Human Eye Structure and Function: A Detailed Look

Human Eye Structure and Function

Key Components:

• Conjunctiva: Thin, transparent membrane lining the front of the sclera. It's moistened by tears, produced in the tear glands, which contain lysozyme, an enzyme that kills bacteria.
• Pupil: Opening in the iris through which light passes. It controls the amount of light entering the eye and appears black due to the black pigment of the choroid.
• Lens: Clear structure that allows light into the eye. It's made of stiff jelly and can change shape to focus light.
• Iris: Colored part of the eye that responds to light.
• Cornea: Transparent, protective layer at the front of the eye.
• Suspensory Ligaments: Fibers supporting the lens. When pulled tight, the lens is thin; when loosened, the lens is thicker.
• Ciliary Muscles: Muscles that change the thickness of the lens by controlling the shape of the suspensory ligaments.
• Retina: Internal lining at the back of the eye, consisting of thousands of light-sensitive cells (rods and cones). These cells send nervous impulses to the brain via the optic nerve.
• Sclera: Tough, white, outer protective layer. The front part is clear, allowing light to enter.
• Choroid: Black layer with many blood vessels, located between the retina and sclera.
• Fovea: Area where light-receptive cells (primarily cones) are most densely packed. It's where light is focused when looking directly at an object.
• Blind Spot: Part of the retina where the optic nerve enters; there are no light-sensitive cells here.
• Optic Nerve: Carries nerve impulses to the brain.
• Aqueous Humor: Watery liquid filling the front part of the eye.
• Vitreous Humor: Transparent jelly that supports the back part of the eye.

Accommodation:

Accommodation is the adjustment in the shape of the lens to focus light from different distances. Light is refracted (bent) to focus on the retina.

The cornea and lens work together to refract light.

The image on the retina is upside down, but the brain interprets it so that we see it the right way up.

• Objects Nearby: Light rays need to be bent more (diverging rays). The lens must be fat. Ciliary muscles contract, and suspensory ligaments loosen.
• Objects at a Distance: Light rays need less bending (nearly parallel rays). The lens must be thin. Ciliary muscles relax, and suspensory ligaments tighten.

Pupil Reflex:

The pupil reflex occurs when the eye adjusts to the light intensity (amount of light) entering the retina.

Pupil size changes with light intensity: it widens (dilates) in low light and constricts in bright light.

Controlled by:

• Circular Muscles: Surround the pupil. Contraction constricts the pupil.
• Radial Muscles: Run outwards from the edge of the pupil. Contraction dilates the pupil.

Rods and Cones:

• Rods: Sensitive to dim light, used for night vision. Lack of Vitamin A can cause night blindness. They only detect black and white light.
• Cones: Detect colors in bright light. Fewer in number than rods, with most found at the fovea.