Eye Model and Dissection Handout

8/13/2019 Eye Model and Dissection Handout

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CJ Shuster Lab Addenum Eye Anatomy 1

EYE ANATOMY(Adapted from Johnson, Weipz and Savage Lab Book)

Introduction

The eyes are the largest and most complex sensory organs in the human body. Theyfunction somewhat as a pair of cameras, recording images of the surrounding world thatcan be interpreted by the brain.

In order to record a clear image the eye must be able to focusequally well on both nearand far objects (accommodation for distance) and must be able to adjust to different lightintensities (accommodation to light). These many varied functions of the eyes requirea variety of specialized and intricate functional mechanisms.

The perception of information received by special cells called PHOTORECEPTORS. Thestimulus is light energy. In other words, vision is the perception of light energy. The EYEis a special organ that allows us to receive light energy, focus it, and transduce it into anaction potential.

The fovea centralis is the region on the photoreceptive layer that receives focused light;here, the cells are the densest.


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To avoid the recording of a double image the eyes must be able to point directly at the

object being viewed (convergence). Muscles control eye position and movement.


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* ACUITY is the ability of the eye to focus on the image.

The 2 fovea are 5-7.5 cm apart, and the nose and eye socket block the view of the opposite side. Also, your

brain learns to interpret up as back. This is termed DEPTH PERCEPTION.

A mechanism also exists that enables the eye to differentiate between light of different

wave lengths (color perception).*low wave he ight = long

wave length = lower

power.

*Higher wave height (=

short length) = higher

f r e q u e n c y ( c l o s e r

together); and since it ist h e m o v e m e n t o f

particles called photons ,

higher frequency =

higher power (or ability

to do work).

NOTE: colors are different wavelengths in the visible spectrum.


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Lab Exercises

Human Eye Models.

Take a look at a model of the eye in the orbit. Study the muscles and exterior anatomy asoutlined on your wordlist. See figure #1a on the Data/Analysis Sheet. Here are somehighlights:

1. Eyelids (palebrae). Keep surface lubricated & free of debris.

2. Medial & lateral canthus. The corners of the eye.

3. Eyelashes

4. Meibomian gland. Secretes a lipid-rich product that helps prevent the eyelids from sticking together.

5. Orbicularis oculi & levator palebrae superioris .... closing & raising eyelid.

6. Conjunctiva. Epithelium covering the inner eyelid & outer surface of eye ball. Mucous mem brane.7. Lacrimal gland. Releases lysosome- filled secretion. Drained by the lacrimal ducts, which lead to the

lacrimal sac. From the sac, through the nasolacrimal duct to the inferior mea tus of the nasal cavity.

8. Orbital fat. Insulation & padding.

Obtain one of the "naked" (that is, eye only not in orbit no major detail of associatedanatomy) models of a human eye. Locate each of the anatomical parts (SEEWORDLIST)on the model, and also on Figure #1B of the Data/Analysis Sheet. In mostcases careful reading of the brief description of the structure given here, combined withsome good old common sense, should enable you to make an accurate identification.

Before opening the eye model, locate the following. The outer coat of the eyeball is a

tough layer of dense fibrous tissue divided into two regions. The sclera("white of the eye")is white and opaque, and covers all but the very anterior portion of the eyeball. Thecornea is the transparent anterior portion. The muscles that move the eyeball (theextrinsic eye muscles) attach to the sclera. Light enters the eye through the cornea. Atthe back of the eye, somewhat off center toward the midline of the body, is the opticnerve. The position of the optic nerve will tell you whether the model is of a right or lefteyeball.

Now open the eye model and locate the following. Looking at the posterior part of theeyeball note two layers in addition to the sclera. The middle layer is the choroid. Thislayer is pigmented and is usually shown as dark colored (blue-black). The inner layer is

the retina. This layer is by far the thinnest and most delicate of the three layers and isusually shown as light colored (tan or pink). Blood vessels can be seen directly in theretina of the living eyes by use of a special reflecting light called an ophthalmoscope. Anervous tissue layer of the retina contains the photoreceptor neurons that form rods andcones.

The continuity of the three layers in the back of the eyeball is interrupted where the optic

nerve exits. This point is the optic discor blind spot, so-called because there are no


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photoreceptor neurons at this particular part of the retina. Slightly toward the midline of theeyeball from the blind spot is a small, lighter colored spot called the macula lutea. The

center of the macula lutea is called the fovea centralis. This is the very center of theretina and is the point of sharpest vision.

The entire posterior cavity of the eye (between the lens and the retina) is filled with a thickgelatinous fluid called vitreous humor. On most of the models a clear plastic insertrepresents the vitreous humor. This fluid helps to maintain the shape of the eyeball andalso to hold the retina in place against the choroid. The anterior cavity of the eye (betweenthe lens and cornea) is filled with a more watery fluid called aqueous humor. Aqueoushumor is continuously produced and is drained from the anterior chamber through thecanal of schlemm, a venous sinus located at the junction of the sclera and cornea.Intraocular pressure is produced mainly by the aqueous humor. Excessive intraocularpressure, called glaucoma, can result in degeneration of the retina and blindness.

The lensis a clear, elliptical structure that, together with the cornea, bends light rays tofocus and form a clear image on the retina. Normally, the lens is perfectly transparent. Aloss of transparency of the lens is known as a cataract. The lens is held in place bysuspensory ligaments that attach it to the ciliary body, the thickest portion of thechoroid. The ciliary body consists of protrusions or folds called ciliary processes thatsecrete aqueous humor, and smooth muscles called ciliary muscles (intrinsic eye muscles),that work to alter the shape of the lens. When the ciliary muscles are relaxed, thesuspensory ligaments pull on the lens making it flatter and enabling the eye to focus onobjects father away. Contraction of the ciliary muscles reduces tension on the suspensoryligaments, allowing the elastic lens to round up and enabling the eye to focus on objects

closer to the eye. Immediately in front of the lens lies the pigmented iris, also a

modification of the choroid. The opening in the iris is called the pupil. Muscle fibers in theiris (intrinsic eye muscles) can cause the pupil to constrict or dilate, thus regulating theamount of light passing through the lens. Genetically, some persons produce a pigmentin the iris that causes it to be brown. Blue-eyed persons do not produce the pigment.Persons with eyes a shade of green have genes to produce pigment (i.e., are geneticallybrown-eyed), but produce only small amounts of the pigment, causing the unusual shading.

Other Human Eye Models.

Locate each of the following structures on Figures #1A & #1B of the Data/Analysis Sheet.

Also, find as many of these structures as possible on the other eye models available in thelab.

The conjunctivais a thin mucous membrane that lines the eyelids and covers the exposedsurface of the eyeball (mostly cornea) with an epithelial layer of cells. The extrinsic

musclesare small and very precisely controlled skeletal muscles that serve to point theeye at an object being viewed. In Figure #2A the muscle which has a portion cut away to

reveal the optic nerve is the lateral rectusmuscle. On the other side of the eye is its


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opposing muscle, the medial rectuswhich is only partially visible. On the superior surfaceof the eyeball is attached the superior rectus, and opposing it on the inferior surface isthe inferior rectus. These four muscles point the eyeball up, down, medially and laterally.

Just above the stub of the lateral rectus is seen the insertion of the superior obliquemuscle, which passes through a cartilaginous loop, the trochlea. Opposing the superioroblique is the inferior obliqueof which only the insertion is seen below the stub of thelateral rectus. These two muscles are used for small rotational movements of the eyeball.The muscle which extends into the upper eyelid is the levator palpebrae. It raises theeyelid.In Figure #2B identify the lacrimal glandslocated superior and lateral to the eye. Theirsecretion is spread over the surface of the eye by the opening and closing of the eyelid.Excess tear fluid is collected in the medial corner (medial canthus) of the eye andchanneled into the lacrimal ducts. These ducts empty into a larger vessel, the lacrimal

sac, which then empties into the nasal cavity via the nasolacrimal duct.


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Figure #1A & B (Exterior Eye & Horizontal Section)


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Beef Eye Dissection.

Obtain one of the preserved or fresh beef eyes available for dissection. Take a moment

to examine it externally before cutting into it. The amount of excess tissue remaining willvary from minimal to lots. Look to see if there are any remnants of the extrinsic muscles,

eyelid, or lacrimal glands. Note the tough, white outer sclera and the transparentanterior cornea. Examine the back of the eyeball to locate the stub of the optic nerve.If the cornea is still relatively clear, look through it (a small penlight might be useful) at theirisand pupilwithin the eye.

Holding the eyeball very firmly in your hand, pierce the sclera with a sharp- pointedscissors at a point about 1 cm away from the edge of the cornea. EXERCISE EXTREMECARE IN DOING THIS. The eye is difficult to hold, and the sclera is difficult to penetrate.The difficulty you have in piercing the sclera should give you an appreciation for theprotective capabilities of a dense connective tissue. Once you have made an openingthrough the wall of the eyeball, use a scissors to cut all around the edge of the cornea.Using a forceps, lift the anterior part of the eye away to expose the interior of the eye.(NOTE! Try to keep the thick vitreous humor in the back of the eye as you do this.Dislodging the vitreous humor will detach the retina.)

Locate and examine the lens. It may remain attached to the vitreous humor or it maycome off with the anterior part of the eye. Use a probe to separate the lens from the othertissue of the eye. Carefully remove the lens from the eye and set it on a scrap piece ofprinted matter. Is the lens transparent? Feel the lens and note whether the surface feelsany different than the central core.

Look at the vitreous humorin the back of the eye and note its consistency and clarity.Locate the filmy, light tan retinathat lines the inside of the eyeball. Determine if it is firmlyattached to the choroid coat or just lying against it. Note the blood vessels in the retina.

Remove the vitreous humor from the back of the eye. Note what happens to the retinawhen you do this. Locate the point at which the retina is firmly attached to the wall of theeyeball and determine if this is the same point where the optic nerve stub was found.

Examine the choroidcoat and separate it partially from the sclera. Note that most of thechoroid coat (including the ciliary body in the anterior part of the eye) is black. Thisprevents light from reflecting around inside the eye and blurring the images being formedon the retina. Note that a portion of the choroid coat is an iridescent blue-green color thatdoes reflect light. This is called the tapetum lucidumand is an adaptation for nightvision. Although the reflection of light tends to blur the images, it also increases theamount of stimulation of the retina. Animals with this adaptation are trading clarity of visionfor the ability to at least see something in lower light conditions. This is also the featurethat causes some animal's eyes to shine back at you at night when you shine a light atthem. Humans do not have a tapetum lucidum.


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Examine the ciliary bodyand irisin the anterior part of the eye. Look at the iris to see ifyou can find any indications of the radial and circular muscle fibers that control pupil size.

When you have completed this dissection, ask your instructor about any structures youwere uncertain of. Answer the questions about the dissection that are on the Data/AnalysisSheet.

Remove excess muscle,glandular tissue, etc.

Notice: Sclera, conjunctiva,cornea, optic nerve

Using your scapula and small,pointy scissors, cut aroundcornea, keeping a couple of mmaway from the cornea. Do notdrift too far away, or youlldetach the retina (see later).

Notice: iris, pupil, lens, vitreoushumor, aqueous humor, ciliarybody, suspensory ligaments


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Remove lens. Vitreous humorshould come along with it. Becareful not to detach retina.

Look into posterior chamber.Notice: retina, choroid, optic disk,tapetum.


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DATA/ANALYSIS SHEET

Beef Eye Dissection.

1. What is it about the wall of the eye that makes it so difficult to penetrate with thepoint of a scissors?

2. Compare the consistencies of the aqueous humor and vitreous humor of the eye.

3. What happens to the retina of the eye when the vitreous humor is removed? Whydoes this happen?

4. What is the reflective portion of the beef eye called? Do people have this structure?What is the advantage of being reflective? What is the disadvantage of beingreflective?

5. Why is most of the inner wall of the eye pigmented black?

6. Does the central core of the lens feel any different from the outer edges? Whatdifference do you notice?


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Matching.

___outer layer of wall of eye

___middle vascular layer of wall of eye

___inner light-sensitive layer of wall of eye

___fluid between lens and cornea

___fluid between lens and back of eye

___small pit in retina of eye

___small area at back of eye that lacksphotoreceptors

___membrane that lines eyelid and covers cornea

___drain tubes for tears in eyelids

___tube that drains tears to nasal cavity

___transparent front portion of eyeball

___circular band of smooth muscle around lens

___connective tissue holding lens in place

___opening in the center of the iris

___region containing muscle to regulate lens shape

A. aqueous humor

B. blind spot

C. choroid coat

D. ciliary body

E. cornea

F. conjunctiva

G. fovea centralis

H. iris

I. lacrimal ducts

J. nasolacrimal ducts

K. pupil

L. retina

M. scleroid coat

N. suspensory ligament

O. vitreous body

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Eye Model and Dissection Handout