Structures of the External Auditory Canal

1. Overall Framework

• The isthmus is the narrowest point, located just medial to the cartilaginous-bony junction (about 5 mm from the tympanic membrane)
• The anterior recess (deep to the isthmus) is a wedge-shaped space between the anterior canal wall and tympanic membrane - a surgically difficult area

2. Structures of the Cartilaginous (Lateral) EAC

• Auricular cartilage - forms the skeletal framework; deficient superiorly at the incisura terminalis (used in endaural incisions)
• Fissures of Santorini - 2-3 anteroinferior perforations/fissures in the cartilage; allow passage of infection or tumour into the parotid gland
• Thick skin with well-developed subcutaneous layer containing:
  • Hair follicles
  • Sebaceous glands
  • Ceruminous (cerumen) glands - modified apocrine sweat glands that produce cerumen (earwax)

3. Structures of the Bony (Osseous) EAC

• Tympanic bone - forms the floor and most of the canal wall; also carries the tympanic sulcus
• Squamous bone - forms the roof of the bony canal
• Two suture lines projecting into the canal with closely adherent overlying skin:
  • Tympanosquamous suture (anteriorly)
  • Tympanomastoid suture (posteriorly) - landmark for "vascular strip" incisions in surgery
• Tympanic sulcus - a groove at the medial end of the bony canal (absent superiorly) that holds the tympanic membrane; the absence superiorly creates the notch of Rivinus
• Thin skin with minimal subcutaneous layer - no hair follicles, sebaceous glands, or ceruminous glands (the absence of hair is what clinically distinguishes the bony from cartilaginous canal)

4. Lining (Entire Canal)

5. Innervation

• Auriculotemporal nerve (branch of CN V3 / trigeminal) - anterior wall and roof
• Greater auricular nerve (C3) - floor and posterior wall
• Lesser occipital nerve (C2, C3)
• Arnold's nerve (auricular branch of CN X / vagus) - posterior inferior wall; stimulation can trigger cough reflex or even vasovagal syncope
• Twigs from CN VII (facial nerve)

6. Blood Supply

Key Clinical Correlations

• Auricular cartilage framework
• Incisura terminalis (superior gap)
• Fissures of Santorini (anteroinferior perforations)
• Thick skin with hair follicles, sebaceous glands, and ceruminous glands

• Tympanic bone (floor/walls) + squamous bone (roof)
• Tympanosquamous suture (anterior) and tympanomastoid suture (posterior)
• Thin skin - no adnexal structures
• Isthmus (narrowest point, ~5 mm from tympanic membrane)
• Anterior recess (wedge-shaped space)
• Tympanic sulcus + notch of Rivinus (holds the tympanic membrane)

Tympanic Membrane - Structure (NEET PG High Yield)

Gray's Anatomy for Students - Right Tympanic Membrane (otoscopic view)

1. Basic Facts (Frequently Tested)

2. Layers (Trilaminar / 3 or 4 Layers - Both Correct)

1. Outer layer - Squamous (keratinizing stratified squamous) epithelium, continuous with skin of EAC
2. Middle layer (Pars Propria / Lamina Propria) - Fibrous layer; contains radial fibres (outer) + circular fibres (inner, parabolic/transverse deeper)
3. Inner layer - Mucous membrane (mucosal epithelium), continuous with middle ear lining

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

Key exam point: The pars flaccida lacks proper radial and circular fibres - its lamina propria has randomly oriented collagen. This makes it weak and prone to retraction pockets and cholesteatoma.

3. Two Parts of TM (Most Important!)

4. Key Landmarks on TM (Otoscopic View)

         Pars Flaccida (Shrapnell's)
              /\
    Post. ---/--\--- Ant. Malleolar fold
    mall. fold      Lateral process of malleus (bulge)
         |
    Handle of Malleus (manubrium)
         |
       UMBO  ← apex of cone, tip of manubrium
         \
          Cone of Light (anteroinferior)

• Umbo - apex of the cone-shaped TM; point of attachment of tip of manubrium of malleus; most concave point
• Cone of Light (Light Reflex) - anteroinferior to umbo; points to 5 o'clock in right ear, 7 o'clock in left ear
• Lateral process of malleus - small projection superiorly, above which pars flaccida sits
• Anterior malleolar fold - runs from lateral process to anterior tympanic spine
• Posterior malleolar fold - runs from lateral process to posterior tympanic spine

5. Prussak's Space (Lateral Epitympanic Recess) ★★ NEET Favorite

• Bounded laterally by pars flaccida (Shrapnell's membrane)
• Medially by neck of malleus + lateral malleal ligament
• Anterosuperiorly by anterior malleal fold
• Inferiorly by posterior malleal fold / lateral process of malleus
• Clinical importance: Primary site for pars flaccida cholesteatoma formation

6. Attachment

• Tympanic annulus (fibrocartilaginous ring) sits in the tympanic sulcus of the tympanic bone
• Sulcus is absent superiorly = notch of Rivinus (where pars flaccida sits, with no annulus)
• TM is firmly attached to malleus at:
  • Lateral process (superiorly)
  • Umbo (at tip of manubrium)
  • Between these two points = only a flimsy plica mallearis connects them

7. Blood Supply

8. Nerve Supply

Exam tip: CN IX (Jacobson's nerve) supplies the inner surface; stimulation can refer pain to the throat/tongue base.

9. Clinical Correlates (High Yield)