Sheep Eye Dissection

Sheep Eye Dissection

Adapted from http://bcis-us-sciencedh-gr6.wikispaces.com/file/view/Sheep+Eye+Dissection.doc

For this lab you will be dissecting a sheep eye to examine the structure of the various parts we have studied. You must follow the procedure carefully and answer all questions.

Materials

Sheep eye Latex/Vinyl Gloves Safety goggles

Lab Apron Dissecting probe Dissecting scissors

Tweezers Dissecting tray Paper towels Plastic Trash bag

Procedure

1.  Prior to making any incisions (cuts), examine the outside layer of the sheep eye. Identify the following parts: The sclera, cornea, and the optic nerve. Notice the amount of external yellow, light and dark tissue around the eye. This tissue is fat and muscle.

2.  Use the scissors to cut away the excess fat and muscle from around the eye so the sclera and the optic nerve are clearly exposed. Be careful when cutting around the optic nerve so that you did not cut it away along with the fat. The optic nerve will be a “tough” tube like structure coming out of the back of the eye.

3.  The sheep eye has 4 external attached muscles which control the eye movement. The human eye has six. The additional muscles allow humans the ability to “roll” and move their eyes in all directions. Follow a circular pattern around the sclera, rotate the eye while continuing to carefully cut the eye in half.

4.  A clear jelly like fluid will be inside the eye. This is the aqueous humor. Carefully lay the two sides of the eye in the dissection pan.

5.  Observe the back half of the eye. You will see a dark reflective lining. This is called the tapetum and is not found in the human eye. This is why an animal’s eyes glow in the dark. This reflective covering reflects light back toward the lens and allows the animals to see better in limited light.

6.  Attached to the optic nerve is a wrinkled saclike structure connected to the back of the eye. This is the retina and is considered the innermost layer of the eye. Just as the tapetum surrounds the inside of the sclera, the living tissue of the retina would have been smooth and would have rested against the tampetum. The point at which the retinal nerve tissue connects to the optic nerve is the eye’s blind spot.

7.  Separate the retina from the back portion of the eye.

8.  Remove the tapetum from the tough shell like outer layer of the sclera and see if you can find the choroid.

9.  Now observe the front half of the eye. Place the eye with the cornea down.

10.  In the front of the eye, or mixed with the vitreous humor will be a small lens. This lens is convex and will have a slightly gray color. In a live organism, the lens would be transparent?

11.  Remove the lens from the vitreous humor. Dry it with a piece of paper towel. Feel how flexible it is. Hold the lens up in front of your eye, look through it. Describe how the lens changes your vision.

12.  A ring of tiny ciliary muscles are located along the inner side of the iris. These muscles connect to the lens.

13.  The pupil surrounded by the iris is on the front of the eye is the actual opening located in the center of the pupil. The pupil is covered and protected by the transparent living tissue of the cornea.

Questions

1.  It is noted above that humans have 6 external muscles as opposed to the sheep’s 4. Describe if this would make our vision better or worse, and indicate why.

2.  In a live sheep the retina would have been smooth; what texture did it have in the eye you dissected? Why would it be smooth in a living sheep?

3.  Did the retina cover the area where the optic nerve connected to the eye? What would this result in?

4.  State two types of cells found on the retina. Where are they most densely packed? (you may not be able to tell from your dissection).

5.  Why would the lens be transparent in a live sheep? What function would the flexibility of the lens serve? What alters the lens’ shape?

6.  The iris is composed of a thin circular muscle, what function would this serve?