Sheep Eye Dissection

External AnatomyUsing your scissors, remove all the fat and muscletissue surrounding the eyeball. Look at the beforeand after pictures below.

before removingfat and muscle

after removingfat and muscle

Once you have removed all the fat and musclesurrounding the sheep eye, identify the structuresin the picture below.

The sclera is the white, outer covering of theeye.

The cornea is the transparent layer at the front of the eye through which light firstenters the eye.

The optic nerve carries impulses from theeye to the brain.

Using your scissors, poke a hole in the sclera justoutside of the cornea. Be careful, it may sprayeye juice on you if you squeeze too hard!

poke herewith scissors

Now, insert the point of your scissors into thehole and cut all the way around the eye followingthe line indicated below.

cut all the wayaround the eye

following this line

Next, remove the portion of the eye containingthe cornea that you just cut out with the scissors.Now, remove the contents of the eye and laythem on the tray next to the part you just cut out.Use the diagram below to identify the eye struc-ures that are now visible.

The iris controls how much light enters the eyeby expanding and contracting to control thesize of the pupil. The pupil is the hole in theiris through which light enters the inner eye.

The ciliary muscle expands and contracts to change the shape of the lens. The lens focusesthe image onto the back of your eye.

The vitreous body or vitreous humor is atransparent jelly that fills the inside of the eye.

Now, look at the empty eyeball and locate theback layer of the eye called the retina. Alsolocate where the optic nerve leaves the eye.This is called the blind spot.

The retina is a tissue at the back of the eye that issensitive to light energy. This is where the imageis focused by the lens. Light energy stimulatesthe cells on the retina which send impulses tothe optic nerve. The optic nerve carries theseimpulses to the brain and then you see.

Using your tweezers, remove the retina whichis the white layer covering the back of the eye.Now you should be able to see a bluish layercalled the tapetum. Look at the diagram on thenext slide to identify the tapetum.

The tapetum is a reflective surface that helpssome animals to see better at night. If you haveever seen a dog or cat’s eyes shine at night it isfrom the tapetum. Humans do not have a tapetumlayer in their eyes.

Cleanup

Throw the eye parts in the garbage, wipe out yourtray with a paper towel, and wipe off the dissectingtools.

Wash your hands with soap and water and thenanswer the questions beginning on the next slideon a separate piece of paper.

Photograph & ID

Questions & Conslusions1. What is the sclera?2. What is the cornea and what does it do?3. What is the function of the optic nerve?4. What does the iris do?5. What is the pupil?6. What is the function of the ciliary muscle?7. What does the lens do?8. What is the vitreous humor?9. Where is the blind spot located?10. What is the retina?11. What is the function of the tapetum?12. Do humans have a tapetum layer?