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The Eye Lab by MikeJenny


									                                  The Eye
                         Laboratory Exercise 35


The eye contains photoreceptors, which are modified neurons located on its
inner wall. Other parts of the eye provide protective functions or make it possible
to move the eyeball. Still other structures serve to focus light entering the eye so
that a sharp image is projected onto the receptor cells. Nerve impulses
generated when the receptors are stimulated travel along the optic nerves to the
brain, which interprets the impulses and creates the sensation of sight.

Materials Needed
Dissectible eye model
Anatomic charts of the human eye
Fresh sheep eye
Dissecting tray
Dissecting instruments

Purpose of the Exercise
Review the structure and function of the eye and dissect a mammalian eye.


1. Label figures 35.1, 35.2, and 35.3.
2. Examine the dissectible model of the eye and locate the following features:
   eyelid, conjunctiva, orbicularis oculi, levator palpebrae superioris,
   lacrimal apparatus (lacrimal gland, canaliculi, lacrimal sac, nasolacrimal
   duct), extrinsic muscles (superior, inferior, medial, and lateral rectus,
   superior and inferior obliques), cornea, sclera, optic nerve, choroid coat,
   ciliary body (ciliary processes and muscles), lens, suspensory ligaments,
   iris, anterior cavity (anterior and posterior chambers, aqueous humor),
   pupil, retina (macula lutea, fovea centralis, optic disc), posterior cavity
   (vitreous humor).
3. Obtain a mammalian eye, place it in a dissecting tray, and dissect it as
   a. trim away the fat and other connective tissues but leave the stubs of the
       extrinsic muscles and of the optic nerve. This nerve projects outward from
       the posterior region of the eyeball.

   b. Note the conjunctiva, which lines the eyelid and is reflected over the
      anterior surface (except cornea) of the eye. Lift some of this thin
      membrane away from the eye with forceps and examine it.
   c. Locate and observe the cornea, sclera, and iris. Also note the pupil and
      its shape. The cornea from a fresh eye will be transparent; when
      preserved, it becomes opaque.
   d. Use sharp scissors to make a coronal section of the eye. To do this, cut
      through the wall about 1 cm from the margin of the cornea and continue all
      the way around the eyeball. Try not to damage the internal structures of
      the eye.
   e. Gently separate the eyeball into anterior and posterior portions. Usually
      the jellylike vitreous humor will remain in the posterior portion, and the
      lens may adhere to it. Place the parts in the dissecting tray with their
      contents facing upward.
   f. Examine the anterior portion of the eye, and locate the ciliary body, which
      appears as a dark, circular structure. Also note the iris and the lens if it
      remained in the anterior portion. The lens is normally attached to the
      ciliary body by many suspensory ligaments, which appear as delicate,
      transparent threads.
   g. Use a dissecting needle to gently remove the lens, and examine it. If the
      lens is still transparent, hold it up and look through it at something in the
      distance and note that the lens inverts the image. (The lens of a
      preserved eye is usually too opaque for this experience.)
   h. Examine the posterior portion of the eye. Note the vitreous humor. This
      jellylike mass helps to hold the lens in place anteriorly and helps to hold
      the retina against the choroid coat.
   i. Carefully remove the vitreous humor and examine the retina. This layer
      will appear as a thin, nearly colorless to cream-colored membrane that
      detaches easily from the choroid coat.
   j. Locate the 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.”
   k. Note the iridescent area of the choroid coat beneath the retina. This
      colored surface in ungulates (mammals having hoofs) is called the
      tapetum fibrosum. It serves to reflect light back through the retina, an
      action that is thought to aid the night vision of some animals. The tapetum
      fibrosum is lacking in the human eye.
4. Complete Parts A, B, C, and D of the laboratory report.

Figure 35.1 Label the structures of the lacrimal apparatus.

Figure 35.2 Label the extrinsic muscles of the right eye (lateral view).

Figure 35.3 Label the structures indicated in this transverse section of the right
eye (superior view).

Figure 35.4 The interior of the eye as seen using an ophthalmoscope: (a)
photograph with an arrow indicating the optic disc; (b) diagram of the retina.

Part A

Match the terms in column B with the descriptions in column A. Place the letter
of your choice in the space provided.

                     Column A                                   Column B

___ 1. Posterior five-sixths of vascular tunic        a. Aqueous humor

___ 2. White part of outer tunic                      b. Choroid coat

___ 3. Transparent anterior portion of outer tunic    c. Ciliary muscle

___ 4. Inner lining of eyelid                         d. Conjunctiva

___ 5. Secretes tears                                 e. Cornea

___ 6. Empties into nasal cavity                      f. Iris

___ 7. Fills posterior cavity of eye                  g. Lacrimal gland

___ 8. Area where optic nerve originates              h. Lysozyme

___ 9. Smooth muscle that controls light intensity    i. Nasolacrimal duct

___10. Fills anterior and posterior chambers of       j. Optic disc
       the anterior cavity of the eye
                                                      k. Retina
___11. Contains visual receptors called rods
       and cones                                      l. Sclera

___12. Connects lens to ciliary body                  m. Suspensory ligaments

___13. Causes lens to change shape                    n. Vitreous humor

___14. Antibacterial agent in tears

Part B

Complete the following:

1. List the structures and fluids through which light passes as it travels from the
   cornea to the retina.

2. List three ways in which rods and cones differ in structure or function.

Part C
After completing the dissection of the eye, complete the following:

1. Which tunic/layer of the eye was the most difficult to cut?

2. What kind of tissue do you think is responsible for this quality of toughness?

3. How do you compare the shape of the pupil in the dissected eye with your
   own pupil?

4. Where do you find aqueous humor in the dissected eye?

5. What is the function of the dark pigment in the choroid coat?

6. Describe the lens of the dissected eye.

7. Describe the vitreous humor of the dissected eye

Part D

Identify the structures (numbered 1–10) of the dissected sheep eye.

 1. _________________________

 2. _________________________

 3. _________________________

 4. _________________________

 5. _________________________

 6. _________________________

 7. _________________________

 8. _________________________

 9. _________________________

10. _________________________


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