Bio4 Lab Manual: Eye Dissection
Biology 4 Lab: Eye Dissection
Lab #8 Table of Contents:
• Expected Learning Outcomes
• Introduction .
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• Cow Eye Dissection .
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Expected Learning Outcomes
At the end of this lab, you will be
able to
• locate and identify structures in
the cow eyeball; and
• explain the functions of parts of
the eye.
Figure 1: Sagittal Section of the Eye
Introduction
The eye is one of the special sensory organs located in the head, along with the ear, nose,
etc. It is especially complex, combining a number or tissues and specialized structures
that allow us to process visual images. The human eye is the product of hundreds of
millions of years of evolution, and has much in common with other mammals, which is
the reason we can examine the structures of the cow eye and appreciate our own.
Although cows see the world much in the same way we do, there are differences.
Primates have color vision, whereas most other mammals do not. Primates are also
mostly diurnal (day-living), whereas many mammals are adapted to maximize light
because they are active at night. For example, you will see an abalone-like layer lining
the back of the cow eyeball, called the tapetum lucidum. This shiny surface collects and
reflects light and is the reason that many animal eyes glow at night. Humans lack a
tapetum lucidum.
Bio 4 Lab—Eye Dissection
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Bio4 Lab Manual: Eye Dissection
What are you required to know for the test?
1. Locate and write down the name of all structures in bold font. (Spelling
counts!)
2. Be able to answer the functional questions that are included in the Check Your
Understanding boxes.
Where does one find the required information?
Consult Chapter 17 in your textbook.
A useful online resource is:
< www.exploratorium.edu/learning_studio/cow_eye/>
A web site sponsored by the San Francisco Exploratorium museum.
Cow Eye Dissection
Work with a partner. Obtain a cow eye, dissection tray, and a bamboo stick to point out
structures.
Figure 2: Anatomy of the Eye
1. Identify the sclera, the outer layer of tough, white dense irregular connective
tissue. There are numerous grayish-pink extrinsic eye muscles attached to the
sclera, that allow the cow to move its eyeball up and down and from side to
side. The anterior portion of the sclera is modified to form a window, the
Cornea, through which light can pass. The preservative changes the tissue,
and causes the cornea to be cloudy. In life, it is crystal clear.
2. Observe the conjunctiva, a thin layer that covers the inside of the eyelids and
the anterior, exposed part of the eye. It attaches to the edge of the cornea
anteriorly. It occasionally becomes infected and results in “pink-eye”, or
Bio 4 Lab—Eye Dissection
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Bio4 Lab Manual: Eye Dissection
conjunctivitis. The conjunctiva resembles wet, sticky tissue paper that may be
gently lifted from the sclera.
3. Locate the pupil, the circle that can be constricted to control the amount of
light that reaches the center of the eye. The pupil is surrounded by the iris,
which has smooth muscles that can control the pupil size.
4. Opposite the pupil, on the posterior surface of the eyeball, find the optic
nerve, an ivory, rope-like structure that exits the back of the eyeball. It carries
nerve impulses from the photoreceptors to the brain for interpretation.
5. Now use a scalpel to cut the eye in half in a coronal or frontal section so that
the eyeball is divided into anterior and posterior portions (See Figure 3).
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Figure 3: Dissection of Cow Eye
7. The eyeball is arranged in layers. The outermost is the sclera, which you
already identified. The inside of the sclera is lined with the choroid. It
consists of pigmented tissue that appears black. Note the iridescent structure at
the back of the eyeball. It is a specialization of the choroid called the tapetum
lucidum (“carpet of light”). This feature is not found in the human eye, but is
found in many other mammals.
Bio 4 Lab—Eye Dissection
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Bio4 Lab Manual: Eye Dissection
8. The black, anterior part of the choroid that looks like the underside of a
mushroom is the ciliary Body. It contains smooth muscle that changes the
shape of the lens so that the eye can focus on objects in the near distance.
9. When you cut the eyeball, a clear watery fluid leaked out. It is the aqueous
humor, and fills the space between the cornea and the lens in the anterior
section of the eyeball.
10. The jelly-like substance filling the posterior section of the eyeball is the
vitreous humor.
11. Find the lens, a clear, marble-like ball that attaches by tiny threads to the
ciliary body. In the cow eye it is opaque because of the preservative, but in
life it is crystal clear because of a unique arrangement of proteins.
12. Locate the retina, the filmy, beige layer that lines the inside of the back of the
eyeball. It houses the photoreceptors, specialized cells (rods and cones), which
are stimulated by light.
Check Your Understanding
Try to answer the following questions without looking at your notes.
1. What structure of the outer part of the eyeball helps to provide protection and support
for the inner part?
2. What structure in the eye has specialized nerve cells that receive information about
light?
3. What is the shiny structure on the choroid at the back of the cow’s eyeball? What is its
function?
4. Compare and contrast structure and function of the ciliary body and iris.
Bio 4 Lab—Eye Dissection
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