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.
Dissectible eye model
Anatomic charts of the human eye
Fresh sheep eye
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
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
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,
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.
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
___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
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.
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
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
Identify the structures (numbered 1–10) of the dissected sheep eye.