SCN3230-ABED_1: Module 1 Lesson 2 - 3 (Lab)

Lesson 2 — Photoreception: The Eye

Lab - Dissection of an Eye

Read page 417

Introduction

In this dissection lab, you will participate in a virtual dissection of an eye. Typically, a cow eye is used for this dissection because its anatomy is very similar to that of the human eye. Refer to Investigation 12A: Dissection of an Eye on page 417 of your textbook as a guide to the procedure.

Procedure

Click the tabs to progress through this virtual lab.

Examine the outside of the eye. How many parts of the eye can you identify?

You should be able to find the sclera, which is the tough, outer covering of the eyeball. You should be able to identify the fat and muscle surrounding the eye, the cornea, the iris, and the pupil. Cut away the fat and muscle, and you will see the optic nerve protruding from the back of the eye.

Front of the eye.

Back of the eye - it is covered in fat and muscle.

Back of the eye after cutting away the muscle and fat. The optic nerve is protruding out from the back.

Use the scalpel to make an incision through the sclera in the middle of the eye. Use the scissors to cut around the middle of the eye, cutting the eye in half. On the front half is the cornea. The cornea is made of tough material. It helps protect the eye and bends the light that comes into the eye.

Cutting the eye in half: the front half will be the cornea.

We will also cut the eye into left and right half.

This will help us visualize the spot where the optic nerve will be exiting -- the blind spot.

Take the front half of the eye. Use a scalpel to make an incision in the cornea. Cut until the clear liquid under the cornea is released. That clear liquid is the aqueous humor. It is mostly water and keeps the shape of the cornea.

The next step is to pull out the iris. The iris is between the cornea and the lens. You can see a hole in the centre of the iris. That is the pupil, the hole that allows light into the eye. The iris contracts or expands to change the size of the pupil. In dim light, the pupil opens wide to let light in. In bright light, the pupil becomes smaller to reduce light entering.

This is the front of the pupil after the cornea has been cut out.

This is the back of the pupil. The hole in the middle of the iris is called the pupil.

Now, look at the back half of the eye, which is filled with a clear jelly. That is the vitreous humor, a mixture of protein and water. It is clear so light can pass through it. It also helps maintain the shape of the eyeball.

Remove the lens. It is a clear lump that looks similar to a squashed marble. The lens of the cow’s eye feels soft on the outside and hard in the middle.

The lens is located behind the iris. Most of vitreous humor was emptied from the eye when we made an incision to cut the eye into two halves.

Removing the lens.

This lens is cloudy because it is from a preserved cow eye.

Now, look at the remainder of the eye. The thin, yellow, fleshy film is the retina, which is composed of photoreceptors called rods and cones that can detect light. The light that comes into the eye through the lens lands on the retina. The cells of the retina react to the light that falls on them and send messages to the brain.

Use your finger to push the retina around. The retina is attached to the back of the eye at just one spot. That is the place where nerves from all the cells in the retina come together. All these nerves go out the back of the eye to form the optic nerve, which is the bundle of nerves that carries messages from the eye to the brain. This spot is called the optic disc or blind spot. Because no light-sensitive cells are at that spot, anything that lands in that place on the retina cannot be seen by the individual.

Try to locate the small depression in the retina termed the fovea centralis. With its a high density of cones, this area is specialized for sensitivity to colour and acute images. The rods, which are sensitive to light intensity, cover the surrounding area of the retina.

The yellow film is the retina. It covers the back of the eye.

Can you see where the retina is attached to the back of the eye? That is where the nerve cells in the retina come together and exit the eye.

After removing the retina, you can see clearly where the nerve cells exit. This is the blind spot.

Under the retina, the back of the eye is covered with shiny, blue-green stuff called the tapetum. It reflects light from the back of the eye. If you shine a light at a cow at night, the cow’s eyes will shine with a blue-green light because the light reflects from the tapetum.

Look at the other side of the back of the eye. Can you find the optic nerve? To see the separate fibres that make up the optic nerve, pinch the nerve with a pair of scissors or your fingers.

You can see how the retina is attached to the back of the eye at the blind spot.

These are the left and right halves of the eye. The cornea is the blueish-grey structure on the inside. The optic nerve is on the right half of the eye.

Looking at the right half of the eye, we can see the blind spot and the optic nerve clearly.

Now, recap the cow eye dissection. Look at the various layers of the cow eye.

As you go through the virtual dissection, the role of the aqueous and vitreous humour should have become obvious. Think about glaucoma, which is the disorder that results when the aqueous humour is not drained successfully from the anterior chamber.

After the first incision, the vitreous humour has been emptied from the interior of the eye.

The back of the eye has the retina. The lens has been removed. The front of the eye has the iris and cornea.

The layers of the eye: (left to right) choroid, retina, tapetum, lens, iris, and cornea.

Analysis

Complete the following diagram of the eye to check your understanding.

Check Your Work.

A. cornea

B. lens

C. pupil

D. aqueous humour

E. vitreous humour

F. choroid

G. retina

H. fovea centralis

I. optic nerve

J. blind spot

K. sclera

Watch This

Watch the following video on the dissection of the cow eye to help you understand the process.

©Alberta Education. The Eye: Vision and Perception: A Whole World to See (2:43 - 8:41);
Biologix Series 6. LearnAlberta.ca