👂 Ear - Summary (Clinical Anatomy Easy Points)

Source: Gray's Anatomy for Students (the closest equivalent to Snell's in the library - same clinical anatomy style and detail)

1. Overview - Three Parts

Part	Location	Function
External ear	Outside + canal	Captures and conducts sound
Middle ear	Petrous temporal bone cavity	Transmits vibrations via ossicles
Internal ear	Petrous temporal bone (medial)	Converts sound to electrical signals + balance

2. External Ear

Auricle (Pinna)

Cartilage covered with skin (except the lobule - only part with NO cartilage)
Key landmarks: helix (outer rim), antihelix (inner parallel rim), tragus (anterior to meatus opening), antitragus (opposite tragus), concha (hollow center)
Muscles: intrinsic (change shape) + extrinsic (anterior, superior, posterior auricular) - all innervated by facial nerve [VII]

Sensory Innervation of the Auricle

Auriculotemporal nerve (V3) - anterosuperior skin
Greater auricular nerve (C2, C3) - lower auricle and lobule
Lesser occipital nerve (C2) - superior auricle
Auricular branch of vagus [X] - some of the posterior surface
Facial nerve [VII] - small contribution

External Acoustic Meatus (EAM)

Length: ~2.5 cm
Outer 1/3 - cartilaginous, inner 2/3 - bony (tympanic part of temporal bone)
Skin of cartilaginous part has ceruminous glands (wax) and hair follicles
The meatus has a slight S-shaped curve - to examine, pull pinna upward and backward in adults
Sensory supply: auriculotemporal nerve [V3] (anterior + superior walls), auricular branch of vagus [X] (posterior + inferior walls)
Clinical note: foreign body in EAM can stimulate vagal branch and cause cough/vomiting (Arnold's reflex)

3. Tympanic Membrane

Separates external meatus from middle ear
Oriented obliquely - slopes medially from top to bottom, and posteriorly to anteriorly
Structure: fibrocartilaginous ring peripherally + connective tissue core + skin outside + mucous membrane inside
Umbo = center of concavity, where the handle of malleus attaches
Cone of light = bright otoscopic reflection, anteroinferior to umbo
Pars flaccida (Shrapnell's membrane) = thin, slack superior portion above the malleolar folds
Pars tensa = thick, taut remainder of the membrane
Handle of malleus runs superior to umbo; the lateral process of malleus creates a small bulge at the top

4. Middle Ear (Tympanic Cavity)

Extent

Extends from tympanic membrane laterally to the medial (labyrinthine) wall
The epitympanic recess is the space above the tympanic membrane

Boundaries (6 walls - easy mnemonic: "The Mighty Jaw Joins Lateral Master")

Wall	Name	Important Structure
Roof	Tegmental wall	Tegmen tympani (thin bone separating middle ear from middle cranial fossa)
Floor	Jugular wall	Separates from internal jugular vein; tympanic branch of CN IX enters here
Lateral	Membranous wall	Mostly the tympanic membrane
Medial	Labyrinthine wall	Oval window (stapes), round window, promontory (cochlear 1st turn), facial canal
Anterior	Carotid wall	Pharyngotympanic tube opening, tensor tympani muscle
Posterior	Mastoid wall	Aditus to mastoid antrum; pyramidal eminence (for stapedius muscle)

Communications

Anteriorly via pharyngotympanic tube (Eustachian tube) → nasopharynx
Posteriorly via aditus → mastoid antrum → mastoid air cells

5. Pharyngotympanic (Eustachian) Tube

Length ~3.5 cm (adults); shorter and more horizontal in children = more prone to middle ear infections
Medial 2/3 is cartilaginous; lateral 1/3 is bony
Opens during swallowing/yawning to equalize middle ear pressure
Lined with mucous membrane continuous with the nasopharynx (infection can spread easily)
Innervation: tympanic plexus (via CN IX)
Arterial supply: ascending pharyngeal artery + branches of maxillary artery (middle meningeal + artery of pterygoid canal)

6. Auditory Ossicles

Three bones forming a chain: Tympanic membrane → Malleus → Incus → Stapes → Oval window (internal ear)

Ossicle	Key Features
Malleus	Largest; head (in epitympanic recess), neck, handle (attached to tympanic membrane), lateral process, anterior process
Incus	Middle bone; body + short crus + long crus + lenticular process (connects to stapes)
Stapes	Smallest bone in body; head, two crura, footplate (in oval window)

Muscles of the Ossicles

Muscle	Bone moved	Nerve	Action
Tensor tympani	Malleus (pulls handle medially)	Medial pterygoid nerve (CN V3)	Stiffens chain, damps vibration
Stapedius	Stapes	Facial nerve [VII]	Stiffens chain, protects from loud sounds (stapedius reflex)

Paralysis of stapedius (facial nerve palsy) → hyperacusis (sounds painfully loud)

7. Innervation of Middle Ear

Tympanic nerve (branch of CN IX, glossopharyngeal) → enters middle ear → forms tympanic plexus
Tympanic plexus supplies mucosa of middle ear, pharyngotympanic tube, and mastoid area
Lesser petrosal nerve branches from tympanic plexus → exits via foramen ovale → otic ganglion → postganglionic parasympathetics to parotid gland
Facial nerve [VII] runs in the facial canal on the medial wall of the middle ear - at risk in middle ear surgery
Chorda tympani (branch of VII) crosses the middle ear between malleus and incus

8. Internal Ear

Located in the petrous part of the temporal bone, between:

Middle ear (lateral)
Internal acoustic meatus (medial)

Two Labyrinths

	Bony Labyrinth	Membranous Labyrinth
Fluid	Perilymph (like CSF - high Na⁺)	Endolymph (like intracellular fluid - high K⁺)
Parts	Vestibule, 3 semicircular canals, cochlea	Utricle, saccule, 3 semicircular ducts, cochlear duct

Bony Labyrinth Parts

Vestibule - central cavity, contains utricle and saccule
3 Semicircular canals - anterior, posterior, lateral (each at right angles to each other)
Cochlea - snail-shaped, 2.5 turns; modiolus = central bony axis

Membranous Labyrinth - Balance Organs

Structure	Responds to
Utricle (macula)	Linear acceleration in horizontal plane + sideways head tilts
Saccule (macula)	Linear acceleration in vertical plane (up-down, forward-backward)
Semicircular ducts (cristae in ampullae)	Rotational (angular) movement in any direction

Hearing Organ - Cochlear Duct

The cochlear duct divides the cochlea into: scala vestibuli (above) and scala tympani (below)
Cochlear duct filled with endolymph; scalae filled with perilymph
Scala vestibuli starts at oval window (stapes footplate); scala tympani ends at round window
Organ of Corti sits on the basilar membrane in the floor of the cochlear duct - the actual hearing receptor
The vestibular membrane (Reissner's) forms the roof of the cochlear duct

Endolymphatic Sac

Endolymphatic duct passes through the vestibular aqueduct → expands into the endolymphatic sac on the posterior surface of petrous temporal bone
Functions in resorption of endolymph

9. Vestibulocochlear Nerve [CN VIII]

Runs in the internal acoustic meatus alongside facial nerve [VII]
Divides into:
- Cochlear nerve - hearing
- Vestibular nerve (superior + inferior divisions) - balance
Cell bodies for vestibular part: vestibular ganglion (Scarpa's ganglion)
Cell bodies for cochlear part: spiral ganglion (in modiolus)

10. Quick Clinical Pearls

Fact	Clinical Relevance
Pars flaccida is the weak part of tympanic membrane	Cholesteatoma typically starts here
Chorda tympani crosses middle ear	Damaged in ear surgery → loss of taste on anterior 2/3 tongue
Children's Eustachian tube is short + horizontal	Prone to acute otitis media
Facial nerve in bony canal on medial wall	At risk in mastoiditis or middle ear surgery
Stapedius reflex	Tests CN VII integrity; loss → hyperacusis
Arnold's reflex (vagal branch in EAM)	Ear instrumentation can cause cough or even cardiac vagal reflex
Round window covered by secondary tympanic membrane	If ruptured → perilymph fistula, sensorineural hearing loss
Endolymph resorbed by endolymphatic sac	Disruption → endolymphatic hydrops (Meniere's disease)

Source: Gray's Anatomy for Students, Head & Neck chapter (Ear section, pp. 1093-1106). Note: Snell's Clinical Anatomy was not available in the library, but Gray's for Students covers the same anatomy at an equivalent level.

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