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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TVI’s Guide to Teaching the ECC: An Activities Based Curriculum for Teaching Students who are Blind or Visually Impaired
The TVI’s Guide to Teaching the ECC is a digital pdf available for download. It is designed specifically for fellow itinerant Teachers of Students with Visual Impairments (TVI’s), and consists of over 400 activities and topic areas of discussion for instructing students in the Expanded Core Curriculum. The activities are age-neutral and multi-sensory and therefore can meet the needs of the broad range of students served on an itinerant caseload serving. The activities can be individualized to the students various learning modalities and scaffold in order to challenge students but ensure success. Select those activities that align with the student’s learning objects based on the student’s unique visual needs and academic and developmental level. In addition to the core activity areas, each of the 32 Thematic Units incorporates additional unique ECC concepts and skills providing you with a years’ worth of activities. These units are cyclical and can be used repeatedly to help students build on prior knowledge and develop a deeper understanding of concepts. Each unit includes suggestions for activity adaptation associated with the unit. These include lists of objects, possible community-based experiences, environmental print, poems, children & young reader books, children's songs, pop culture songs, movies, and websites.


In addition to the book, the following two supplements are included:
·      TVI’s Guide to the ECC Concept Checklist
·      TVI’s Guide to the ECC Activities Checklist


For complete product details and additional images, visit the TVI's Guide product page.


TVI's Guide to the ECC Product details
·      Digital pdf download: 346 pages (11 pt font)
·      Publisher: Teaching Students with Visual Impairments
·      Author: Carmen Willings
·      Language: English


Visual Efficiency & Near Magnifier Fluency Worksheets: TVI's Guide to the ECC Grab and Go Supplements
The Near Magnifier Fluency Worksheets is a Grab and Go supplement to the TVI's Guide to Teaching the ECC. This workbook is a pdf download that can be printed on demand for use with students. It contains five different types of worksheets for developing visual motor skills and near magnifier fluency skills particularly with the use of a video magnifier. As a companion to the TVI’s Guide to the ECC, the worksheets correspond to each of the 32 ECC Thematic units. The worksheets, along with a list of environmental print for each thematic unit, are designed to help students refine their visual motor skills while reinforcing ECC concepts presented in the thematic units.  


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Visual Efficiency & Near Magnifier Fluency Worksheet Details:
·      Digital pdf download: 210 pages
·      Publisher: Teaching Students with Visual Impairments
·      Author: Carmen Willings
·      Language: English


Thematic Keyboarding & Braille Fluency and Maintenance Worksheets: A TVI’s Guide to Teaching the ECC Grab & go Supplement
The Thematic Keyboarding & Braille Fluency and Maintenance Grab and Go ECC worksheet packet is a print on demand workbook that contains worksheets in print and simulated braille to use for data collection. Present the student with a hard braille copy and use the worksheets to document accuracy and fluency in reading braille. As a companion to the TVI’s Guide to the ECC, the worksheets correspond to the ECC Thematic activities presented in the curriculum guide. These word lists and sentences will help the student refine their keyboarding or braille reading skills while bridging ECC concepts presented in the thematic units. Although the lists and passages can be produced using a manual braillewriter, you may also copy and paste the passages into any braille translation software for quick production on an embosser. They may also be entered into a typing program such as APH’s Talking Typer to build keyboarding fluency.


For complete product details and additional images, visit the Thematic Keyboarding & Braille Fluency & Maintenance Product Page


Thematic Keyboarding & Braille Fluency Worksheet Details:
·      Digital pdf download: 338-page print and simulated braille worksheets
·      Publisher: Teaching Students with Visual Impairments
·      Author: Carmen Willings
·      Language: English


Thematic Vocabulary Cards and Checklists: Grab & Go ECC Supplement
The Thematic Vocabulary Cards and Checklists Grab and Go ECC supplement is a 446 page pdf downloadable print on demand workbook that includes 2940 vocabulary card on 3 1/2" x 2" templates divided by each of the 32 Thematic Units along with a checklist of each unit words in print and simulated braille. As a companion to the TVI's Guide to the ECC, the vocabulary cards correspond to the ECC Thematic activities presented in the curriculum guide. 


Additional Use:
Teachers of Students with Visual Impairments can have a extremely varied caseloads. It is possible to go several years without having a student on your caseload who requires braille instruction. Like other skills, if you do not practice your braille regularly, it is easy to lose those skills. Use these checklists and cards to maintain your braille skills. Fold the checklist along the line dividing the print from the braille. Choose to either read the print and produce the word in braille using a braillewriter or read the simulated braille and decode it. Finally, these vocabulary cards and checklists may be used with family members or peers who are interested in learning and practicing the braille code.


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Thematic Vocabulary Cards and Checklists Details:
·      Digital pdf download: 446-page print on demand resource
·      Publisher: Teaching Students with Visual Impairments
·      Author: Carmen Willings
·      Language: English


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Lotto Cards
The LOTTO Cards Grab and Go Supplement includes 37 theme related unit cards along with activity suggestions that support activities within the TVI's Guide to Teaching the ECC.


On My Way File Folder Cards
Print and use these cards to represent locations the student may visit that are related to the current thematic unit. Use these with the On My Way File Folder Game outlined in the TVI's Guide to Teaching the ECC p. 27.

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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.