Structure & Function of the Eye

By: Carmen Willings
teachingvisuallyimpaired.com
​Updated June 13, 2025
To better understand a student’s vision, it is important to know how each part of the eye contributes to a person's ability to see. Each part plays a critical role and must function properly for vision to occur. There are three primary reasons a person may have a visual impairment:
  • Damage or injury to one or more structures of the eye, interfering with how the eye receives or processes visual information.
  • Developmental differences in eye shape or size (e.g., unusually long or short eyeball), which may affect how the eye focuses light.
  • Neurological causes, where the brain cannot properly process visual signals, even if the eyes are structurally healthy (e.g., in Cortical Visual Impairment or CVI).

​Structures of the Eye and Their Functions

Tear Layer (Lacrimal System)
​The Tear Layer (The Lacrimal System). This is the first layer of the eye that light strikes.  It is clear, moist, and salty.  Its purpose is to keep the eye smooth and moist.  
Cornea​
​The Cornea is the second structure that light strikes.  It is the clear, transparent front part of the eye that covers the iris, pupil, and anterior chamber and provides most of an eye’s optical power (if too flat = hyperopia/farsightedness; if too steep = myopia/nearsightedness).  It needs to be smooth, round, clear, and tough. It is like a protective window. The function of the cornea is to let light rays enter the eye and converge the light rays.
​​​Anterior Chamber (with Aqueous Humor)
The Anterior Chamber is filled with Aqueous Humor.  Aqueous Humour is a clear, watery fluid that fills the space between the back surface of the cornea and the front surface of the vitreous, bathing the lens (The anterior and posterior chambers.  Both are located in the front part of the eye, in front of the lens.  The eye receives oxygen through the aqueous.  Its function is to nourish the cornea, iris, and lens by carrying nutrients, to remove waste products excreted from the lens, and to maintain intraocular pressure, thus maintaining the shape of the eye.  This gives the eye its shape.  It must be clear to function properly.
Iris and Pupil
The iris is the pigmented tissue lying behind the cornea that gives color to the eye and controls the amount of light entering the eye by varying the size of the pupil.  It functions like a camera. The color of the iris affects how much light gets in.  The iris controls light constantly, adapts to lighting changes, and is responsible for near point reading (to see close, pupils must constrict)
 
Pupil: It is a variable-sized black circular opening in the center of the iris that regulates the amount of light that enters the eye.  The pupil needs to be round to constrict.
  • Constricted A constricted pupil occurs when the pupil’s size is reduced to constriction of the iris or relaxation of the iris dilator muscle. The iris constricts in response to bright illumination, certain drugs, and can be a consequence of ocular inflammation. 
  • Dilated A dilated pupil is an enlarged pupil, resulting from contraction of the dilator muscle or relaxation of the iris sphincter.  It occurs normally in dim illumination or may be produced by certain drugs (mydriatics) or result from blunt trauma.
Lens
The lens is the natural lens of the eye (crystalline lens).  Transparent, biconvex intraocular tissue that helps bring rays of light to focus on the retina (It bends light, but not as much as the cornea).  Suspended by fine ligaments (zonules) attached between ciliary processes. It has to be clear, has to have a power of about +16, and has to be pliable so it can control refraction (This becomes less pliable as you age, leading to presbyopia). 
Ciliary Body
​The circumferential tissue (a ring of tissue between the end of the choroids and the beginning of the iris) inside the eye is composed of the ciliary muscle (involved in lens accommodation and control of intraocular pressure and thus the shape of the lens) and 70 ciliary processes that produce aqueous fluid. 
Vitreous Humor (Chamber)
Vitreous Humour (Chamber).  It is the transparent, colorless, gelatinous mass that fills the rear two-thirds of the eyeball, located between the lens and the retina.  It must be transparent so that light can pass through it, and it must be present; otherwise, the eye would collapse.
Retina
The retina is the light-sensitive nerve tissue in the eye that converts images from the eye’s optical system into electrical impulses that are sent along the optic nerve to the brain, to be interpreted as vision.  Forms a thin membranous lining of the rear two-thirds of the globe; it consists of layers that include two types of cells: rods and cones.  There is no retina over the optic nerve, which causes a blind spot (This is the sightless area within the visual field of a normal eye.  It is caused by the absence of light-sensitive photoreceptors where the optic nerve enters the eye.)
  • Cones The cones are the light-sensitive retinal receptor cell that provides the sharp visual acuity (detail vision) and color discrimination; they are most numerous in the macular area.  Function under bright lighting.
  • Rods are the light-sensitive, specialized retinal receptor cell that works at low light levels (night vision).  The rods function in response to movement and provide light and dark contrast.  It makes up peripheral vision.
  • Macula It is the “yellow spot” in the small (3 °) central area of the retina surrounding the fovea.  It is the area of acute central vision (used for reading and discriminating fine detail and color). Within this area is the largest concentration of cones
  • Fovea The fovea is the central pit in the macula that produces the sharpest vision. It contains a high concentration of cones within the macula and no retinal blood vessels.
Choroid
The vascular (major blood vessel), central layer of the eye lies between the retina and the sclera. Its function is to provide nourishment to the outer layers of the retina through blood vessels. It is part of the uveal tract.
Sclera
The sclera is the opaque, fibrous, tough, protective outer layer of the eye (“white of the eye”) that is directly continuous with the cornea in front and with the sheath covering the optic nerve behind. The sclera provides protection and form.
Optic Nerve
The Optic Nerve is the largest sensory nerve of the eye. It carries impulses from the retina to the brain for sight.  Composed of retinal nerve fibers that exit the eyeball through the optic disc, traverse the orbit, pass through the optic foramen into the cranial cavity, where they meet fibers from the other optic nerve at the optic chiasm.
Extraocular Muscles
There are six extraocular muscles in each eye: 
  • Rectus Muscles: The four Rectus muscles are responsible for straight movements: Superior (upward), Inferior (downward), Lateral (toward the outside, or away from the nose), and Medial (toward the inside, or toward the nose). 
  • Oblique Muscles: Two Oblique muscles are responsible for angled movements. The superior oblique muscles control angled movements upward toward the right or left. Inferior oblique muscles control angled movements downward toward the right or left.

Surrounding Areas Impacting Vision

Optic Chiasm and Visual Pathways
  • The point where optic nerve fibers from each eye partially cross.
  • Ensures visual input from both eyes reaches both hemispheres of the brain.
  • Damage along the pathway (e.g., optic tract, lateral geniculate nucleus, or visual cortex) may result in visual field cuts or cortical blindness.
Eyelids and Eyelashes
  • Protect the eye from debris and regulate light exposure.
  • Assist with tear distribution and eye moisture via blinking.
Meibomian Glands (Lid Margin Glands)
  • Located in the eyelids.
  • Secrete oils that prevent tear evaporation and help maintain a stable tear film.
Conjunctiva
  • Thin, transparent membrane covering the sclera and inside of the eyelids.
  • Helps lubricate the eye and protect against infection.
Foveal Avascular Zone (FAZ)
  • Part of the fovea with no blood vessels, allowing unobstructed light access to photoreceptors.
  • Critical for sharp, detailed vision.
​Zonules (Suspensory Ligaments)
  • Attach the lens to the ciliary body.
  • Essential for changing the lens shape during accommodation.

Additional Resources...

Exploratorium
Dissecting a cow eye was the ABSOLUTE worst part of my VI coursework in learning about the structure & function of the eye...it didn't help that I was 6 months pregnant at the time! I will say it is fascinating, however, to examine the eye. The Exploratorium site demonstrates the process of a proper cow's eye dissection (Don't watch this if you're eating!!! You've been warned!).
Having an eye model can help the Teacher of Students with Visual Impairments identify the area(s) of the eye affected by certain visual conditions to help team members understand the area of the eye affected.

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Structure and Function of the Eye Handout
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