Introduction:
How do we see? The eye processes the light through photoreceptors located in the eye that send signals to the brain and tells us what we are seeing. There are two types of photoreceptors, rods and cones. These photoreceptors are sensitive to the light. Rods are the most sensitive to light and therefore provide gray vision at night. Cones are mainly active in bright light and enable you to see color. There are 100 million rods compared to the 3 million cones located in your retina. The photoreceptors help you adjust to night and day. For example, if you walk inside from the sun, you can not initially see anything. This is due to the activity of the cones and the lack of activity of the rods. The rods become activated and adapted to the dim light, resulting in gray images formed in the dark. The same thing happens when you leave a dark movie theatre during the day. The rods are mainly activated and the cones have to adjust to sunlight when you leave the theatre.

Objective: 
By dissecting the eye of a cow, which is similar to the eyes of all mammals including humans, you will gain an understanding of the structure and function of the parts of the eye.

Materials:
Cow eye, dissecting pan, dissecting kit, safety glasses, lab apron, and gloves

Procedure (External Structure):

1. Obtain a cow eye, place it in your dissecting pan, & rinse the eye with water.
2. Rotate the eye until the larger bulge or tear gland is on the top of the eye. The eye is now in the position it would be in a body as you face the body.
3. On the outside of the eye, locate the following parts:

• fat- surrounds the eye & cushions it from shock
• tear or lacrimal gland - forms a bulge on the top outer area of the eye & produces tears to wash the surface of the eye
• tear ducts - tubes to carry the tears from the gland to the eye
• optic nerve - a white cord on the back of the eye about 3mm thick just toward the nasal side; carries messages between the eye & brain
• muscles - reddish, flat muscles found around the eye to raise, lower, & turn (right & left) the eye

4. Turn the eye so that it is facing you & examine these structures on the front surface of the eye:

• eyelids - two moveable covers that protect the eye from dust, bright light, and impact
• sclera - this is the tough, white outer coat of the eye that extends completely around the back & sides of the eye
• cornea - a clear covering over the front of the eye that allows light to come into the eye (preservative often makes this appear cloudy)
• iris - round black tissue through the cornea that controls the amount of light that enters the inner part of the eye (may be colored in humans)
• pupil - the round opening in the center of the eye that allows light to enter and whose size is controlled by the iris

Click here for labeled eye model

Procedure (Internal Structure):

5. Place the eye in the dissecting pan so it is again facing you. Using your scalpel, pierce the white part of the eye or sclera just behind the edge of the cornea. Make a hole large enough for your scissors.
6. Using your scissors, carefully cut around the eye using the edge of the cornea as a guide. Lift the eye & turn it as needed to make the cut and be careful not to squeeze the liquid out of the eye.
7. After completing the cut, carefully remove the front of the eye and lay it in your dissecting pan.
8. Place the back part of the eye in the pan with the inner part facing upward.
9. Locate the following internal structures of the eye:

• cornea - observe the tough tissue of the removed cornea; cut across the cornea with your scalpel to note its thickness
• aqueous humor - fluid in front the eye that runs out when the eye is cut
• iris - black tissue of the eye that contains curved muscle fibers
• ciliary body - located on the back of the iris that has muscle fibers to change the shape of the lens
• lens - can be seen through the pupil; use your scalpel & dissecting needle to carefully lift & work around the edges of the lens to remove it
• vitreous humor - fluid inside the back cavity of the eye behind the lens
• retina - tissue in the back of the eye where light is focused; connects to the optic nerve; use forceps to separate the retina from the back of the eye & see the dark layer below it

10. Answer the worksheet questions on the cow eye dissection.

Click here for eye dissection questions

11. Dispose of the eye as your teacher advises and rinse and return all equipment to the supply cart. Wash your hands thoroughly.