The mammalian eye consists of many specialized cells and tissues that make up several different structures. The structures have certain functions and together, they form images that are interpreted by the brain. In this lab, you identify the structures of a horse eye and learn their functions.
A. Locate External features of the eye: cornea, sclera, and optic nerve.
a. The white part of the eye, the sclera , is a tough, outer covering of the eyeball. The sclera gives the eye its shape and helps to protect the delicate inner parts.
b. The blue covering over the front of the eye is the cornea. When the horse was alive, the cornea was clear. Together with the lens, the cornea refracts light and helps the eye to focus. The cornea gives a larger contribution to the total refraction than the lens. The curvature of the cornea is fixed while that of the lens is changeable. In your horse's eye, the cornea may be cloudy.
c. You may be able to look through the cornea and see the iris, the colored part of the eye, and the pupil, the dark oval in the middle of the iris. At the back of the eye is the optic nerve. To see the separate fibers that make up the optic nerve, pinch the nerve with a pair of scissors or your fingers. If you squeeze the optic nerve, you may get some white goop. That is myelin, the fatty layer that surrounds each fiber of the nerve. It is the nerve that transmits visual information from the eye to the brain.
1. List two functions of the sclera.
2. The cornea covers and helps protect the eye. List two other functions of the cornea.
3. What is the function of the optic nerve?
B. Examine the fat and muscle surrounding the eyeball.
Without moving your head, look up. Look down. Look all around. Six muscles attached to your eyeball move your eye so you can look in different directions. Cows have only four muscles that control their eyes. They can look up, down, left, and right, but they can’t roll their eyes like you can. Locate the externally attached muscles . These muscles control eye movement and help focus images.
C. Cut away the fat and muscle.
D. Remove the cornea.
Use your scissors to cut around the middle of the eye, cutting the eye in half. You’ll end up with two halves. On the front half will be the cornea . The cornea is made of pretty tough stuff—it helps protect your eye. It also helps you see by bending the light that comes into your eye.
E. The next step is to pull out the iris.
The iris is between the cornea and the lens. It may be stuck to the cornea or it may have stayed with the back of the eye. Find the iris and pull it out. It should come out in one piece.
F. Now you want to remove the lens.
a. It’s a clear lump about the size and shape of a squashed marble. The lens is a transparent structure in the eye that, along with the cornea, helps to refract and focus light.
b. A ring of tiny ciliary muscles , located along the inner side of the iris, connects the lens to the middle layer of the eye. Ciliary muscles contract to change the curvature of the lens.
c. The lens of the horse’s eye feels soft on the outside and hard in the middle. Hold the lens up and look through it. In a living organism, it is completely transparent. To focus on closer objects, it gets fatter so it can refract more light.
d. Put the lens down on a text and look through it at the words on the page. If your lens is transparent, it should magnify.
List the two functions of the lens.
Describe the iris and explain its function.
Describe the pupil.
If you enter a very bright room after being in the dark, what would happen to your pupils – get larger or get smaller?
G. Now it’s time to examine the retina.
a. On the inside of the back half of the eyeball, you can see some blood vessels that are part of a thin fleshy film. That film is the retina. Before you cut the eye open, the vitreous humor pushed against the retina so that it lay flat on the back of the eye. It may be all pushed together in a wad now.
b. Use your finger to push the retina around. The retina is attached to the back of the eye at just one spot. Can you find that spot? That’s the place where nerves from all the cells in the retina come together. All these nerves go out the back of the eye, forming the optic nerve, the bundle of nerves that carries messages from the eye to the brain. The brain uses information from the retina to make a mental picture of the world.
Why does the optic nerve cause a blind spot?