Structure & Function of the Eye

By: Carmen Willings
teachingvisuallyimpaired.com

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 area of the eye has an important role and must function properly in order for the eye to function properly. The following is a quick lesson in the structure and functions of the eye.

There are three ways areas, or underlying reasons for, a visual impairment. The first, there may be damage or a result of an injury to one or more parts of the eye essential to vision.  This damage may interfere with the way the eye receives or processes visual information. Second, the eyeball may be proportioned incorrectly (have different dimensions than usual), making it harder to focus on objects or may not have developed correctly. And finally, the part of the brain that processes visual information may not work properly.  The eye may be perfectly normal, but the brain is not able to analyze and interpret visual information so that the person can see.

Parts of the eye...

As part of the coursework for becoming certified as a Teacher of Students with Visual Impairments, it is necessary to take a class on the structure and function of the eye. Understanding the significance of each area of the eye can help a TVI understand the possible effects of various visual diagnosis.

1. Tear Layer

The Tear Layer (The Lacrimal System) 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.  

2. 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.

3. Anterior Chamber

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, it removes waste products excreted from the lens, and maintain intraocular pressure and thus maintains the shape of the eye.  This gives the eye its shape.  It must be clear to function properly.

4. Iris

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 papillary opening.  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 in order to constrict.
• Constricted. A constricted pupil occurs when the pupil size is reduced to constriction of the iris or relaxation of the iris dilator muscle. The iris constricts with bright illumination, with 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.

5. Lens

The lens is the natural lens of the eye (chrystaline 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 presbiopia).

• Ciliary Body. The circumferential tissue (a ring of tissue between the end of the choroids and the beginning of the iris) inside the eye 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.

6. Vitreous Humour (Chamber)

Vitreous Humour (Chamber) is the transparent, colorless gelatinous mass that fills rear two-thirds of the eyeball, between the lens and the retina.  It has to be clear so light can pass through it and it has to be there or eye would collapse.

7. 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 interpret as vision.  Forms a thin membranous lining of the rear two-thirds of the globe; 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 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; most numerous in macular area.  Function under bright lighting.
• Rods The light-sensitive, specialized retinal receptor cell that works at low light levels (night vision).  The rods function with movement and provide light/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.

8. Choroid

The vascular (major blood vessel), central layer of the eye lying between the retina and sclera. Its function is to provide nourishment to the outer layers of the retina through blood vessels. It is part of the uveal tract.

9. 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.

10. Optic Nerve

The Optic Nerve is the largest sensory nerve of the eye. It carries impulses for sight from the retina to the brain.  Composed of retinal nerve fivers 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.

11. Extraocular Muscles

There are six extraocular muscles in each eye:

• Rectus Muscles. There are four Rectus muscles that are responsible for straight movements: Superior (upward), Inferior (lower), Lateral (toward the outside, or away from the nose), and Medial (toward the inside, or toward the nose). 
• Oblique Muscles. There are two Oblique muscles that are responsible for angled movements. The superioroblique muscles control angled movements upward toward the right or left. Inferior oblique muscles control angled movements downward toward the right or left.

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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. 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 in order for them to help team members understand the area of the eye affected.