Eye Dissection

Objectives

Dissect a mammalian eye and identify its major features.

Procedure

  1. Prelab Discussion Questions:
  2. To receive your eyeball, you need to complete the prelab discussion questions as a group.
  3. Once you finish prelab discussion questions, send your leader with your tray and he/she will receive the eye.

  1. Before cutting anything, identify:
  2. Orbital fat around the eyeball
  3. Extrinsic eye muscles (6 in total)
  4. Optic nerve: white cord projecting from the posterior region of the eyeball.
  5. Cornea: clear in a fresh eye butopaque in a preserved eye.
  6. Sclera: tough white wall of theeyeball.
  7. Iris: the contractile structure that forms thecolored portion of the eye.
  8. Pupil: the pupil is just the hole in the iris.
  9. Conjunctiva: (may or may not be present)
  10. Lift some of this thin membrane from the eye with forceps from the anterior surface (except cornea)of the eye.

CHECKPOINT 1 – Identify external structures of the eye and its purpose

  1. Removal of eyelids, muscle, and fat surrounding eye:
  2. Trim away the orbital fat and muscles: leaving the eyeball itself and the optic nerve..Scissors work best.
  3. The eyeball should be quite bare, mostly down to the sclera.
  4. Identify:
  5. Blood vessels that enter/leave the posterior half of the eyeball.
  6. There is also a blood supply down the center of the optic nerve.

CHECKPOINT 2 – Removal of fat and muscle, Identify blood vessels

  1. Cut through the sclera in a “meridional” circle:

  1. About 1 cm from the margin of the cornea, make an incision with a scalpel. Go over the incision several times until you are through all of the tunics.
  2. When you are through, continue with scissors all the way around the eyeball.
    Be careful not to damage the interior structures!
  3. Gently separate the eyeball into anterior and posterior portions.
  4. Usually the jellylike vitreous humor will remain in the posterior portion. Keep the vitreous humor intact and do not break it.

CHECKPOINT 3 – Cut and incisions

  1. Identify the following in the anterior portion:
  1. Aqueous humor: clear liquid in the anterior chamber(space between the iris and the cornea) if it’s still there, it may have drained out already.
  2. Vitreous humor: clear jellylike liquid that fills the eye. Although most of this should remain in the posterior portion, some may be in the anterior portion attached to the lens.
  3. Lens
  4. Ciliary body: muscles that surround the lens equator to control lens shape.
  5. Use a dissecting needle/probe to gently remove the lens and examine it. If the lens is not damaged the lens can be seen through and used to form images of something in the distance.
  6. Iris: after the lens is removed the iris and pupil are visible.
  1. In the posterior portion remove the vitreous humor and identify:

  1. Retina: nearly colorless to cream-colored membrane that detaches easily from the choroid.
  2. Tapetumlucidum: This iridescent area of the choroid coat which is beneath the retina, serves to reflect light back through the retina and is responsible for night vision in animals. The tapetumlucidum is lacking in humans.
  3. Optic Disc: the point where the retina is attached to the posterior wall of the eyeball and where the optic nerve originates. Because there are no receptor cells in the optic disc, this region is also called the “blind spot.”
  4. Choroid: behind the retina and tapetumlucidum but in front of the sclera. Detach the densely pigmented choroid layer from the sclera by probing from the cut edge of the eye cup.

CHECKPOINT 4 – Identify structures and functions

  1. Extra Credit – take a group picture
  2. I must receive the picture by 11:59pm on the day of the lab.
  3. Picture should be labeled with your group leader and period.
  4. Everyone in the group must be in the picture
  5. Everyone must be smiling with your teeth showing! No teeth, No credit!
  1. Clean – up
  2. Dispose of all animal parts and used paper towels in the designated trash can
  3. Thoroughly wash, rinse and bleach all instruments and the dissection tray
  4. Clean your table with 409 or Bleach cleaner
  5. Thoroughly DRY ALL instruments, the dissection tray, and your table
  6. Dispose of all aprons, gloves, and used paper towels in the large grey trash can.
  7. When everything is CLEAN and DRY and BACK IN ORDER, Mr. DeGuzman will sign off on your clean-up.

Checkpoint 5 – Clean – up, lab questions completed and stapled to labsheet

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