THE EYE
Human senses are categorized as either general senses, or
special senses. General senses include somatic (pain,
proprioceptive, touch/tactile, and temperature/thermal),
and the visceral senses. Receptors are scattered throughout
the body, have simple structures, and can send nerve
impulses to the ANS, ENS, or CNS.
Special senses include smell, taste, hearing, equilibrium,
and vision. Special sense receptors are located at specific
locations in the head only, are associated with complex
organs, and send nerve impulses to the CNS exclusively.
The eye is a special sense organ containing photoreceptors
called rods and cones. When stimulated, photoreceptors
send impulses to the brain via the optic nerve where they
are interpreted as an image. Of all human senses, we rely
most heavily on sight to interact with our environment.
OBJECTIVES: WHAT TO KNOW FOR PRACTICAL
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Identify external anatomical structures of the cow
eye.
Identify internal anatomical structures and
cavities/chambers of the cow eye.
KNOW FOR LAB PRACTICAL:
EYE: EXTERNAL ANATOMY
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extrinsic eye muscles
optic nerve
conjunctiva
cornea
sclera
EYE: INTERNAL ANATOMY
CAVITIES; CHAMBERS; HOLES
• anterior cavity: internal space from lens to cornea;
filled with a fluid called aqueous humor
o anterior chamber: internal space from iris to cornea
o posterior chamber: internal space from lens to iris
• posterior cavity: internal space from retina to lens; filled
with gelatinous vitreous body
• pupil: hole in the iris
STRUCTURES
• cornea: first structure to focus light
• iris: contaction/relaxation of its smooth muscle regulates
diameter of the pupil
• ciliary body and muscle
• suspensory ligaments
• lens: primary light focusing structure
• retina: interior-most of the 3 eye layers
o optic disc
• vitreous body – not shown or labeled in photos
• choroid: middle eye layer
o tapetum lucidum – not found in human eye;
irridescent silver-blue color
• sclera: outer-most, thickest eye layer
PROCEDURE
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Consult the glossary in your text for pronunciation of
structures. By this time, it should be second nature.
REMEMBER: correct pronunciation will lead to correct
spelling. Don’t be lazy; look it up!
Follow the link below to: 2651 Laboratory Page
http://www.valdosta.edu/~dodrobin/2651/2651Lab.htm
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On the drop-down menu to the left entitled Lab Study
Aids, CLICK: Eye Review
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Read the instructions along the left column.
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You will work in pairs with the same lab partner
with whom you studied the sheep brain.
Take your jar over to a sink, remove the cow eye, and
run water over the eye removing surface preservative;
thus, decreasing odor in the lab.
Immediately after removing the eye, tighten the lid
on your jar securely so preservative will not be
spilled if the jar is accidently dropped, or if it gets
knocked off your lab table. There have been cases in
the past where jars with un-tightened (or very loosely
tightened) lids have been knocked to the floor,
resulting in preservative splashing across the floor and
on fellow students.
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EXTERNAL ANATOMY
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Examine external anatomy of the cow eye. Identify
the structures listed above.
The conjunctiva is a thin, vascularized membrane
attached directly to, and covering the cornea, and the
portion of the sclera outside the eye orbit.
Vasodilation of conjunctiva blood vessels results in
“bloodshot” eyes. Inflammation of the conjunctiva is
called “pink eye”, or more precisely, conjunctivitis.
NOTICE: the cornea appears milky-colored and nearly
opaque; in a live eye, the cornea is transparent. The
cornea is composed of protein that becomes
denatured and opaque when preserved.
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There usually is a large amount of adipose tissue
around the posterior portion of the eye. You may need
to remove some fat so that the optic nerve is visible.
DO NOT remove adipose tissue unless it is
necessary; you can view all other external anatomical
features without its removal. A greasy mess will result
if you remove all fat.
INTERNAL ANATOMY
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Place the eye in your dissecting pan, with the cornea
facing upward. Hold the eye firmly against the pan.
Keep your fingers out of harm’s way, apply direct
downward force and puncture (to the inner cavity)
the cow eye with a needle probe or one scissor blade
approximately ½cm outside the cornea perimeter.
(DO NOT use your scalpel or hold the eye in your
hand).
BE CAREFUL: it will take considerable pressure to
puncture the sclera; it is thick, dense, and slick.
Now use scissors; place one blade through the
puncture and proceed to cut around the cornea
completely at the same ½cm distance until you have
separate anterior and posterior portions. The anterior
portion will be a smaller portion of the eye than the
posterior.
Remove the lens that attaches to the ciliary body by
tiny, difficult to see suspensory ligaments.
Nonetheless, you can establish where they attach to
the lens at a noticeable ridge around the lens.
NOTICE: the lens is composed of protein that
becomes opaque and denser as a result of
denaturation from preservation. A live eye has a
transparent lens, able to change shape from nearly
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spherical to ovoid as ciliary muscles contract and pull
on suspensory ligaments.
Pick up the posterior portion of the eye. Gently
squeeze the sides to allow the gelatinous vitreous
body to be extricated. The vitreous body is composed
of protein (like the “white” of an egg) and normally
transparent, but might be slightly opaque due to
preservation.
The vitreous body provides a medium through which
light passes to be focused on retinal photoreceptors,
and provides intraocular pressure needed to keep
the retina pressed to the choroid.
The retina is physically attached at a single point only
- to the optic nerve. The attachment point is called
the optic disc (“blind spot”) because it is the only
location on the retina where there are no
photoreceptors. The optic disc is recognized by a
“puckered” appearance on the retina surface if it
remains pressed against the choroid. The retina may
collapse when the vitreous body is removed, but will
remain attached at the optic disc.
Examine other internal anatomy of the eye. Identify
the structures, cavities, and hole listed above.