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Term Paper on the Human Ear
Term Paper # 1. External Ear:
This is formed by:
i. Auricle or pinna.
ii. External acoustic meatus.
Formed by yellow elastic, single crumpled plate of cartilage, covered by skin.
Lowest part is soft and consists only of connective tissue covered by skin called lobule.
Large depression is called concha which leads into external auditory meatus.
The ear develops as six tubercles around the first branchial cleft.
Helix, antihelix, scaphoid and triangular fossa concha, tragus, anti-tragus and incisura terminalis.
Nerve Supply of Pinna:
1. Great auricular nerve (C2 and C3) – supplies lower 1/3 of pinna both sides.
2. Lesser occipital nerve (C2) – upper 2/3 of posterior – surface of pinna.
3. Auriculo temporal nerve – upper 2/3 of anterior surface of pinna.
4. Auricular branch of vagus – This supplies root of auricle
a. Superficial temporal vessels supply anterior surface of pinna.
b. Posterior auricular vessels supply posterior surface of pinna.
1. Anterior Surface:
This is drained by preauricular group of lymph nodes.
2. Posterior Surface:
This is drained by posterior auricular or mastoid group of lymph nodes.
3. Superficial Cervical:
Group of lymph nodes.
Muscles of Auricle:
These are extrinsic and intrinsic.
1. Extrinsic Muscles:
Move the auricle as a whole, e.g., Auricularis anterior, Auricularis superior and Auricularis posterior.
2. Intrinsic Muscles:
Alter the shape of the auricle, e.g., Helicis major and minor, Tragicus and anti-tragicus, Transversus auriculae and Oblique auriculae.
Ligaments of Auricle:
a. Extrinsic ligaments- These connect the auricle with temporal bone.
b. Intrinsic ligaments- These connect various cartilages of the pinna.
ii. External Auditory Meatus:
Development- First branchial cleft.
Length: About 2.5 cm.
Extent: From concha to tympanic membrane.
This is outer 1/3 of external auditory meatus.
This is inner 2/3 of external meatus.
Canal is “S” shaped.
1st directed – medially, upwards and forwards.
Then directed – medially, backwards and upwards.
Finally directed – medially, forwards and downwards.
Narrowest part of the canal is isthmus – This is 5 mm lateral to the tympanic membrane.
During examination – Pinna is pulled upwards and backwards.
It conducts the sound waves to the tympanic membrane.
Anteriorly – Temporo-mandibular joint.
Posteriorly – Mastoid air cells and mastoid antrum.
Superiorly – Middle cranial fossa.
Inferiorly – Parotid gland (parotid abscess may burst into external auditory meatus).
Inflammation of external auditory meatus causes painful opening of the mouth – Trismus.
Anterior wall of the canal is longer than posterior wall.
Bony part is narrower than the cartilaginous part.
Formed by “C” shaped cartilage.
In the floor – fissures of santorini is present, filled by fibrous tissue, they permit free mobility of pinna and abscess burst through these fissures into external auditory meatus.
“C” shaped tympanic plate of temporal bone forms squamous part completes the deficiency in bony part.
Skin lining the external auditory meatus has – sebaceous glands, modified sweat gland called ceruminous glands and hair.
1. Superficial temporal artery
2. Posterior auricular artery.
i. Pre-auricular lymph nodes
ii. Post auricular lymph nodes
iii. Infra auricular lymph nodes.
1. Auriculo temporal nerve supplies anterior ½ of meatus.
2. Auricular branch of vagus (Arnolds’ nerve) – supplies posterior half of meatus. Stimulation of this nerve causes ear cough and even vaso vagal symptoms.
1. Pre-auricular sinus – formed due to incomplete fusion of auricular tubercles.
2. Partial or total agenesis of pinna.
3. Accessory auricles.
4. Bat’s ear (protruding ears)
5. Peri chondritis of pinna.
6. Cauliflower ear – Permanent deformity due to organized haematoma under perichondrium (Boxers).
7. Rodent ulcer (Basal cell CA).
8. Blaindile’s ear – Congenital asymmetry of ears.
9. Cagot’s ear – Congenital absence of lobule of ear.
10. Treacher – Collin’s syndrome – poor development of external and middle ear, eyes, zygomatic, maxillary and mandible bone.
Term Paper # 2. Middle Ear Cavity:
It is an air conditioning chamber, situated within the petrous part of the temporal bone – slit like cavity.
Connects external ear with internal ear.
Communicates with nasopharynx through auditory tube anteriorly and posteriorly communicates with mastoid antrum via aditus.
Biconvex – Antero posterior – 15 mm.
Vertical diameter- 15 mm.
a. Upper part – 6 mm
b. Central part – 2 mm
c. Lower part – 4 mm
Parts of Middle Ear:
Situated above tympanic membrane also known as attic part of cavity. Contains- Head of malleus and body of incus with its short process.
2. Meso-Tympanum or Tympanic Cavity Proper:
Situated behind the tympanic membrane. Contains- Handle of malleus and long process of incus parallel to handle and stapes.
Situated below tympanic membrane.
Development of Middle Ear:
1. Tubo tympanic recess.
2. Malleus, incus, and tensor tympani muscle are formed from 1st pharyngeal arch (V3) mandibular nerve supplies.
3. Stapes and stapedius muscle develops from IInd pharyngeal arch (VII) facial nerve supplies it.
Contents of Middle Ear:
1. Three bony ossicles – Malleus, incus and stapes.
2. Two muscles – Tensor tympani and stapedius muscle.
3. Chorda tympanic nerve branch of facial nerve (VII).
4. Tympanic plexus of nerves.
5. Blood vessels and lymphatics of middle ear.
6. Air fills the cavity.
All these structures are covered by mucous membrane.
Boundaries of Middle Ear Cavity:
It is like a six sided box having – anterior, posterior, medial and lateral walls with a roof and a floor.
I. Anterior Wall or Carotid Wall:
Having openings of:
1. Anterior canliculus for chorda tympani nerve.
2. Canal for tensor tympani muscle.
3. Opening of pharyngo tympanic tube (from above downwards).
II. Posterior Wall or Mastoid Wall:
Having following features:
1. Upper part has an opening – leading to aditus to mastoid antrum.
2. Pyramid – Is a thin bony elevation has an opening at its apex – through this tendon of stapedius passes.
3. Facial nerve prominence (facial nerve canal).
4. Posterior canaliculus – for chorda tympani nerve.
5. Fossa Incudes is a shallow depression – lodging short process of incus.
III. Medial Wall or Labyrinthine Wall:
Having following features:
1. Prominence caused by lateral semicircular canal.
2. Impression of facial nerve canal (prominence).
3. Promontary – smooth rounded elevation formed by basal turn of cochlea. Tympanic plexus of nerves lies on it.
4. Fenestra vestibuli – The opening is closed by foot piece of stapes and annular ligament. It transmits sound waves from ear ossicles to the perilymph of scala vestibuli. Situated above and behind the promontary.
5. Fenerstra cochlearis – This is the round window situated below and behind promontary – closed by secondary tympanic membrane. It accommodates the pressure waves transmitted to the perilymph of scala tympani.
6. Sinus tympani are a small depression situated behind the promontary between fenestra vestibuli and fenestra cochlearis.
Deep to it, ampulla of posterior semi-circular canal is situated.
IV. Lateral Wall or Membranous or Tympanic Wall:
(a) Formed by tympanic membrane bulges medially.
(b) Handle and lateral process of malleus are embedded in fibrous layer of tympanic membrane.
(c) Chorda tympanic nerve branch of facial nerve passes across the tympanic membrane lying lateral to long process of incus and medial to handle of malleus. Enters the tympanic cavity through posterior canaliculus in posterior wall and leaves through anterior canaliculus present in anterior wall.
V. Roof of Middle Ear Cavity:
It is formed by a thin plate of bone known as tegmen tympani. This separates middle ear from middle cranial fossa.
VI. Floor or Jugular Wall:
Related to superior bulb of internal jugular vein.
Tympanic branch of glossopharyngeal nerve enters through a canaliculus present in the posterior part of the floor.
1. Incudo Malleolar Joint:
Formed in between head of malleus and body of incus.
2. Incudo Stapedial Joint:
It is formed between lentiform process of incus and head of stapes.
Functions of Middle Ear:
1. Transmits sound waves from external to internal ear through chain of ear ossicles.
2. Thus transforms air born vibrations from tympanic membrane to liquid born vibration in the internal ear.
3. Intensity of sound waves is increased ten times by ossicles.
4. Chain of ear ossicles moves as a whole –
Joints of Middle Ear:
1. Incudo Malleolar Joint:
Saddle type synovial joint between head of malleus and body of incus.
2. Incudo Stapedial Joint:
Ball and socket, synovial type joint formed between long process of incus and head of stapes.
3. Foot piece of stapes is fixed to oval window by annular ligament.
Muscles of Middle Ear:
1. Tensor Tympani:
Arises from bony and cartilaginous part of auditory tube.
Into handle of malleus.
Branch from mandibular nerve (medial pterygoid nerve).
Damping the sound waves reaching the internal ear.
Arises from internal wall of pyramid present in posterior wall of middle ear.
Into neck of stapes.
Branch of facial nerve.
Damping the sound waves reaching middle ear.
Blood Supply of Middle Ear:
1. Anterior tympanic artery – branch of maxillary artery.
2. Posterior tympanic artery – branch of stylomastoid artery branch of posterior auricular artery.
3. Petrosal branch of middle meningeal artery.
4. Superior tympanic artery branch of middle meningeal artery.
5. Inferior tympanic artery branch of ascending pharyngeal artery.
6. Artery of pterygoid canal.
7. Tympanic branch of internal carotid artery.
1. Pterygoid venous plexus
2. Superior petrosal sinus.
Retro pharyngeal group of lymph nodes.
Tympanic plexus is formed by:
1. Tympanic branch of glossopharyngeal nerve (sensory).
2. Carotico tympanic nerve (sympathetic) from plexus around internal carotid artery.
Tympanic Membrane (or Ear Drum):
It is a thin, translucent partition between external and middle ear.
Oval in shape.
Pearly white or grayish white in colour.
Placed obliquely at an angle of 55° with the floor of meatus.
To a sulcus of temporal bone, called tympanic sulcus of riveni.
Forwards and medially.
Vertically – 10 mm
Transverse – 8 mm.
Parts of the Membrane:
There are two parts:
1. Pars tensa and
2. Pars flaccida.
Medial and lateral surface.
Lateral surface is concave.
Medial surface is convex, maximum convex point is called Umbo – Handle of malleus is attached to the inner surface of umbo.
Cone of light is light reflex area present along antero-inferior part of membrane.
These are layers:
1. Outer layer is cuticular – formed by skin epithelium (Ectodermal in origin).
2. Middle layer is fibrous – consisting of radial fibres which are superficial and circular fibres lies – deep (Mesodermal origin).
3. Inner layer is mucosal layer lined by ciliated columnar epithelium (Endodermal origin).
1. External Surface:
Deep auricular artery
2. Internal Surface:
Anterior tympanic branch of maxillary artery.
Posterior tympanic branch of stylomastoid artery, branch of posterior auricular artery.
1. External Surface:
Into external jugular vein.
2. Internal Surface:
Into transverse sinus and venous plexus of pharynx tympanic tube.
1. External Surface:
Into posterior auricular lymph node.
2. Internal Surface:
Into retro pharyngeal lymph node.
I. External surface is supplied by Auriculo temporal nerve – anterior half and Auricular branch of vagus – posterior half.
II. Internal surface is supplied by tympanic plexus, formed tympanic branch of glossopharyngeal nerve.
1. Myringitis- Inflammation of tympanic membrane, it becomes reddish.
2. Perforation of tympanic membrane due to acute suppurative otitis media (ASOM) usually – antero inferior quadrant is perforated.
3. Myringotomy- Tympanic membrane is incised to drain the pus in middle ear.
4. Tympanoplasty- Reconstruction of tympanic membrane and ossicular chain after treating disease of middle ear.
5. Congenital atresia of meatus.
6. Ear wax- May close meatus and causes deafness.
7. Otorrhea- In head injury, C.S.F. or blood leaks from ear.
8. Foreign bodies- Insects, maggots, peas, grains, pearl and stones etc.
9. Inflammatory stenosis- Causes deafness.
10. Otoscope is an instrument used for ear examination (for external auditory meatus and tympanic membrane).
It is one of the five parts of temporal bone:
1. Squamous part
2. Mastoid part
3. Petrous part
4. Tympanic part
5. Styloid part.
Is situated postero inferiorly.
It has two surfaces:
i. External surface
ii. Internal surface.
There are two borders:
i. Superior border
ii. Posterior border.
It is the downward prolongation of external surface of temporal bone – rough and convex in nature, gives insertion for the following muscles – superior to inferior.
1. Sternocleido mastoid
2. Splenius capitis
3. Langissimus capitis.
Superior to insertion of sternocleidomastoid it gives origin to:
a. Auricularis posterior.
b. Occipital belly of occipito frontalis muscle.
c. Mastoid process is a nipple shaped projection.
d. It is absent in foetus, so facial nerve is very much superficially situated.
e. Along the medial surface of the process – mastoid notch is found – gives origin to posterior belly of digastric muscle.
f. Medial to the notch groove for occipital artery is situated.
Bounds the posterior cranial fossa. It is grooved by sigmoid sinus.
1. Superior border is thick and serrated, it articulates with inferior border of parietal bone at the mastoid angle.
2. Posterior border is thick and serrated, it articulates with the squamous part of occipital bone.
Mastoid air cells and mastoid antrum are found in the mastoid part. They communicate with the middle ear through aditus to mastoid antrum, when mastoid air cells are not developed the mastoid process is solid and sclerotic.
Mastoid antrum is a small air filled space situated in the posterior part of petrous temporal bone.
Diameter: 10 mm
Capacity: 1 ml.
Superior: Tegmen tympani (2 mm thick).
Inferior: Mastoid process and air cells.
1. Medial part of external auditory meatus.
2. Epitympanic recess of middle ear.
Aditus is related to VIIth nerve above and behind.
Thin plate separates the antrum from sigmoid sinus and cerebellum.
1. Facial nerve canal.
2. Lateral semi-circular canal.
3. Posterior semi-circular canal.
Infections affecting medial wall damages VIIth nerve or semi-circular canals.
Cortex of mastoid bone medial to supra-meatal triangle.
In New Born:
Lateral wall thickness is about 2 mm and increases 1 mm per year. (Antrum is of adult size at birth – size of small pea.)
About 15 mm (no increase after puberty) thick.
Mastoid air cells.
Mastoid Air Cells:
These are variable in size and arrangement situated in mastoid antrum communicates with middle ear.
According to the Site:
1. Zygomatic cells
2. Subdural cells
3. Cells of petrosal angle
4. Peri sinus cells
5. Tip cells
6. Facial cells.
Types of Mastoid Process:
According to air cells:
1. Cellular Mastoid Process:
Air cells are larger in size and numerous in numbers – found in 80%.
2. Diploic Mastoid Process:
Air cells are smaller in size and less in number.
3. Sclerotic Mastoid Process:
No air cells are found 20%.
Blood Supply (Arterial Supply):
Posterior tympanic artery arises from stylomastoid artery branch of posterior auricular artery.
Venous Drainage of Mastoid Antrum:
1. Mastoid emissary vein
2. Sigmoid sinus
3. Posterior auricular vein.
Posterior auricular nodes.
1. Tympanic plexus (IX)
2. Nervi spinosus (V3).
Supra Meatal Triangle of Macewen:
Surgical exposure of mastoid antrum is done through this triangle.
It is bounded:
Supra mastoid crest.
b. Perpendicular line drawn from supra- mastoid crest to the posterior border of external auditory meatus.
c. Antero Inferiorly:
Superior and posterior borders of external auditory meatus.
On the surface of triangle – supra meatal spine of Henla is found.
1. Congenital absence of mastoid process.
2. At the time of birth, mastoid antrum is well developed, mastoid air cells are rudimentary. Only by 2nd year mastoid process develop, and only by 4th year mastoid air cells enter the process. At puberty mastoid air cells are fully grown. In 20% mastoid has no air cells (Sclerotic).
3. Mastoiditis may cause labyrinthitis or facial palsy.
Sigmoid sinus thrombosis, subdural abscess etc.
Applied Anatomy of Middle Ear:
1. A.S.O.M. – Acute suppurative otitis media.
Perforation of tympanic membrane at antero inferior quadrant.
2. C.S.O.M. – Chronic infection.
Tympanic membrane ruptured at:
a. Pars flaccida or
b. Pars tensa.
3. Haemotympanum – Blood is present in tympanic cavity – head injury.
4. Oto-sclerosis – Fixation of stapes at oval window due to new bone formation – causes conductive deafness.
5. Hyper acousia – Paralysis of stapedius muscle after VIIth nerve injury – severe hizzing noise in ear present.
6. Tympano sclerosis – Due to C.S.O.M. deposition of chalky white patches on tympanic membrane and on ear ossicles.
Auditory Tube or Pharyngo Tympanic Tube or Eustachian Tube:
It is a trumpet shaped tube which connects the middle ear cavity with naso pharynx.
From medial part of tubo tympanic recess.
In children- Tube is shorter, wider and straighter.
About 3.6 to 4 cm.
Downwards, forwards and medially.
Posterior 1/3 – Bony part – 12 mm long, lies in petrous temporal bone.
Anterior 2/3 – Cartilaginous part – 25 mm long lies in sulcus tube (a groove between greater wing of sphenoid and apex of petrous temporal).
Isthmus is the junction of anterior 2/3 and posterior 1/3.
Oval on cross-section.
Relations of Bony Part:
Superior- Canal for tensor tympani muscle.
Inferiorly- Tympanic plate of temporal bone.
Medial- Carotid canal.
Laterally- Chorda tympani nerve.
Spine of sphenoid
Temporo mandibular joint
Lower portion of tegmen tympani.
Cartilaginous Part of Tube (Anterior 2/3 = 25 mm):
A triangular cartilage forms – superior and medial walls of the tube – lateral wall and floor is completed by fibrous membrane.
Cartilagenous part is attached to the anterior part of bony tube.
Passes through the space above the upper border of superior constrictor of pharynx (i.e., sinus of Morgagni).
It pierces the pharyngobasilar and buccopharyngeal fascia and opens into the lateral wall of nasopharynx.
Opening is present about 1.25 cm behind the inferior nasal choncha and guarded by an elevation formed by tubal tonsil.
Relations of Cartilaginous Part:
Tensor palatini muscle.
Spine of sphenoid.
Mandibular nerve with its branches.
Otic ganglion and chorda tympani nerve.
Middle meningeal artery.
Medial pterygoid plate.
Apex of petrous temporal bone.
Muscles Acting on Tube:
Tensor palati, levator palati and salpingopharyngeus – dilates the tube.
Muscles attached on inferior surface of tube are:
i. Portion of levator palati and
Artery of pterygoid canal.
Middle menigeal artery branches.
Branches of ascending pharyngeal artery.
Drain into – Pterygoid venous plexus and Pharyngeal venous plexus.
Go to retropharyngeal group of lymph nodes.
Nerve Supply via:
i. Pharyngeal branch of pterygo palatine ganglion (maxillary nerve) at ostium.
ii. Cartilaginous part by nervi spinosus (mandibular nerve).
iii. Bony part by tympanic plexus (Glossopharyngeal nerve).
It communicates middle ear cavity with exterior, thus ensuring equal air pressure on both sides of tympanic membrane.
Tube is usually closed.
It opens during swallowing, yawning and sneezing by the action of tensor and levator palati.
Applied Anatomy of Auditory Tube:
1. Infection from throat may pass to middle ear via tube, common in children because tube is shorter, straighter and wider.
2. When tympanic membrane is ruptured, fluid entering the external ear enters the pharynx through middle ear and then through auditory tube.
3. Inflammation of tube is known as salpingitis.
4. Valselva’s test.
5. Eustachian catheterization.
6. Eustachian catarrah – Allergic condition.
Term Paper # 3. Internal Ear or Labyrinth:
Lies in petrous part of temporal bone. Consists of a bony labyrinth within which lies a membranous labyrinth. Membranous labyrinth is filled with fluid called endolymph. Membranous labyrinth is separated from bony labyrinth by another fluid called perilymph.
Consists of three parts:
(a) Cochlea – anteriorly
(b) Vestibule – in the middle
(c) Semicircular canals posteriorly:
(i) Anterior semicircular canal.
(ii) Posterior semicircular canal.
(iii) Lateral semicircular canal.
Has a conical central axis. Modiolus around which cochlear canal makes two and three quarter turns.
A spiral bony ridge called spiral lamina projects from modiolus and divides the cochlear canal partially into scala vestibuli above and scala tympani below. Division is completed by basilar membrane.
Scala vestibuli communicates with scala tympani at the apex of cochlea – Helicotrema.
Vestibule is the middle part of internal ear.
Laterally related to middle ear.
Fenestra vestibuli communicates vestibule with the middle ear.
Medial wall of vestibule has an opening for aqueduct of vestibule. It opens through a fissure on posterior surface of petrous temporal bone and is closed by ductus endolymphaticus.
(c) Semicircular Canals:
Three in number arranged as anterior, posterior and lateral.
Situated above and behind the vestibule.
Canals are arranged at right angles with each other.
Dilated lower ends called ampullae.
Crus commune is the site of fusion of posterior end of anterior semicircular canal with anterior end of posterior semicircular canal.
Anterior canal is also known as superior canal.
A closed system of inter communicating membranous sacs and ducts within the bony labyrinth and is filled with endolymph.
Duct of cochlea, utricle, saccule and Semicirular ducts.
Parts of epithelium of membranous labyrinth are specialized to form sensory receptors for sound.
1. Organ of Corti:
Present in duct of cochelea anteriorly.
Within vestibule lies utricle and saccule – lined by specialized neuro epithelium called maculae is responsible for linear acceleration and gravitational pull.
For static balance – thickened neuro epithelium on medial wall of saccule and utricle is responsible.
2. In Semicircular Ducts:
Specialized neuro epithelium is present in ampula – as cristae are responsible for kinetic balance – angular acceleration and caloric stimulation.
Duct of Cochlea or Scala Media:
Situated in between scala vestibuli and scala tympani. Floor of duct is formed by vestibular membrane on the upper surface of basilar membrane the duct of organ of corti is arranged.
Organ of corti is an end organ formed by neuro epithelium of auditory function. Peripheral processes of spiral ganglion cells supply the organ of corti and central processes become the cochlear nerve Endolymph is secreted by stria vascularis situated on the outer wall.
Tunnel of corti is made up of two rows of cells – still cells (Pillars of corti), on the medial side of tunnel of corti a single row of hair cells present, rest in cup shaped depressions in the bodies of cells – Deitier’s cells lateral to tunnel four parallel rows of cells (outer hair cells) supported by Henson cells.
Tectorial membrane is made up of jelly like material.
About 3,500 inner hair cells.
About 20,000 outer hair cells present.
Arrangement of cells on the upper surface of basilar membrane from medial to lateral are:
1. Border cells.
2. Inner hair cells.
3. Inner phalangeal cells.
4. Outer phalangeal cells and outer hair cells
5. Cells of Henson → cells of Claudius or supporting cells.
Is connected with saccule via – Ductus utriculo – saccularis, utricle has a lining of neuro epithelium called macula → end organ → responding to gravitational pull and linear acceleration.
Saccule has a patch of neuro epithelium called macula – responding to gravitational pull and linear acceleration (static balance).
Floating in perilymph, ampullae contain → crista ampularis → hair present gets displaced during movements of endolymph respond to angular acceleration and caloric stimulates (kinetic balance).
Vestibular part of VIIIth nerve arise form macula and crista ampularis, vestibular ganglion having:
i. Superior nucleus
ii. Inferior nucleus
iii. Medial and lateral nucleus.
Chochlear part of VIIIth nerve arises from spiral ganglion of organ of corti, central processes arising from ventral cochlear nucleus and dorsal cochlear nucleus.
Endo-lymph is secreted by stria vascularis.
I. Spiral duct of cochlea- Organ of hearing (organ of corti).
II. (a) Utricle and saccule- Organ of static balance (Maculae).
(b) Semicircular ducts- Organ of kinetic balance (Cristae Ampularis).
Blood Supply of Labyrinth:
1. Labyrinthine branch of basilar artery accompanies vestibulo cochlear nerve.
2. Stylomastoid branch of posterior auricular artery.
Into superior and inferior petrosal sinus or transverse sinus and internal jugular vein.
1. Changes in secretion and absorption of endolypmph results in Mennier’s disease with vertigo and deafness.
2. Certain drugs, e.g., Streptomycin, quinine may affect cochlear nerve and causes deafness.
3. Vestibular nerve involvement produces:
iii. Nausea and vomiting, tachycardia.
4. Cochlear nerve involvement:
iii. Hearing scotoma (deafness for certain pitches)
iv. Word deafness (sensory aphasia).
5. Fracture of petrous part of temporal bone may involve VIIth and VIIIth nerve.
6. Acoustic neuroma is a tumour affecting (VIII) vestibulo cochlear nerve – cochlear part especially.