What part of the eye contains photoreceptors

EXPERIMENT 3: COW EYE DISSECTION Survival depends on the meaningful interpretation of stimuli. The five senses (taste, sight, touch, sound, and smell) are completely dependent on the nervous system. Nearly 70% of the sensory receptors in the body are found in the eye. Photoreceptors are specialized cells that sense and encode light patterns from which the brain can construct images. The cow eye is quite similar to the human eye. In this experiment, you will gain an understanding of how the eye forms images and communicates with the brain through dissection and examination of the cow eye.

DEFINITIONS • Aqueous Humor: Watery fluid that provides shape to the front of the eye.

• Sclera: The white, outermost protective part of the eye.

• Cornea: A clear protective layer that refracts light.

• Pupil: Opening regulates the amount of light entering the eye.

• Iris: The colored muscle of the eye that regulates pupil size to protect the retina from light.

• Lens: The structure that refracts light onto the retina.

• Choroid: The middle layer of the eye that contains blood vessels.

• Retina: Posterior portion of the eye that contains photoreceptors (rods and cones).

• Fovea Centralis: An area at the center of the retina that contains photoreceptors used for color interpretation.

• Optic Nerve: The pathway for impulses to the occipital lobe of the brain.

• Vitreous Humor: A viscous liquid that provides shape to the eye.

PROCEDURE 1. To begin, lay down your underpad and place your dissecting tray on top of it. Be sure you have all of your safety equipment

on before beginning.

2. Once prepared, gently open the bag containing your cow eye and place it on the dissection tray.

3. Carefully observe the cow eye, and remove any fat or extrinsic muscle with the scalpel. After removing this tissue, the sclera and optic nerve should be visible.

4. Make an incision with the scalpel into the cornea until the aqueous humor (a clear liquid) is released.

Experiment 3Cow Eye Dissection Materials Cow Eye

Dissection Tray

Dissection Tools

*Piece of Paper (should have writing on it)

Experiment Inventory

Note: You must provide the materials listed in *red.

https://vimeo.com/190319505
5. Make an incision with the scalpel along the coronal plane through the sclera so that one-half will have the anterior features of the eye (cornea, lens, iris, ciliary body), and the other half will contain the posterior features of the eye (most noticeably where the optic nerve is attached to the eye).

6. Position the anterior half with the cornea so that it is in front of you on the dissection tray.

7. Using a scalpel, cut through the cornea.

8. Using forceps, remove the iris, located between the cornea and the lens.

9. The anterior half of the eye contains the lens. Notice the vitreous humor, the clear gel posterior to the lens.

10. Using the scalpel and forceps, remove the lens from the specimen.

11. Place the lens on a piece of paper with writing on it. Note the lens color, clarity, and malleability.

Note: In a living creature, the lens is clear and flexible. After preservation, it may have yellowed and become more rigid.

12. Move to the posterior half of the eyeball. Remove the vitreous humor and note the blood vessels on the posterior wall. This is the retina.

13. Using gloved fingers, gently press on the retina and slide it around the eyeball to get a sense of where the optic nerve connects to the tissue.

14. Use forceps to gently lift the retina off the inside wall of the eye. You should observe a colorful, shiny tissue, the choroid coat (vascular tunica).

15. Turn the eye over, and observe where the optic nerve enters the eyeball.

16. Take a photograph of the dissected cow eye with the lens removed. Make sure your name and access code are handwritten on a piece of paper that is clearly visible in the background of the photography.

17. Clean off your dissecting tray and dissecting tools with soap and water. Biological scraps should not be thrown into the garbage. Securely store the biological scraps until the end of the term so that they can be properly disposed of at one time.

18. Clean the area in which you worked with soap and water. As long as the underpad has not been damaged, keep it for future experiments.