Materials

■ safety goggles

■ disposable gloves

■ preserved sheep's eye

■ dissection tray

■ fine scissors

Background

1. The sheep's eye is very similar to the human eye, as shown in the diagram below. The wall of the eyeball

Ciliary body

Ciliary body

is made up of three layers. The outer layer, the sclera, is a tough tissue that forms the white of the eye. At the front of the eye, the sclera becomes thin and transparent to form the cornea.

2. The middle layer, the choroid, is dark and rich with blood vessels. At the front of the eye behind the cornea, the choroid is modified into the iris and the ciliary body. The pigment in the iris determines eye color. The opening in the center of the iris is the pupil. What is the function of the pupil?

3. The inner layer, the retina, is sensitive to light. What are the photoreceptors in the retina called?

4. Directly behind the iris is the elastic, transparent lens. The suspensory ligament attaches the ciliary muscle, the main part of the ciliary body, to the lens. This ring of muscle changes the shape of the lens to focus on near and far objects.

5. The lens and its suspensory ligament divide the eye into two chambers. The large vitreous chamber extends from the retina to the lens and ligaments. It is filled with a gelatinous mass, the vitreous humor. The vitreous humor helps to maintain the shape of the eye and hold the retina in place. The second chamber, which extends from the iris to the cornea, is subdivided into two parts, the anterior and posterior chambers. The anterior chamber extends from the cornea to the iris; the posterior chamber from the iris to the suspensory ligament. Both chambers contain aqueous humor, a watery substance that bathes the front part of the eye.

6. Ganglion cells form synapses with bipolar cells, which, in turn, synapse with rods and cones. In the retina, the axons of ganglion cells bundle together at the optic nerve. The region of the retina where its nerve fibers and blood vessels enter the optic nerve is the small optic disk, which contains no rods or cones. Why is the optic disk also called the blind spot?

7. A very short distance away is a yellowish spot, the fovea. It is the site of sharpest vision because it has a high concentration of photoreceptors.

Procedure

J^L Put on safety goggles, gloves, and ^^ ^^ a lab apron. ^^ CAUTION Use extreme care when handling ^^ all sharp and pointed instruments, such as scalpels.

Muscles

Cornea Pupil

Optic nerve

Muscles

Optic nerve

Cornea Pupil

Muscles

Locate the six main muscles on the outside of the eye, as shown in the diagram above. These muscles move the eye. Use a scalpel to carefully cut the muscles near the eye. This will expose the sclera.

3. Observe the fatty tissue that cushions the eye in its socket, especially around the optic nerve. This fatty tissue helps to prevent shock. With a tweezer and scalpel, remove the fatty tissue. This will expose the optic nerve more fully.

4. Using a scalpel, carefully cut the sclera about 1 cm behind the cornea. Using fine scissors, extend the cut to make a flap that you can lift, as shown in the diagram below.

Incision line

Optic nerve

Scleral flap

Edge of cornea

Edge of cornea

Incision line

Optic nerve

Scleral flap

Blood vessels of choroid Incision line

5. With forceps, carefully remove the sclera in this area and observe the dark choroid layer immediately below the sclera.

6. Next, use a scalpel to make an incision through the eye. Following along the incision you made in step 4, cut almost completely around the eye. You have separated the eye into an anterior and a posterior portion.

7. In the posterior section, observe the whitish retina. It is probably shriveled and may have fallen into the vitreous chamber.

8. In the anterior section, use a blunt probe to expose the lens. In a preserved eye, the lens is no longer clear.

9. In the anterior section, also locate the ciliary muscle and as many other structures as possible.

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