Makindo Medical Notes

Related Subjects: |Anatomy of the Ear |Anatomy of the Oesophagus |Anatomy of the Diaphragm |Anatomy of Large Bowel |Anatomy of Small Bowel |Anatomy of the Biliary system |Anatomy of the Eye

👁️ Anatomy of the Eye

• Eyeball (Globe): Spherical (~24 mm diameter), composed of three main layers:
  • 🟤 Fibrous Tunic: Outer layer → sclera (white) + cornea (transparent, refractive power).
  • 🩸 Vascular Tunic (Uvea):
    • 🌈 Iris: Controls pupil size & regulates light entry.
    • 🔄 Ciliary Body: Produces aqueous humour & controls lens shape (accommodation).
    • ❤️ Choroid: Vascular layer supplying oxygen & nutrients to retina.
  • 📸 Retina: Photoreceptors (rods & cones) → convert light to electrical signals.
• 🌐 Cornea: Transparent, avascular → provides ~70% of eye’s refractive power.
• 💧 Aqueous Humor: Nourishes anterior eye, maintains intraocular pressure.
• 🔍 Lens: Biconvex, transparent → fine focus of light onto retina.
• ⚫ Pupil: Central iris opening controlling light entry.
• 🟣 Vitreous Humor: Gel that maintains eyeball shape.
• 🎯 Macula & Fovea: Central vision, high cone density for colour/sharp detail.
• ⭕ Optic Disc: "Blind spot" — exit of optic nerve (no photoreceptors).
• 🧠 Optic Nerve (CN II): Transmits signals to brain.

🏰 Anatomy of the Orbit

• The orbit is a bony cavity housing the eye, muscles, nerves, vessels, & fat. Formed by 7 bones:
  • ⬆️ Frontal (roof)
  • ⬇️ Maxilla (floor)
  • ➡️ Zygomatic (lateral wall)
  • 🔑 Sphenoid (posterior wall, optic canal)
  • 💧 Lacrimal (medial wall, lacrimal sac)
  • 🦴 Ethmoid (medial wall)
  • 📐 Palatine (posterior orbital floor)
• 🎯 Extraocular Muscles (EOM): "LR6 SO4 AO3" mnemonic
  • Medial Rectus → adduction
  • Lateral Rectus → abduction (CN VI)
  • Superior Rectus → elevation (CN III)
  • Inferior Rectus → depression (CN III)
  • Superior Oblique → intorsion + depression + adduction (CN IV)
  • Inferior Oblique → extorsion + elevation + abduction (CN III)
• 💧 Lacrimal Apparatus: Tear production & drainage
  • Lacrimal gland (superolateral orbit)
  • Lacrimal canaliculi → lacrimal sac
  • Nasolacrimal duct → drains into nasal cavity

🩸 Arterial Blood Supply

• Main supply = Ophthalmic Artery (branch of ICA) → travels via optic canal.
• Key branches:
  • 🔴 Central Retinal Artery → inner retina (occlusion = sudden vision loss, "cherry-red spot").
  • 💧 Lacrimal Artery → gland, lids, lateral rectus.
  • 🌌 Posterior Ciliary Arteries:
    • Short → choroid & outer retina (photoreceptors).
    • Long → ciliary body & iris (aqueous humour regulation).
  • 🌿 Ethmoidal Arteries → nasal cavity & ethmoid sinus.
  • 🟡 Supraorbital & Supratrochlear → forehead & scalp.
  • 👃 Dorsal Nasal → nose skin & lacrimal sac.

💡 Clinical Significance

• 🔴 Central Retinal Artery Occlusion (CRAO): Sudden painless vision loss, cherry-red spot.
• ⚡ Posterior Ciliary Artery Occlusion: Causes anterior ischaemic optic neuropathy (AION).
• 🧩 Orbital fractures (blow-out fractures) → commonly involve floor (maxilla) & entrap inferior rectus.
• 🦠 Venous drainage → via ophthalmic veins to cavernous sinus (risk of cavernous sinus thrombosis from facial infections).

🩸 Venous Drainage

• Superior Ophthalmic Vein: → cavernous sinus (connects to facial vein).
• Inferior Ophthalmic Vein: → cavernous sinus or pterygoid venous plexus.