Anatomy of the Eyeball

Introduction

• Understanding the anatomy of the eye is crucial before delving into phototransduction and optic pathways.
• Focus areas include the tunics of the eye, segments, aqueous humor flow, lacrimal flow, and relevant clinical correlations.

Tunics of the Eye

1. Fibrous Tunic

   • Composed of:
     • Sclera: Opaque connective tissue. Has three layers:
       • Episclera: Dense fibrous connective tissue.
       • Sclera Proper: Rich in collagen fibers, providing toughness.
       • Lamina Fusca: Pigmented layer.
     • Cornea: Transparent, five layers - epithelial (rich in nociceptors), Bowman’s membrane, stromal layer, Descemet's membrane, endothelial layer.
     • Function: Allows light passage, minimal blood supply, low risk of rejection in transplants.

2. Vascular Tunic (Uvea)

   • Composed of:
     • Choroid: Pigmented, prevents light scattering, and has a rich vascular supply.
     • Iris: Controls pupil size; contains dilator (sympathetic) and sphincter (parasympathetic) pupillae muscles.
     • Ciliary Body: Ciliaris muscle (involved in accommodation), ciliary processes (produce aqueous humor).

3. Sensory Tunic

   • Primarily the Retina, consists of:
     • Neural layer with photoreceptors (rods and cones), bipolar, ganglion cells, amacrine, and horizontal cells.
     • Pigmented layer absorbs light, prevents scattering.
     • In proximity to choroid for vascular supply.

Segments of the Eye

• Anterior Segment: Contains anterior and posterior chambers, separated by the iris.
• Posterior Segment: Posterior to the lens, filled with vitreous humor.

Aqueous Humor Flow

• Produced by ciliary processes, flows from posterior to anterior chamber, drained via scleral venous sinus (canal of Schlemm).

Vitreous Humor

• Fills the posterior segment, maintains retinal position, transmits light, contributes to intraocular pressure.

Clinical Correlations

• Sclera: Jaundice can cause yellowing.
• Cornea: Minimal immune presence allows transplant success.
• Lens: Cataracts result from crystalline protein clumps, influenced by factors like diabetes and smoking.
• Retina: Detachment can occur when vitreous humor leaks between neural and pigmented layers.

Lacrimal Flow

• Lacrimal gland produces tears, flow across the eye and drain into the nasal cavity.

Other Eye Structures

• Palpebra (Eyelids): Conjunctiva covers inner lid, tarsal plates provide structure and gland secretions.
• Optic Nerve: Exits eye at the optic disc, passes through lamina cribrosa.

Conclusion

• In the next session, focus will be zooming into the retinal layers and phototransduction process.