Structure of the Eyeball

Outline

1. Tunics of the Eye
2. Segments (Anterior and Posterior)
3. Flow of Aqueous Humor
4. Lacrimal Flow
5. Clinical Correlations

Tunics of the Eye

Fibrous Tunic

• Sclera: Three layers
  • Epi Sclera
  • Sclera Proper
  • Lamina Fusca
• Cornea: Five layers
  • Epithelial Layer
  • Bowman's Membrane
  • Stromal Layer
  • Descemet's Membrane
  • Endothelial Layer

Vascular Tunic (Uvea)

• Choroid: Pigmented membrane, prevents light scattering, rich vascular supply
• Iris: Controls pupil size
  • Two muscles: Dilator Pupillae (Sympathetic) & Sphincter Pupillae (Parasympathetic)
• Ciliary Body: Consists of ciliary muscle and ciliary processes
  • Ciliary Muscle: Controls lens shape for accommodation (Parasympathetic - contracts, Sympathetic - relaxes)
  • Ciliary Processes: Secretes aqueous humor

Sensory Tunic

• Retina: Two parts
  • Neural Layer: Photoreceptors (rods and cones), bipolar cells, ganglion cells, amacrine and horizontal cells
  • Pigmented Layer: Single layer of epithelial cells, rich in melanin

Segments of the Eye

• Anterior Segment: Cornea to lens
  • Divided into anterior and posterior chambers
• Posterior Segment: Lens to retina

Flow of Aqueous Humor

1. Secreted by ciliary processes
2. Moves through posterior chamber
3. Flows through pupil into anterior chamber
4. Drains into the scleral venous sinus (canal of Schlemm)

Vitreous Humor

• Jelly-like material in the posterior segment
• Functions: Transmits light, holds retina layers together, maintains intraocular pressure

Lens

• Refracts light rays
• Consists of two components: Lens fibers (crystallins) and lens epithelium
• Clinical Correlation: Cataracts (clumping of crystallins)

The Optic Nerve (Cranial Nerve II)

• Pierces the back of the sclera through the lamina cribrosa
• Forms the optic disc visible in ophthalmoscopy

Lacrimal Flow

1. Lacrimal gland produces fluid
2. Fluid moves across cornea
3. Enters lacrimal punctum
4. Flows through lacrimal canaliculi
5. Collects in lacrimal sac
6. Drains through nasolacrimal duct into the nasal cavity (inferior meatus)

Eyelids and Conjunctiva

• Eyelids (Palpebrae): Superior and inferior
• Conjunctiva: Two types
  • Palpebral Conjunctiva (lines the inner surface of eyelids)
  • Bulbar Conjunctiva (covers part of the sclera)
• Tarsal Plate: Support structure within the eyelid; attaches muscles (levator palpebrae superioris and orbicularis oculi) and contains tarsal glands for lubrication

Clinical Correlations

• Jaundice in Sclera: Bilirubin accumulation
• Cataracts: Diabetes, smoking, congenital, excessive Vitamin C
• Retinal Detachment: Separation of retinal layers, urgency required
• Conjunctivitis (Pink Eye): Inflammation of conjunctiva from bacteria or virus

Final Notes

• Next lecture: Detailed look at the retina and phototransduction process