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Eye Dissection

This dissection is a slow process and some of the structures are very delicate. You must follow the step by step procedure very carefully. Answer the questions!

Materials: cow eye, dissecting tray, hand lens, forceps, scalpel, scissors

Procedure:
a. Examine the cow eye to find the optic nerve, which will be a whitish stump protruding from the back of the eyeball. Holding the optic nerve, rotate the eyeball until two of the eye muscles are on top. If the optic nerve pierces the eyeball to the left of the center of the back side, you have a right eye. If it pierces the eyeball to the right of the center, you have a left eye.

1) How many eye muscles control the movement of the eyeball.
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b. Pull on one of the muscles attached to the eyeball. Examine how it is attached to the eye via connective tissue. Remove one eye muscle and save it for later identification.

c. Using forceps, scalpel, and scissors, remove all white fatty tissue around the optic nerve and t he eyeball. When you have finished, the purple outside of the sclerotic coat should be visible.

2) Describe the location of the optic nerve entering the eyeball.
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d. Examine the eye and observe that it is covered with a tough outer covering called the sclerotic coat. The front portion of the coat is called the cornea.

3) How does the cornea differ from the rest of the sclerotic coat.
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e. Place the eye in your dissecting pan with the optic nerve pointing up. Make a circular incision around the nerve and remove it. This is difficult to do because the outer coat is very tough. Place a cover glass to cover the hole to keep the jelly like, vitreous humor, from coming out of the chamber of the eye which it occupies.

f. Place the eye now cornea side in your dissecting pan. Hold the eye firmly with one hand (do not squeeze it) and using a scalpel, make an incision at the edge of the cornea. When you have cut through the cornea, a watery fluid, the aqueous humor, will begin to flow though the incision. Carefully insert the blunt end of your scissors into the incision. Lifting upward on the cornea, with forceps and scissors, cut completely around the cornea. The cornea will be removed as a thick transparent tissue.

4) Using a metric ruler measure the diameter of the cornea of the eye. Holding the ruler up to your lab partners eye, estimate the diameter of the human cornea.
sheep cornea: ________ human cornea: _________

g. After you have removed the cornea you will notice a black ring covering the lens. This is the iris. Using forceps carefully lift one end of the iris away from the lens. Lift and pull the iris away from the sclerotic coat; the iris should come off as a black ring.

h. Carefully using scissors make several cuts into the sclerotic coat; see diagram .
Be careful not to touch the lens with the scissors. Lift the eyeball upside down and carefully pushing from the back, push out the vitreous humor. The lens and part of the ciliary muscle (lens muscle) will come out with the vitreous humor.
Note: A beige pink membrane may come out with the vitreous humor. Save this it is the retina!

5) Describe the vitreous humor
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i. Rotate the vitreous humor until the lens is pointing up at you. Carefully observe the lens and the ciliary muscle with the hand magnifying lens.

6) Describe the appearance of the ciliary muscle.
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k. Leave the lens in the dissecting plan and take a piece of wet newsprint and place the lens over some printing.

7. Explain what you observe happens to the newsprint image.
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8. What type of lens is the human lens?
Biconcave, biconvex, or concave-convex: _______________________

9. Measure the diameter of the lens: __________________

l. Using scissors, extend the incisions that you made in the sclerotic coat. Carefully turn the eyeball inside out. Look to see if there is a thin beige pink membrane remaining on the inside. This is the retina. The retina can be removed for closer examination. Observe that it attaches to the back of the eye. This is the blind spot leading to the optic nerve.

10. Explain what type of photoreceptors the retina is composed of, and how they function.
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m. At the back of the eye (under the retina) is a bluish layer called the tapetum. This layer acts as a reflective surface and is only found in certain animals. This dark black pigment on the inside of the eyeball is the pigment of the choroid layer.
Push the tapetum aside at its cut edge to find the choroid layer directly below.

11. What is the function of this dark pigment?
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Terms To Know

Eye Lid: helps protect the eye and contains special tear glands that secrete tears. The tears contain antibiotics that attack any bacteria or fungi.

Cornea: The transparent part of the sclerotic coat located on the front of the eye.

Iris: The colored part of the eye, that regulates the amount of light entering the eye. In bright light the iris contracts and makes the pupil smaller, In dim light the iris muscle relaxes and the pupil gets larger.

Pupil: A transparent opening into the inner eye made by the relaxing and contracting of the iris muscle.

Lens: A transparent tissue in the front of the eye that focuses light on to the retina.

Lens Muscle: Controls the thickness of the lens, thus enabling you to focus on close and far objects

Eye Muscle: Controls the movement of the eye

Retina: Contains rods and cones, the rods are sensitive to dim light and the cones are sensitive to bright light, with the cones we perceive colors. Within the rods and cones lights impulses are converted into nervous impulse which are sent to the sight center in the brain.

Fovea: The part of the retina where we have the most rods and cones. It is the area of the retina where objects are focused.

Blind Spot: An area of the retina where there are no rods or cones. It is the area where the optic nerve enters the back of the eye.

Optic Nerve: Carries nerve impulses from the eye to the sight centers in the brain
Sclerotic Coat: The tough outer part of the eyeball.

Vitreous humor: A transparent jelly fluid that fills the eyeball.

Using key choices, identify the parts of the eye described in the following statements. Choose the correct term and letter.


___1. Attaches the lens to the ciliary body
___2. Fluid that fills the anterior chamber of the eye; provides nutrients to the lens and cornea
___3. The “white” of the eye
___4. Area of retina that lacks photoreceptors; the blind spot
___5. Contains muscle that controls the shape of the lens
___6. Vascular covering of the eye
___7. Drains the aqueous humor of the eye
___8. Covering containing the rods and cones
___9. Gel-like substance filling the posterior cavity of the eyeball;
helps to reinforce the eyeball
___10. Heavily pigmented covering that prevents light scattering within the eye
___11. ___12. Smooth muscles structures
___13. Area of acute or discriminatory vision
___14. ___ 15. ___ 16. ___ 17. : Refractory media of the eye
___18. Anterior most part of the sclera- your “window on the world”
___19. Covering composed of tough, white fibrous connective tissue

Match the terms provided with the appropriate descriptions

___1. Light bending
___2. Ability to focus for close vision (under 20 feet)
___3. Normal vision
___4. Inability to focus well on close objects: farsightedness
___5. Reflex-constriction of pupils when they re exposed to bright light
___6. Clouding of the lens, resulting in loss of sight
___7. Nearsightedness
___8. Blurred vision, resulting from unequal curvatures of the lens or cornea
___9. Condition of increasing pressure inside the eye, resulting from blocked
drainage of aqueous humor
___10. Medial movement of the eyes during focusing on close objects
___11. Reflex constriction of the pupils when viewing close objects
___12. Inability to see well in the dark; often a result of vitamin A deficiency