Anatomy of the Ear

1. External Ear

Auricle (Pinna)

• A funnel-shaped cartilaginous structure covered by keratinizing squamous epithelium over a framework of elastic cartilage with perichondrium
• Intricate ridges and depressions include the helix, antihelix, tragus, antitragus, concha, and lobule
• Sebaceous glands and hair follicles are present in the subcutaneous layer; adipose tissue is restricted to the cartilage-free lobule
• Blood supply: branches of the posterior auricular and superficial temporal arteries
• Innervation: auriculotemporal nerve (V3), great auricular nerve (C2/C3), lesser occipital nerve, auricular branch of the vagus (Arnold's nerve), and facial nerve (VII)

External Auditory Canal (EAC)

• Extends from the lateral surface of the tympanic membrane (TM) to the external auditory meatus; approximately 2.5 cm long in adults
• Lateral one-third: cartilaginous skeleton with hair follicles, sebaceous glands, and apocrine glands. Cerumen (earwax) forms here - a hydrophobic, slightly acidic (pH 6.0-6.5) substance
• Medial two-thirds: surrounded by bone (tympanic part of the temporal bone), covered by thin squamous epithelium tightly adherent to bone - no glands or hair follicles
• The isthmus (bony-cartilaginous junction) is the narrowest point of the canal
• Fissures of Santorini: transverse slits in the cartilaginous canal - a pathway for spread of infection or neoplasm to surrounding soft tissue
• Foramen of Huschke: a defect in the anterior bony canal from incomplete ossification that allows disease spread to the deep lobe of the parotid gland
• Self-cleansing mechanism: squamous epithelium migrates centrifugally from the TM toward the cartilaginous portion, carrying debris outward

2. Tympanic Membrane (Eardrum)

• A four-layered concave membrane connected centrally to the manubrium of the malleus and peripherally to the tympanic sulcus
• Two fibrous layers: an outer radiate layer inserting onto the malleus manubrium, and an inner circular layer arranged circumferentially at the periphery
• These integrate peripherally into the annular ligament anchoring the TM into the bony tympanic sulcus
• Pars tensa: the larger, thicker lower segment (has two fibrous layers)
• Pars flaccida (Shrapnell membrane): the smaller, thinner superior segment above the mallear folds; lacks a fibrous layer - clinically significant as the site of retraction pockets and cholesteatoma formation
• Umbo: the central depression where the tip of the malleus manubrium attaches
• Mallear stria: the line of the manubrium visible through the membrane
• Cone of light: a bright otoscopic reflection anteroinferior to the umbo
• Innervation: outer surface - primarily auriculotemporal nerve (V3), with contributions from CN VII and vagal auricular branch; inner surface - tympanic branch of CN IX (Jacobson's nerve)

3. Middle Ear

• Hypotympanum: below the TM level
• Mesotympanum: the central space just medial to the TM; extends from the Eustachian tube (ET) opening anteriorly to the facial nerve posteriorly
• Epitympanum (attic): superior compartment; separated from the mesotympanum by the ossicular chain and mucosal folds

Ossicular Chain

Muscles of the Middle Ear

• Tensor tympani: originates in the semicanal superior to the ET; inserts on the manubrium of the malleus; pulls the malleus medially to tense the TM (innervated by CN V3)
• Stapedius: originates in the pyramidal eminence; inserts on the stapes; the smallest skeletal muscle in the body; dampens loud sounds - acoustic reflex (innervated by CN VII)

Eustachian (Pharyngotympanic) Tube

• Connects the middle ear to the nasopharynx; angled approximately 45 degrees
• Proximal one-third: bony (petrous bone); distal two-thirds: fibrocartilaginous, collapsed at rest
• Opens during swallowing/yawning by contraction of tensor veli palatini (CN V3)
• The bony-cartilaginous junction is its narrowest point
• The internal carotid artery is closely related to its medial wall at the tympanic opening

Other Middle Ear Structures

• Chorda tympani: branch of CN VII carrying taste from anterior 2/3 of tongue; crosses the middle ear between malleus and incus; exits through the petrotympanic fissure
• Promontory: bulge on the medial wall overlying the basal turn of the cochlea; crossed by Jacobson's nerve (CN IX)
• Oval window (fenestra vestibuli): covered by the stapes footplate
• Round window (fenestra cochleae): allows pressure relief from cochlear fluid
• Facial nerve (CN VII): runs in the fallopian canal through the middle ear above the oval window - may be dehiscent here

4. Inner Ear (Labyrinth)

Bony Labyrinth

• Vestibule: central chamber containing the oval and round windows; houses the utricle and saccule
• Semicircular canals (3): superior (anterior), posterior, and lateral - detect angular acceleration
• Cochlea: a spiral structure (~2.75 turns); divided into scala vestibuli (above), scala media (cochlear duct), and scala tympani (below); connected at the apex by the helicotrema

Membranous Labyrinth

• Utricle and saccule: otolith organs in the vestibule that detect linear acceleration and head position (gravity)
• Semicircular ducts: inside the semicircular canals; each has an ampulla containing the crista ampullaris (detects rotational acceleration)
• Cochlear duct (scala media): contains the organ of Corti - the actual hearing organ, with inner and outer hair cells resting on the basilar membrane

Key Features of the Cochlea

• Basilar membrane: tonotopic organization - high frequencies encoded at the base, low frequencies at the apex
• Organ of Corti: hair cells with stereocilia in contact with the tectorial membrane; inner hair cells (single row) are the primary sensory cells; outer hair cells (3 rows) amplify signals
• Reissner's membrane: separates scala vestibuli from scala media
• Stria vascularis: maintains endolymph composition

Internal Auditory Canal (IAC)

• Contains CN VII (facial nerve) and CN VIII (vestibulocochlear nerve - divided into cochlear, superior vestibular, and inferior vestibular branches)
• The fundus of the IAC has separate foramina for each division (identified at surgery as cochlear, inferior vestibular, superior vestibular, and facial nerve openings)

5. Temporal Bone

• Squamous part: lateral wall of the middle cranial fossa
• Mastoid part: contains air cells; site of mastoid antrum (connected to middle ear)
• Petrous part: pyramid-shaped; houses the labyrinth and IAC
• Tympanic part: forms the anterior and inferior walls of the bony EAC

Summary Table

• Cummings Otolaryngology Head and Neck Surgery, Chapter 126 (Temporal Bone, External Ear, and Middle Ear Anatomy), p. 2443-2450
• Gray's Anatomy for Students, p. 1096
• Cummings Otolaryngology, Chapter on External Ear Diseases, p. 2648