Eye Structure and Function

Outer Layer

• Sclera: Tough, white outer layer with a protective function.

Middle Layer

• Choroid: Supplies nutrients to blood vessels and contains melanin to prevent internal reflection.

Inner Layer

• Retina: Contains photoreceptors (rods and cones).
  • Fovea: An area in the retina with a high concentration of cones, most sensitive part of the retina.

Ciliary Body and Muscles

• Connective tissue called ligaments attach ciliary muscles to the lens.
• Accommodation:
  • Muscles contract to flatten ligaments, allowing the lens to bulge.
  • Muscles relax, tightening ligaments, causing the lens to flatten.

Lens

• Transparent, biconvex for fine light adjustment.

Iris

• Colored disc made of muscles.
  • Circular Muscles: Relax to widen the pupil, contract to narrow it.
  • Radial Muscles: Antagonistic to circular muscles.

Aqueous Humor

• Colorless fluid in front of the lens, maintains shape of the cornea and aids in light refraction.

Vitreous Humor

• Jelly-like substance helping maintain the shape of the eyeball.

Conjunctiva

• Thin, transparent layer protecting the cornea.

Optic Nerve

• Carries impulses from the retina to the brain.
• Blind Spot: No light-sensitive cells where the optic nerve leaves the eye.

External Structures

• Eyelashes, eyelid, eyebrow—serve protective functions.

Image Formation

• Light rays enter the eye, refract, and form an inverted image on the retina.
• Information sent to the brain via the optic nerve.

Photoreceptors

• Rods: Function in low light, detect black and white.
• Cones: Function in high light, detect colors.

Pupil Control

• Controlled by the iris muscles, adapting to dim and bright light by altering pupil size.

Accommodation

• Lens shape changes to allow focus on near and distant objects.
• Distance Vision: Ciliary muscles relax, ligaments tighten, lens flattens.
• Near Vision: Ciliary muscles contract, ligaments slacken, lens bulges.

Eye Defects and Corrections

Myopia (Nearsightedness)

• Lens too convex; distant objects fall in front of the retina.
• Corrected with a concave lens to diverge light rays.

Hypermetropia (Farsightedness)

• Lens too flat; near objects fall behind the retina.
• Corrected with a convex lens to converge light rays.

Presbyopia

• Loss of lens elasticity; corrected with bifocal or converging lenses.

Astigmatism

• Uneven lens curvature causing blurred vision; corrected with a cylindrical lens.

Glaucoma

• Pressure build-up in aqueous humor damaging the optic nerve.
• Treated with eye drops or surgery.

Cataract

• Lens opacity due to protein deposits; treated by lens replacement surgery.