Anatomy of the Ear

1. External Ear

Auricle (Pinna)

Surface anatomy of the auricle - Cummings Otolaryngology

External Auditory Canal (EAC)

• Total length: ~2.5 cm in adults
• Lateral one-third: cartilaginous (membranous), lined with thick skin containing sebaceous glands, apocrine (ceruminous) glands, and hair follicles; this is where cerumen is produced
• Medial two-thirds: bony, formed by the tympanic part of the temporal bone; lined by thin skin adherent to bone, with no glands or hair follicles
• The isthmus (bony-cartilaginous junction) is the narrowest point
• Fissures of Santorini: transverse slits in the cartilaginous portion that can allow spread of infection to surrounding soft tissue
• Foramen of Huschke: developmental defect in the anterior bony canal that may allow disease to spread to the parotid gland

2. Middle Ear

Tympanic Membrane (TM)

1. Squamous epithelium (outer)
2. Radiating fibrous layer
3. Circular fibrous layer
4. Mucosal layer (inner)

• Total area: 70-80 mm²
• Functional vibrating surface: ~55 mm²
• Divided into pars tensa (the larger, taut portion) and pars flaccida (Shrapnell membrane, the small superior portion above the lateral process of the malleus)

Middle Ear Cavity

• Epitympanum (attic): superior compartment, contains the head of the malleus and body of the incus; bounded superiorly by the tegmen tympani (bone forming the floor of the middle cranial fossa)
• Mesotympanum: main cavity at the level of the TM
• Hypotympanum: below the TM
• Prussak space: just medial to the pars flaccida and lateral to the head and neck of the malleus - a common site for cholesteatoma formation

Ossicles

• Head, Neck, Manubrium (handle), Anterior process, Lateral process
• The manubrium is embedded in the TM

• Body, Short process (posteriorly to incudal fossa), Long process with lenticular process (articulates with stapes)
• Most vulnerable ossicle: its long process has a single nutrient vessel with no collateral circulation

• Anterior crus, Posterior crus, Capitulum (head), Footplate
• Footplate average dimensions: 1.41 mm × 2.99 mm
• Footplate sits in the oval window, secured by the annular ligament

• Malleo-incudal joint: diarthrodial
• Incudo-stapedial joint: diarthrodial
• Stapedial-labyrinthine joint: syndesmotic

• Tensor tympani: attaches to the medial surface of the upper manubrium; innervated by the medial pterygoid nerve (V3)
• Stapedius: smallest skeletal muscle; attaches to the posterior neck of the stapes from the pyramidal eminence; innervated by the facial nerve (CN VII)

Eustachian Tube

• Connects the middle ear to the nasopharynx
• Maintains equalization of middle ear pressure
• The superior/lateral semicanal contains the tensor tympani; the deeper canal contains the Eustachian tube

Key Middle Ear Landmarks

• Cochleariform process: landmark for the midportion of the tympanic facial nerve segment
• Pyramidal eminence: landmark for the second genu of the facial nerve
• Oval window: where the stapes footplate articulates
• Round window: covered by the secondary tympanic membrane; allows hydraulic compensation of cochlear pressure waves
• Aditus ad antrum: opening connecting epitympanum to the mastoid antrum

3. Inner Ear

Cochlea

• Bony spiral making 2.5 turns
• Divided into three fluid-filled compartments:
  • Scala vestibuli: filled with perilymph, above the basilar membrane
  • Scala media (cochlear duct): filled with endolymph, contains the organ of Corti
  • Scala tympani: filled with perilymph, below the basilar membrane; ends at the round window

• Sits on the basilar membrane
• Contains inner hair cells (IHCs) and outer hair cells (OHCs)
• IHCs: responsible for 95% of all afferent nerve responses
• OHCs: motile, act as a "cochlear amplifier" by adjusting basilar membrane stiffness
• Covered by the tectorial membrane

• Endolymph (in scala media): high K+, low Na+ (similar to intracellular fluid); produced by the stria vascularis
• Perilymph (in scala vestibuli and tympani): high Na+, low K+ (similar to extracellular fluid)

Vestibular Labyrinth

• Superior (anterior) SCC
• Lateral (horizontal) SCC
• Posterior SCC
• Each has an ampulla containing a crista ampullaris with hair cells embedded in the cupula

• Utricle: macula in horizontal plane; detects horizontal linear acceleration
• Saccule: macula in vertical plane; detects vertical linear acceleration
• Maculae contain hair cells embedded in the otolithic membrane (contains calcium carbonate crystals called otoliths/otoconia)

Vestibular Nerve Innervation

Vestibular nerve distribution - KJ Lee's Essential Otolaryngology

• Superior vestibular nerve: innervates the utricle, superior SCC, lateral SCC, and (via the Voit nerve) part of the saccule
• Inferior vestibular nerve: innervates the saccule and posterior SCC

Cochlear (Auditory) Pathway

1. Hair cells → Spiral ganglion cells → Cochlear nerve
2. Cochlear nucleus (ipsilateral, at pontomedullary junction)
3. Superior olivary complex (bilateral)
4. Lateral lemniscus → Inferior colliculus
5. Medial geniculate body (thalamus)
6. Primary auditory cortex (Heschl's gyri, temporal lobe)

Temporal Bone Overview

• Cummings Otolaryngology Head and Neck Surgery - Chapter 126: Anatomy of the Temporal Bone, External Ear, and Middle Ear
• KJ Lee's Essential Otolaryngology - Chapter 13-14: Ear Anatomy and Physiology
• Miller's Anesthesia (10e) - Chapter 66: Synopsis of ENT Anatomy