DISSECTED SHEEP’S EYE

1. DISSECTED SHEEP’S EYE PETER REONISTO, MD MOORPARK COLLEGE

2. SHEEP EYE The sheep eye has four externally attached muscles (1-4), which control eye movement. The human eye has six (1-6). These additional muscles allow humans the ability to "roll" and move their eyes in all directions.

3. DISSECTION In the area near the top of the eye where the sclera and cornea join (identified with an X mark in the photo), carefully make a stab wound incision using the dissecting probe.  At the point of incision you will notice a small discharge of a watery substance. This important fluid was internally contained within the aqueous humor.

4. Once the tough outer layer of the sclera has been pierced, remove the dissecting probe and carefully insert the dissecting scissors at the point of incision.

5. Following a circular pattern around the sclera, rotate the eye while continuing to carefully cut the eye in half.

6. Attempt to open and separate the front from the back of the eye without damaging its internal structures.

7. Cutting the sclera in a circular pattern, as followed in Step 5 of this procedural tutorial, resulted in also cutting the thin reflective middle layer lining of the sheep eye. This colorful reflective lining is called the tapetum and is not found in the human eye.

8. Attached to the optic nerve, observe a wrinkled saclike structure connected to the back portion of the eye. This is the retina and is considered the inmost layer of the eye. Just as the tapetum surrounds the inside of the sclera, the living tissue of the retina would have been smooth and would have rested against the reflective tapetum. The point at which the retinal nerve tissue connects to the optic nerve is the eye’s blind spot.

9. Separate the retina from the back portion of the eye and again observe the colorful reflective layer of the tapetum.

10. Removing the tapetum from the tough shell like outer layer of the sclera exposes the choroid. With the dissection of the back portion of the eye nearing completion, return your attention to the frontal structures and tissues of the eye.

11. Rest the front section of the eye with the cornea facing downward. Working back to front, a clear jelly like body of fluid should be evident. This is the vitreous humor. The vitreous humor is attached to a convex lens. In the preserved eye, the lens will have a slightly discolored yellow appearance but would be perfectly transparent as a living structure.

12. Remove the lens and vitreous humor by turning the front portion of the eye inside out. A ring of tiny ciliary muscles, located along the inner side of the iris, connects the lens to the middle layer of the eye.

13. The black specks on the outer edge of the lens are small pieces of the connecting ciliary muscles.

14. Remove the lens from the vitreous humor. Use a paper towel and dry the lens of any excess fluid.  

15. Although the yellowed preserved lens is not clear, it remains transparent enough to see through.

16. Having turned the front portion of the eye inside out and removing the lens and vitreous humor; allows the ciliary muscles, pupil, and cornea to be more easily observable. The pupil, surrounded by the iris on the front side of the eye and by the ciliary muscles within the eye, is actually an opening located in the center of the iris. The pupil is covered and protected by the transparent living tissue of the cornea.

17. 1. Cornea 4. Iris 7. Ciliary Body 10. Retina 2. Sclera 5. Pupil 8. Choroid 11. Lens 3. Optic Nerve 6. Ora Serrata 9. Tapetum Lucidum 12. Vitreous Humor

18. END