Understanding the Eye: Structure and Function

• The eye is a paired sense organ that reacts to light and allows vision.
• Rod and cone cells in the retina are photoreceptive cells that detect visible light.
• The eye enables the body to take in light and perceive objects in color, detail, and depth.
• Damage to the eye can impair or eliminate vision.

Related Eye Structures

• Eyebrows direct sweat away.
• Eyelids protect against dust and light.
• Tear glands produce tears containing lysozyme to kill bacteria and wash away dust.
• Eyelashes keep dust out.
• The pupil allows light to enter.
• The iris controls pupil size and gives the eye color.
• The sclera is a protective white coat.

Key Eye Parts

• The cornea is the outer, dome-shaped covering that protects the eye. It has multiple regenerative layers.
• The sclera, or "whites" of the eye, is a white layer with a brown inner layer, providing structure, safety, and flexibility.
• The pupil is a black hole that takes in light.
• The iris contains pigment, giving the eye its color and controlling pupil size with dilator pupillae muscles.
• Conjunctiva glands secrete mucus to keep the eye moist, preventing itchiness, pain, damage, and infection. Infection can lead to "pink eye".
• The lens, located behind the pupil, focuses light and changes shape via the ciliary muscles.
• The retina contains rods and cones, transmitting light as chemical and electrical pulses to the optic nerves.
• The macula helps interpret object details and the fova increases the detail for perception.
• The ciliary body controls the movement and shape of the lens.
• The choroid provides blood supply between the retina and sclera.
• Vitreous humor is a gel that helps maintain eye shape and delivers nutrients from the ciliary body, aqueous humor, and retinal vessels. Debris in it causes perceived "floaters".
• Aqueous humor is a watery substance in two chambers that maintains eye shape.

How Vision Works

• Light rays travel to the eye, passing through structures and focusing on the retina.
• Light-sensitive cells in the retina send impulses to the brain via the optic nerve.
• The brain forms an image based on size, shape, color, and distance.
• The cornea and lens bend light to focus on the retina, with the iris controlling the amount of light entering and protecting retinal cells.
• The lens adjusts its shape (flattening or rounding) to focus on objects at different distances.
• Accommodation is the lens's adjustment for near and distant objects, influenced by ciliary muscle contraction and relaxation.

Distance vs Close Vision

• For distant objects, relaxed ciliary muscles flatten the lens and cause less light refraction.
• For near objects, contracted ciliary muscles round the lens and cause greater light refraction.

Common Sight Defects

• Sight defects are conditions preventing proper light focus on the retina due to factors like eyeball shape or lens hardness.

Long-Sightedness (Hypermetropia)

• This occurs when the eyeball is too short, or the lens is too flat.
• Distant objects are seen more clearly.
• Light from near objects focuses behind the retina.
• It can be corrected by convex or converging lenses.

Near-Sightedness (Myopia)

• This occurs when the eyeball is too long or the lens is too curved.
• Near objects are seen more clearly.
• Light from distant objects focuses in front of the retina.
• It can be corrected by concave or diverging lenses.

Astigmatism

• This is caused by irregularly curved lens or cornea surfaces.
• Requires specially shaped lenses to balance irregularities and provide a clear image on the retina.

Cataract

• This occurs when the lens becomes opaque, blocking light.
• It is treatable with surgery to remove the lens, followed by vision correction with spectacles, contact lenses, or an intraocular lens implant.

Glaucoma

• This occurs due to pressure build-up in the aqueous humor, which can damage the optic nerve.
• There are symptoms of painful, inflamed eyes, halos around objects, and poor vision with sightless areas.
• Risk factors include age, heredity, myopia, and general diseases like stroke.
• Early stages are treatable with medication (eye drops, oral medication), but advanced stages may require surgery. Untreated glaucoma can cause blindness. Damage to the optic nerve is irreversible.