Procedure Beye Dissection

1. Obtain a mammalian eye, place it in a dissecting tray, and dissect it as follows: 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 (fig. 35.8).

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

Figure 35.5 The opthalmoscope is used to examine the interior of the eye.

Figure 35.5 The opthalmoscope is used to examine the interior of the eye.

Eyepiece

Rotating disk

Lens selection window

Handle of ophthalmoscope

Eyepiece

Rotating disk

Lens selection window

Handle of ophthalmoscope

Figure 35.6 (a) Rotate the disk of lenses until sharp focus is achieved. (b) Move the ophthalmoscope to within 5 cm of the eye to examine the optic disk.

Optic Disc DiagramOphthalmoscopy Procedure

Figure 35.7 The interior of the eye as seen using an ophthalmoscope: (a) photograph; (b) diagram.

Figure 35.7 The interior of the eye as seen using an ophthalmoscope: (a) photograph; (b) diagram.

Coronal Section Human Eye
(a)

Figure 35.8 Prepare a coronal section of the eye.

Coronal Eye Anatomy

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. Compare the structures identified to figure 35.9. j. Locate the optic disk—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 disk, this region is also called the "blind spot." k. Note the iridescent area of the choroid coat beneath the retina. This colored surface is called the tapetum lucidum. It serves to reflect light back through the retina, an action that is thought to aid the night vision of some animals. The tapetum lucidum is lacking in the human eye.

l. Discard the tissues of the eye as directed by the laboratory instructor.

2. Complete Parts B and C of the laboratory report.

Figure 35.9 Internal structures of the beef eye dissection.

Choroid coat (reflected) —> Sclera

Tapetum lucidum (iridescent)

Figure 35.9 Internal structures of the beef eye dissection.

Choroid coat (reflected) —> Sclera

Tapetum lucidum (iridescent)

Retinal Anatomy Dissection

Pupil Iris

Ciliary body

Vitreous humor (transparent)

Lens

Pupil Iris

Ciliary body

Vitreous humor (transparent)

Lens

Critical Thinking Application

I A strong blow to the head might cause the retina to detach. From observations made during the eye dissection, explain why this could happen.

Web Quest

How do we see? Search this and review the anatomy and physiology of the eye at http://www.mhhe.com/biosci/

abio/martinlmwq.mhtml

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Responses

  • milo
    When dicecting the eye what layer appears creamcolored thin and fragle?
    5 years ago

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