone of light radiating from the tip of malleus to the periphery in the anteroinferior quadrant is usually seen during otoscopy.
- ▸ Pars flaccida (Shrapnell's membrane): It is situated above the lateral process of malleus between the notch of Rivinus and the anterior and posterior malleal folds. It is not as tense as pars tensa and may appear little pinkish.
- Structure: Tympanic membrane consists of the following three layers (Fig. 14):
- Middle fibrous layer: It encloses the handle of malleus and consists of three types of fibers: radial, circular and parabolic. In comparison to pars tensa, this layer is very thin in pars flaccida and not organized into various fibers.
- Inner mucosal layer: It is continuous with the middle ear mucosa.
- Otoscopy (Fig. 13): Normal TM is shiny and pearly gray in color. Its lateral surface is concave, which is more marked at the tip of malleus (umbo). Attic area lies above the lateral process of malleus and is slightly pinkish. Its transparency varies from person to person. Some middle ear structures can usually be seen through TM such as incudostapedial joint, round window (RW) and Eustachian tube.
- Mobility (Seigalization): A normal TM is mobile. It can be tested with pneumatic otoscope or Siegel's speculum.
- Nerve supply:
- ▸ Auriculotemporal nerve (CN V3): It is a branch of mandibular division of trigeminal nerve and supplies anterior half of lateral surface of TM.
- ▸ CN X (vagus nerve): Its auricular branch (Arnold's nerve) supplies to posterior half of lateral surface of TM.
- ▸ CN IX (glossopharyngeal nerve): Its tympanic branch (Jacobson's nerve) supplies to medial surface of TM.
The middle ear cleft (Fig. 15), which is lined by mucous membrane and filled with air, consists of the middle ear, Eustachian tube, aditus ad antrum, mastoid antrum and mastoid air cells. Middle ear is a 1–2 cm3 air-filled cavity that houses ossicles (malleus, incus, stapes), muscles (tensor tympani, stapedius) and nerves (chorda tympani, tympanic plexus).
Relations of Middle Ear Cleft
- Roof: Tegmen plate separates it from middle cranial fossa and its contents like meninges and temporal lobe of cerebrum.
- Floor: Jugular bulb
- Medial: Labyrinth. Lateral SCC lies posterosuperior to facial nerve.
- Posterior: Sigmoid intracranial venous sinus
- Anterior: Petrous part of internal carotid artery lying in carotid canal
- Posteromedial: Posteromedial to mastoid air cells is situated cerebellum in the posterior cranial fossa.
- Cranial nerves:
- ▸ CN V and CN VI: They lie close to the apex of the petrous pyramid.
- ▸ CN VII: The horizontal tympanic part is situated in the medial wall of middle ear, while vertical mastoid part runs between the middle ear and mastoid air cells system.
Parts of Middle Ear (Tympanum)
The dimensions of middle ear are shown in Figure 16. The tympanum (Fig. 17) is traditionally divided into three parts—mesotympanum, epitympanum and hypotympanum.
- Mesotympanum: This is the portion of middle ear that lies at the level of pars tensa.
- Epitympanum (attic): This is the portion of middle ear that lies above the level of pars tensa and medial to Shrapnell's membrane and the bony lateral attic wall.
- Hypotympanum: This is the portion of middle ear that lies below the level of pars tensa.
- ▸ Protympanum: The portion of middle ear around the Eustachian tube opening is termed as protympanum.
Boundaries of Middle Ear (Fig. 18)
Middle ear has six boundaries: roof, floor, medial, lateral, anterior and posterior walls.
- Roof (tegmental wall): It is formed by tegmen tympani (a thin plate of bone), which extends posteriorly to form the roof of the aditus and antrum (tegmen antri). Tegmen tympani separates middle ear from the middle cranial fossa.
- Floor (jugular wall): A thin plate of temporal bone separates tympanic cavity from the jugular bulb.
- Anterior (carotid wall): The anterior wall, a thin plate of bone, which separates the middle ear cavity from internal carotid artery, has following features:
- ▸ Eustachian tube: It connects the middle ear with nasopharynx. It aerates and drains the middle ear. (Eustachian tube is discussed in detail in Chapter “Disorders of Eustachian Tube”).
- ▸ Canal of tensor tympani muscle: It is situated in the roof of Eustachian tube.
- ▸ Canal for chorda tympani nerve.
- ▸ Attachment of anterior malleolar ligament.
- Posterior (mastoid wall): It lies close to the mastoid air cells and presents following structures:
- ▸ Pyramid: It is a bony projection through the summit of which appears the tendon of the stapedius muscle that is inserted to the neck of stapes.
- ▸ Aditus ad antrum: It is an opening through which mastoid antrum opens into the attic. It lies above the pyramid. Its relations are following:
- Medial: Bony prominence of the horizontal SCC
- Lateral: Fossa incudis (short process of incus lies and attached here).
- Inferior: Fallopian canal for facial nerve
- Superior: Tegmen antri.
- ▸ Facial nerve: The vertical mastoid part of the fallopian canal for facial nerve runs in the posterior wall just behind and below the pyramid.
- ▸ Facial recess or suprapyramidal recess (Fig. 19): This recess is a depression in the posterior wall lateral to the pyramid. Its boundaries are following:
- Medial: Vertical part of CN VII
- Lateral: Chorda tympani (branch of 7th CN) and tympanic annulus
- Superior: Fossa incudis (short process of incus lies and attached here).
- ▸ Sinus tympani or infrapyramidal recess: This deep recess lies medial to the pyramid. It is bounded by the bony ridges (subiculum below and the ponticulus above).
- Medial (labyrinthine wall) (Figs 21 and 22): It is formed by the lateral wall of labyrinth. It presents following structures:
- ▸ Promontory: It is a bony bulge which is due to the basal coil of cochlea.
- ▸ Oval window (fenestra vestibuli): The footplate of stapes is placed in this window.
- ▸ Round window (fenestra cochleae): It is covered by the secondary TM.
- ▸ Horizontal tympanic part of fallopian canal for facial nerve: It lies above the oval window.
- ▸ Lateral SCC: It lies above the fallopian canal of facial nerve.
- ▸ Processus cochleariformis: It is a hook-like projection, which lies anterior to the oval window. The tendon of tensor tympani takes a turn on this process and then is inserted on the neck of malleus.
- Lateral (membranous wall) (Fig. 23):
- ▸ Tympanic membrane: Lateral wall is formed mainly by the TM. Some structures of the middle ear (such as long process of incus, incudostapedial joint, RW and Eustachian tube) can be seen through the normal semitransparent TM.
- ▸ Scutum: An upper part of epitympanum is formed by outer bony attic wall called scutum.
The ossicles (Fig. 24) conduct sound energy from the TM to the oval window. There are three middle ear ossicles—malleus, incus and stapes.
- Malleus (hammer): It consists of a head, neck, handle (manubrium), a lateral and an anterior process. It is the largest ossicle and measures 8 mm in length.
- ▸ Head and neck: They lie in the attic.
- ▸ Manubrium: It is embedded in the fibrous layer of the TM.
- ▸ Lateral process: It appears as a knob-like projection on the outer surface of the TM and provides attachments to the anterior and posterior malleal folds.
- Incus (anvil): It consists of following parts:
- ▸ Body and short process: They lie in the attic.
- ▸ Long process: It hangs vertically and forms incudostapedial joint with the head of stapes.
- Stapes (stirrup): This smallest bone of body measures about 3.5 mm. It consists of head, neck, anterior and posterior crura and footplate. The footplate is positioned in the oval window by annular ligament.
There are two middle ear muscles: tensor tympani and the stapedius.
- Tensor tympani: It runs above the Eustachian tube. Its tendon turns around the processus cochleariformis and passes laterally. It tenses the TM.
- ▸ Origin: Bony tunnel above the osseous part of Eustachian tube.
- ▸ Insertion: Just below the neck of malleus.
- ▸ Nerve supply: It develops from the 1st branchial arch and is supplied by a branch of mandibular division of trigeminal nerve (CN V3).
- Stapedius: On contraction, it dampens the loud sounds and prevents noise trauma to the inner ear.
- ▸ Origin: Conical cavity and canal within pyramid.
- ▸ Insertion: It inserts to the neck stapes.
- ▸ Nerve supply: It is developed from the second branchial arch and is supplied by nerve to stapedius of facial nerve.
- Functions: Acoustic reflex protects ear from loud sounds.
- ▸ Dampening of middle ear mechanics: Loud sounds (80 dB and above) cause contraction of stapedius that limits stapes movement.
- ▸ Gain control mechanism: Acoustic reflex keeps cochlear input more constant and expands dynamic range.
- ▸ Reduction in self-generated noise: Stapedius muscle contracts with chewing and vocalization.
Intratympanic Nerves (Fig. 25)
- Tympanic plexus (nerve supply of middle ear): The tympanic nerve plexus, which lies on the promontory, supplies to the medial surface of the TM, tympanic cavity, mastoid air cells, and the bony Eustachian tube. It is formed by following nerves:
- ▸ Tympanic branch (Jacobson) of glossopharyngeal: It carries secretomotor fibers to the parotid gland. The pathway of secretomotor fibers to the parotid gland consists of inferior salivary nucleus > CN IX > Jacobson's tympanic branch > Tympanic plexus > Lesser petrosal nerve > Otic ganglion > Auriculotemporal nerve > Parotid gland.
- ▸ Sympathetic fibers: Caroticotympanic nerves come from the sympathetic plexus, which is present around the internal carotid artery.
- Chorda tympani nerve: This branch of the facial nerve enters the middle ear through posterior canaliculus. It runs on the medial surface of the TM. It lies between the malleus and long process of incus, above the insertion of tensor tympani. It carries gustatory fibers from the anterior two-thirds of tongue and parasympathetic secretomotor fibers to the submaxillary and sublingual salivary glands.
Middle Ear Mucosa
Middle ear mucosa wraps ossicles, muscles, ligaments and nerves like peritoneum wraps various viscera in the abdomen. These mucosal folds divide the middle ear into various compartments. So, all the middle ear structures lie outside the mucous membrane.
Fig. 25: Nerves in relation with the middle ear. Note secretomotor pathway of salivary, lacrimal and nasal glands
Mucous membrane of the nasopharynx is continuous with that of the middle ear cleft.
Middle ear cavity is lined by ciliated columnar epithelium in its anterior and inferior part and mucosa changes to cuboidal type in the posterior part. Attic and mastoid air cells are lined by flat, nonciliated epithelium. Eustachian tube is lined by ciliated pseudostratified columnar epithelium with several mucous glands in the submucosa.
Ossicles and their mucosal folds separate mesotympanum from epitympanum (attic).
- Compartments of epitympanum:
- ▸ Prussak's space: Its boundaries limit spread of cholesteatoma to other compartments.
- Lateral: Membrana flaccida (Shrapnell's membrane)
- Medial: Neck of malleus
- Floor: Lateral process of malleus
- Roof: Fibers of lateral malleolar ligament arising from neck of malleus and inserting along the rim of notch of Rivinus
- ▸ Attic compartments: Transversely placed superior malleolar fold divides attic into two compartments—smaller anterior and larger posterior. The gap between the lateral malleolar fold and lateral incudal fold provides communication between Prussak's space and posterior attic compartment.
- Anterior attic compartment
- Posterior attic compartment: Superior incudal fold divides this space into following two divisions: medial and lateral spaces.
- Compartments of mesotympanum: In the upper part of mesotympanum, there are following three compartments:
- ▸ Inferior incudal space: Its boundaries are following:
- Superior: Lateral incudal fold
- Medial: Medial incudal fold
- Lateral: Posterior malleolar fold extending from neck of malleus to posterosuperior margin of tympanic sulcus.
- Anterior: Interossicular fold that lies between long process of incus and upper two-thirds of handle of malleus.
- ▸ Anterior pouch of von Troeltsch: It lies between the following boundaries:
- Medial: Anterior malleolar fold extending from neck of malleus to anterosuperior margin of tympanic sulcus
- Lateral: Portion of the TM anterior to handle of malleus
- ▸ Posterior pouch of von Troeltsch: It is situated between the following boundaries:
- Medial: Posterior malleolar fold extending from neck of malleus to posterosuperior margin of tympanic sulcus.
- Lateral: Portion of the TM posterior to handle of malleus.
This air-containing space (9 mm height, 14 mm width and 7 mm depth) is situated in the upper part of mastoid. Its boundaries are following:
- Roof: It is formed by the tegmen antri, which separates mastoid antrum from the middle cranial fossa.
- Lateral wall: It is formed by a 1.5 cm thick plate of squamous part of temporal bone which is marked on the lateral surface of mastoid by suprameatal (Macewen's) triangle (Figs 2 and 28). It is covered by postaural skin.
- ▸ Boundaries of Macewen's triangle:
- Linea temporalis (temporal line): A ridge of bone extending posteriorly from the zygomatic process (marking the lower margin of temporalis muscle and approximating the floor of middle cranial fossa)
- EAC: Posterosuperior margin of EAC.
- Tangent: A tangent to the posterior margin of EAC.
- Medial wall: It is formed by the petrous bone and related to the:
- ▸ Posterior SCC
- ▸ Endolymphatic sac
- ▸ Dura of posterior cranial fossa
- Anterior: Anteriorly mastoid antrum communicates with the attic through the aditus ad antrum. Medial to lateral, the relations are following:
- ▸ Facial nerve canal
- ▸ Aditus ad antrum and facial recess lie between tympanum and mastoid antrum (see posterior wall of middle ear in the section of boundaries of middle ear)
- ▸ Deep bony EAC
- Posterior wall: It is formed by mastoid bone and communicates with mastoid air cells.
- ▸ Sigmoid sinus curves downwards.
- Floor: It is formed by mastoid bone and communicates with mastoid air cells. Other deeper relations from medial to lateral sides are:
- ▸ Jugular bulb medial to facial canal
- ▸ Digastric ridge gives origin to posterior belly of digastric muscle
- ▸ Origin of SCM muscle.
Types of Mastoid (Fig. 29)
The mastoid consists of “honeycomb” air cells, which lie underneath the bony cortex. Depending on its development, three types of mastoid are described: cellular, diploic and acellular.
- Cellular (well-pneumatized): Mastoid cells are well developed with thin intervening septa. Eighty percent people have pneumatized mastoids.
- Diploic: Mainly there are marrow spaces with few air cells.
- Acellular (sclerotic): There are neither cells nor marrow spaces.
Theories of Deficient Mastoid Pneumatization
- Tumarkin: Failure of aeration of middle ear cleft that is the cause of “frustration of pneumatization”, results from the blockage of the Eustachian tube. It happens in patients with upper respiratory tract infections.
- Diamond and Dahlberg: Different types of mastoids are the normal congenital variations.
- Wittmack: Infantile otitis media interferes with normal absorption of diploic bone and leads to failure of pneumatization and dense mastoid bone. Evidence for this theory is lacking.
Mastoid antrum is present in all types of mastoids. It is the most constant mastoid air cell. In sclerotic mastoid, antrum is usually small and sigmoid sinus may be anteriorly positioned. In cases of mastoiditis, abscesses may form in these air cells and result in various types of intra- and extracranial complications For further details, see Chapter 20 “Complications of Suppurative Otitis Media”.
Fig. 31: X-ray mastoid left showing normal pneumatizationSource: Dr Jayesh Patel, Consultant Radiologist, Anand, Gujarat.
Fig. 32: X-ray mastoid right showing partial loss of pneumatizationSource: Dr Jayesh Patel, Consultant Radiologist, Anand, Gujarat.
The mastoid air cells are traditionally divided into several groups, which include:
- Zygomatic cells: In the root of zygoma.
- Tegmen cells: In the tegmen tympani.
- Perisinus cells: Present over the sinus plate.
- Retrofacial cells: Present behind the fallopian canal of facial nerve.
- Perilabyrinthine cells: They are located above, below and behind the labyrinth. The cells, which are present in the arch of superior SCC, may communicate with the petrous apex.
- Peritubal: They are present around the Eustachian tube. These and the hypotympanic cells communicate with the petrous apex.
- Tip cells: These large cells lie in the tip of mastoid, medial and lateral to the digastric ridge.
- Marginal cells: These cells, which lie behind the sinus plate, may extend into the occipital bone.
- Squamous cells: They lie in the squamous part of temporal bone.
Mastoid develops from the squamous and petrous parts of temporal bone. In some cases, petrosquamosal suture persists as a bony plate called Korner's septum, which separates superficial squamosal cells from the deep petrosal cells. During the mastoid surgery, Korner's septum causes difficulty in locating the antrum and the deeper cells.
Following branches of external and internal carotid arteries supply blood to middle ear:
- External carotid artery:
- ▸ Maxillary artery:
- Anterior tympanic artery: Major contributor
- Middle meningeal artery
- Petrosal branch
- Superior tympanic artery: It traverses along the canal for tensor tympanic muscle.
- Artery of pterygoid canal: Branch that runs along Eustachian tube.
- ▸ Posterior auricular artery:
- Stylomastoid artery: Major contributor
- ▸ Ascending pharyngeal artery:
- Tympanic branch
- Internal carotid artery: Petrous part
- ▸ Caroticotympanic branches.
Veins from the middle ear cleft drain into pterygoid venous plexus, superior petrosal sinus and sigmoid sinus.
Lymphatic Drainage of Ear
The lymphatics of middle ear drain into retropharyngeal and parotid nodes. Eustachian tube lymphatics drain into retropharyngeal group of lymph nodes (Table 1). Internal ear does not have any lymphatics.
The internal ear (labyrinth), which has organs of both hearing and balance, is divided into bony and membranous labyrinth. The membranous labyrinth is filled with endolymph. Perilymph is filled in the space present between membranous and bony labyrinths.
Bony labyrinth (Fig. 33) consists of three parts: vestibule, semicircular canals (SCCs) and cochlea. The lateral wall of labyrinth is medial wall of middle ear. The medial wall of labyrinth is the lateral limit of IAC.
- Vestibule: This central chamber of the labyrinth (5 mm) has following structures:
- ▸ Lateral wall: It has oval window.
- Oval window (Fenestra vestibuli): It lies in the lateral wall and is closed by footplate of stapes surrounded by annular ligament.
- ▸ Medial wall (Fig. 34): It shows following structures:
- Spherical recess: It is situated anteriorly and lodges the saccule. Perforations of maculae cribrosa media provide passage for fibers of inferior vestibular nerve.
- Elliptical recess: It is situated posteriorly and lodges the utricle. The perforations of maculae cribrosa superior (Mike's dot) provide passage to nerve fibers that supply to utricle and ampulla of superior and lateral SCC.
- Vestibular crest and cochlear recess: The spherical and elliptical recesses are separated from each other by vestibular crest. Inferiorly, vestibular crest splits to enclose cochlear recess for cochlear nerve fibers.
- Opening of aqueduct of vestibule: It is present below the elliptical recess. Through this passes the endolymphatic duct.
- ▸ Posterosuperior region:
- Five openings of SCCs: They are present in the posterosuperior part of vestibule.
- ▸ Anterior: Cochlea opens into the anterior region of vestibule.
- Semicircular canals (Fig. 35): There are three SCCs: lateral (horizontal), posterior and superior (anterior). Each canal occupies two-thirds of a circle and has a diameter of 0.8 mm. They lie in planes at right angles to one another. Each canal has two ends: ampullated and nonampullated. All the three ampullated ends and nonampullated end of lateral SCC open independently and directly into the vestibule.
- ▸ Superior SCC: It is 15–20 mm long and situated transverse to the axis of petrous part of temporal bone. Its anterolateral end is ampullated and opens in the superolateral part of vestibule.
- ▸ Lateral SCC: It is 12–15 mm long and projects as a rounded bulge into the middle ear, aditus and antrum. It makes an angle of 30° with the horizontal plane. Its anterior end is ampullated and opens into the upper part of vestibule. The posterior nonampullated end opens into the lower part of vestibule below the orifice of crus commune.
- ▸ Posterior SCC: It is 18–22 mm long and situated parallel and close to the posterior surface of petrous part of temporal bone. Its lower end is ampullated and opens into the lower part of vestibule. Its upper limb joins the crus commune.
- Crus commune: The nonampullated ends of posterior and superior canals join and form a crus commune (4 mm length), which then opens into the medial part of vestibule. So, the three SCCs open into the vestibule by five openings.
- ▸ Cochlea (Figs 36 and 37): The bony cochlea, which is a coiled tube, looks like snail. Cochlear canal makes 2.5–2.75 turns round a central pyramid of bone called modiolus. The cochlear tube is 30 mm long. It is 5 mm from base to apex and 9 mm around its base.
- Modiolus: The base of modiolus, which is directed towards internal acoustic meatus, transmits vessels and nerves to the cochlea. The apex lies medial to tensor tympani muscle.
- Osseous spiral lamina: A thin plate of bone called osseous spiral lamina, winds spirally around the modiolus like the thread of a screw. This bony lamina gives attachment to the basilar membrane and divides the bony cochlear tube into three compartments: scala vestibuli, scala tympani and scala media (membranous cochlea).
- Rosenthal's canal: The spiral ganglions are situated in Rosenthal's canal, which runs along the osseous spiral lamina.
- Scala vestibuli: This upper-most channel is continuous with vestibule and closed at oval window by the stapes footplate.
- Scala tympani: This lowermost channel is closed by secondary TM of RW.
- Promontory: The promontory, a bony bulge in the medial wall of middle ear, represents the basal coil of cochlea.
- Helicotrema: This opening is situated at the apex of