DISSECTION EXERCISE: COW EYE
Introduction:
The eyes contain receptors for light stimuli, and they are well protected by the surrounding skull bones and
eyelids. The eye itself is a hollow ball, roughly spherical in shape. Its wall is composed of three distinct layers.
The outer layer is composed of the fibrous layer consisting of two parts: the sclera and cornea. The sclera which
forms the white portion of the eye and the transparent cornea is continuous with the sclera forms the anterior
bulge where light enters the eye. Attached to the sclera are the extrinsic eye muscles. These move the eye in
order to focus on objects without having to move the entire head.
The (middle) Vascular layer consists of three parts. The choroid coat, a black layer, which absorbs excess light
that passes through the retina and contains blood vessels tat nourish the eye. Anteriorly, the ciliary body forms a
ring of muscle that controls the shape of the lens, which is suspended from the ciliary body by the suspensory
ligaments (ciliary zonules). The iris also contracts and relaxes changing the diameter of the pupil thus
controlling the amount of light entering the eye.
The retina composes the inner Neural layer of the wall of the eye. It contains the photoreceptors: rods for blackand-white vision and cones for color vision. The neuron fibers coalesce at the optic disc, where they form the
optic nerve exiting the eyeball and carries impulses to the brain. The optic disc is known as the blind spot since
it has no receptors. A tiny depression just lateral to the optic disc, the fovea centralis contains densely packed
cones for sharp, direct vision.
The interior space or posterior segment of the eye behind the lens is filled with vitreous humor, a transparent
jellylike substance that helps hold the retina in place and gives shape to the eye. A watery fluid, the aqueous
humor, fills the anterior segment, the space between the lens and cornea.
In this exercise we will be identifying these structures of the eye using a cow eye.
Materials (per pair of students): Cow eye, dissection instruments, dissection tray
Procedure and Observations:
1) Review the structures of the eye from the diagram in your text book and
on the model of the human eye.
2) Working in pairs, each partner should participate in the dissection
procedure.
3) Prepare yourself and your area for the dissection. You will need to be
gloved, have a dissection tray with instruments and the specimen.
4) Identify the exterior structures of the cow eye:
a. Fibrous Tunic: Sclera and cornea
b. Conjunctiva
c. adipose
d. Extrinsic muscles (may be attached)
e. Optic nerve
5) Use a razor to make a small incision in the clear, about 5 mm from the edge of the cornea. Holding the eye
firmly, but gently insert the scissors (the blunt side of the scissors should be inserted) into this incision and cut
through the sclera around the eyeball while holding the cornea upward. The fluid that exudes when making
this cut is the aqueous humor.
6) Now gently lift off the anterior part of the eye and place it on the dissecting tray with its inner surface upward.
In the preserved eyes, the lens often remains with the cornea, but in fresh eyes it usually remains with the
vitreous humor (or vitreous body).
Dissection Exercise: Cow Eye, page 135
7) Examine the anterior portion. Locate the lens and ciliary body (thickened black ring). Remove the lens (as
applicable) from the ciliary body by gently cutting the tiny suspensory ligaments. Once removed, the iris
and pupil can be seen clearly. There may be a little aqueous humor remaining next to the inner surface of the
cornea.
Observe the color of the iris and the shape of the pupil in your specimen and your neighbors specimen, what
do you notice about the color of a cow’s eyes? How does this compare with human eye color and the shape
of our pupil?
8) There are two spaces within the eye. The anterior segment is the space between the cornea and lens. It is
filled with aqueous humor. The posterior segment is the space between the lens and the retina which is
filled with vitreous humor.
Observe the vitreous humor within the posterior segment of the eyeball. Looking through the vitreous humor,
the beige retina held in place against the posterior wall of the eyeball. Identify the blind spot or the optic
disc. The fovea centralis is a region of high acuity densely populated with cones for color vision. The cow
has only black and white vision so this is not found as part of the cow eye retina.
9) Pour the vitreous humor onto the dissecting pan and note its jellylike consistency. The delicate retina easily
separates from the underlying choroid but it is attached at the blind spot. (The junction of the optic nerve with
the retina).
10) Note the iridescent portion of the choroid layer; it is a modified choroid called the tapetum lucidum which
aids dim-light vision by reflecting light that has passed through the retina back through the retina. This
iridescence causes animals’ eyes to ‘shine’ at night by reflecting the light back to its light source.
11) Complete your lab notebook dissection steps and answer the analysis questions.
Once you have completed all of the identifications, dispose of the eye as directed by your instructor.
Dissection Exercise: Cow Eye, page 136