SNC 2D1 – Optics Culminating Task



SNC 2P – Sheep Eye Dissection

/5 KU /12 TI /18 C /10 A

Name:

/45 MARKS

Introduction

The anatomy of the human eye can be better shown and understood by the actual dissection of an eye. One eye of choice for dissection, that closely resembles the human eye, is that of the sheep. Sheep eyes are removed at the time the animal is slaughtered and then preserved for later use.

The sclera is the tough, outer coat of the eyeball, which helps keep the shape of the eye, and protect it from injury. The pinkish parts around the sclera are the remnants of extrinsic muscles. Theses muscles move the eyeball in its socket- from left to right, up and down, and a little bit of rotation. Whitish, lumpy masses around the sclera are fat tissue, which help to cushion the eye in its socket. The cornea would be transparent in life, showing the pupil behind it. The preservative in which this eyeball was kept has made the cornea cloudy. The optic nerve carries the information about what the eye is seeing, from the retina (inside they eye) to the brain.

After the front of the eye has been removed from the back, the internal parts can be seen. This view of the back of the eyeball, seen from inside, shows the retina as a translucent greyish membrane, with wrinkles in it. Although some of the wrinkles formed from taking the eye apart, many of them are the remains of blood vessels, which supply the retina with nutrients and oxygen. Traces of blood show up in some of these vessels across the middle of the retina. The point where all these blood vessels, and also all the nerves of the retina, gather together to leave the eye and become the optic nerve, is the blind spot. There is no room for any light receptors in the blind spot, because of all the nerves and blood vessels here.

Because sheep have to watch out for predators, such as wolves, which might attack when it is getting dark, they have a band of very reflective choroid, called the tapetum. The tapetum makes the retina in front of it twice is sensitive to light, by bouncing back light rays which may have missed a light-sensitive cell, and giving them a second chance to be picked up. Behind the retina here, the tapetum lucidum may be silvery, pearly white, or a beautiful iridescent blue when exposed. In this dissected eye, the tapetum behind the retina is clearly blue. The blind spot is not visible, but its approximate position can be guessed at by following the direction of the optic nerve. The blind spot is never located over the tapetum, as this is the zone where sensitive vision is needed for sheep to see danger coming. Humans do not have a tapetum. Instead, in the human eye, a small central area in each eye, called the fovea, is packed with colour-sensitive cells called cones. This gives us sharp, full-colour vision in the centre of our visual field. Sheep do not see colour, and do not have a fovea.

Sheep do not have a round pupil like humans. Instead of varying from a small circular opening in bright light, to a large circular opening in dim light, the sheep's pupil remains wide, and only varies in height. In bright light, the pupil may have a peanut-like shape. As the light becomes dimmer, the sheep's pupil will chance to an oval shape, then finally to a full circle. This gives the sheep wide-angle vision, to scan the horizon for predators without moving its head.

The iris is made of muscles, which change the size (and shape, in sheep) of the pupil. Pigment such as melanin in the iris prevents light from entering the eye, except through the pupil. The ciliary muscle (or ciliary body) surrounds the edge of the lens, and causes the lens to change in thickness, to focus on near or far-away objects. The vitreous body (which used to be called the vitreous humour) is a clear, transparent jelly, which fills most of the eyeball, between the lens and the retina. In some preserved eyes, the vitreous body is almost gone, due to the action of the preservative solution. In this specimen, most of the vitreous body is intact. The lens is clear and transparent as crystal in a living eye, but the preservative changes it to a cloudy orange color, almost opaque to light. In life, it is flexible, and able to change shape to focus. In preserved eyes, it becomes hard and rigid.

Purpose: (1 mark – Knowledge)

Materials: (2 marks - Knowledge)

Safety: (2 marks - Knowledge)

Sheep Eye Dissection Procedure

Part 1 – External Structures (8 marks – Communication)

• Wash the sheep eye in running water to remove the preservative fluid. Dry the eye with paper towel.

• Examine the front of the eye and locate the eye-lid, cornea, sclera (thin tissue surrounding the cornea) and fatty tissue.

• Examine the back of the eye and find extrinsic muscle bundles, fatty tissue and the optic nerve. The four extrinsic muscles (humans have six) move the sheep eye while the fatty tissue cushions the eye.

• If the optic nerve is not visible use the probe to move the fatty tissue around until the nerve is exposed.

|Structure |Description (4 marks) – 2 details each |

|Optic Nerve | |

|Muscles | |

|Sclera | |

|Cornea | |

Draw a titled, labelled sketch of the external structures of the eye, including the cornea, fatty tissue, optic nerve and sclera. (4 marks – title and neatness, accuracy of diagram, correct labels)

Part 2 – Internal Structures (10 marks – Communication)

• Place the eye in the dissecting pan so it is facing you. Using the tip of your probe, gently pierce the white part of the eye about 1 cm away from the edge of the cornea. Make a hole large enough for your scissors.

• Using your scissors, carefully cut around the cornea until you have made a complete circle. Try not to disturb the inside of the eye as you cut (don’t cut too deep – keep the scissors horizontal). Use the edge of the cornea as a guide. Fluid called aqueous humour will come out of the eye as you cut.

• Carefully remove the cornea from the front of the eye. Try looking through it. Observe the lens, the iris, and the ciliary muscles at the front of the eye with the cornea removed. Observe and record the shape of the lens.

• Carefully remove the lens and vitreous humour from the eye. Use your tweezers and probe to carefully work around the edges of the lens.

• Cut the back of the eye partly away from the iris and ciliary body to expose the retina, the blind spot and the tapetum.

|Structure |Description (6 marks – 2 details each) |

|Iris | |

|Lens | |

|Ciliary Muscle | |

|Vitreous Humour | |

|Retina | |

|Tapetum | |

Draw a labelled sketch of the inside parts of the sheep eye, including the iris, lens, retina, tapetum, and vitreous humour. (4 marks – title and neatness, accuracy of diagram, correct labels)

Sheep Eye Dissection Analysis Questions

Discussion Questions (7 marks – Thinking & Inquiry)

1. What is the purpose of the sclera? (1 mark)

2. Why is the cornea cloudy in the dissected sheep eye? (1 mark)

3. Explain two functions of the tapetum for a sheep’s survival. (2 marks)

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2 -

4. Explain three differences between a sheep eye and a human eye. (3 marks)

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3 -

Conclusion (5 marks – Thinking & Inquiry)

Summarize the lab, including what went well, what was challenging, and what you would change if done again. Compare learning about the eye in class versus the actual dissection. How could this lab help you in the future?

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