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Dissecting sheep's eyeballs

Introduction
Public summary: 

Only at "Crash, Bang, Squelch!": see the inner workings of the eye first-hand by dissecting a sheep's eyeball! Feel the smoothness of the lens, prod the gelatinous fluid and see the amazing coloured sheen of the layers that help sheep see better in the dark.

Dissecting a sheep's eyeball to show the structure of the eye
Useful information
Kit List: 

Dissection kit
Chopping board
Eye protection for demonstrators

Consumables:
Sheep's Eyeballs, ordered from Blades (~25 required for CBS)
Non-latex gloves
Yellow clinical waste bags/other disposal system as appropriate and agreed with Zoology staff

Packing Away: 

Dispose of biological waste in appropriate way (consult with committee/zoology staff). Do not run experiment if disposal facilities not available.

Explanation
Explanation: 

***BASIC PROCEDURE AND EXPLANATION***

STEP 1: Look at intact eye:

- see where the eye muscles insert. We have a small model with elastic bands to demonstrate how these work to move the eye in the socket, combinations of muscles mean you can move them in all directions.

- See where the optic nerve enters the eye, explaining what it does, plus talking about blind spot

- Curvature of cornea - also acts like a lens, but we can't change its shape like we can the lens inside the eye to focus on objects. In humans, the cornea contributes about ¾ of the eye’s total refractive power (ability to focus light).

STEP 2: Cut around the junction between the cornea and the scelera to remove the cornea:

- Can then see the iris lying on top of the lens.

- Pupil just a hole, can change its size to let more or light in. So when you’re in a dark room need bigger pupils to get more light to see things. When you’re in bright light it need to be smaller to stop the light damaging inside your eyes. It’ s the iris that actually changes the size of the pupil. There are tiny muscles attached to the iris inside the eye that contract and relax to change the size of the pupil.

- This is a reflex (you don’t have to think about making your pupil bigger, it just happens!). A good way to demonstrate this is in front of a mirror in a bright room. If you cover your hand over one eye and then move it away quickly you will see your pupil shrink as it is exposed to bright light from under the shadow of your hand.

- ‘Red-eye’ in photos happens because the flash makes the iris constrict, but not quickly enough to stop most of the bright light entering the eye. The light from the flash goes into the eye and reflects off the retina at the back of it, making the pupil appear bright red.

STEP 3: Cut the globe in half (as in so cornea on one half, optic nerve on the other):

- look inside!

STEP 4: Remove lens to look at it:

- What is the function of the lens? There are muscles around the lens that allow it to change its thickness. If it’s fatter, it can refract light more and is therefore good for looking at things close to you. When people get older the lens is less elastic so they’re not as good at doing this and need glasses to read the paper...

STEP 5: In the back half of the eye:

- see where the optic nerve leaves – ask the child where it goes (to the back of the brain where the visual cortex is). This is an opportunity to talk about the function of nerves if the child is keen.

- the point where the optic nerve leaves is called the blind spot. There are no light sensitive cells here so you are actually blind in this bit of your eye. You can talk here about when Mum/Dad is driving the car and they have to turn around to check their blind spot before overtaking.

- see it’s dark, this is so it absorbs the maximum amount of light

- in sheep's eye there’s an iridescent coloured part, called the tapetum lucidum (‘bright carpet’ in Latin), just behind the retina. It reflects light back to the retina, increasing the amount of light that the retina receives. This helps animals such as the sheep to see better at night. We don’t have a tapetum lucidum so we can hardly see at all in the dark. Ask the child if they have ever seen a photo of their/their friend’s cat/dog. Animal’s eyes often appear to glow blue, yellow or green in photos because of the light from the flash reflecting off the coloured tapetum.

***OTHER THINGS TO TALK ABOUT***

Once you’ve discussed the main features of the eye there are a couple more things you could touch on:

Short/long sightedness and how it is corrected. Short sightedness is when the lens is too thick so light focuses in front of the retina, and is corrected with a CONCAVE lens. Long sightedness is when the lens is too flat so light focuses behind the retina and is corrected with a CONVEX lens. This is discussed in the camera obscura experiment which can be found in the dark room with the Physics experiments.

The retinal cells (rods and cones), how they work (in simple terms!) and the differences between them.

Risk Assessment
Date risk assesment last checked: 
Wed, 08/02/2017
Risk assesment checked by: 
Fiona Coventry
Date risk assesment double checked: 
Fri, 10/02/2017
Risk assesment double-checked by: 
Jess G
Risk Assessment: 
DESCRIPTION Dissect a sheep's eyeball to show structure of the eye
RISKS
  • 1. Splash of preservative in eye, on skin, or ingestion (preservative is Propylene phenoxytol 1% - slightly harmful by ingestion, may be mildy irritating to skin and eye)
  • 2. Stealing of parts
  • 3. Fainting
  • 4. Scalpel cuts
  • 5. Minute risk of prion disease transmission from sheep eyes.
  • ACTION TO BE TAKEN TO MINIMISE RISKS
  • 1+2. Two demonstrators present, one to demonstrate, the other to mainly check that children aren't trying to take things or get hold of scalpels etc, and to look after faints. Demonstrator to wear eye protection and to ensure that cutting does not take place close to where children are standing, especially during the first incision, when spurting is possible. Nitrile or neoprene gloves (NOT LATEX) to be worn by all who might touch the eyeballs.
  • 3. Clearly signed and "separated" area for dissection, to keep away squeamish people. Chair nearby for light-headed-feeling people
  • 4. Scalpels to be kept out of way of kids, demonstrator is experienced and takes care
  • 5. Ensure children are not taking eyes or handling the scalpel, as for 1+2
  • ACTION TO BE TAKEN IN THE EVENT OF AN ACCIDENT
  • 1. Call first aider if preservative goes in eyes. Demonstrator can administer eye wash if trained and confident to do so. Wash preservative off skin with ample water.
  • 2. If equipment gets stolen, asssisting demonstrator to relocate it and inform committee (especially if a scalpel has been taken).
  • 3. Call first aider and close activity
  • 4. Call first aider in case of injury
  • 5. Effects will be too delayed to be seen. There is no known treatment for prion diseases.
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