Physiology Lab: Cow Eye Dissection Guide
The Exterior of the Globe:
The eye with
The eye with
orbital fat removed clearly showing optic nerve
In dissecting the eye, it is important to note the aspect in which the structures
are located (anterior, posterior, superior, inferior, lateral, medial etc).
Examine the external appearance of the eye. Some of the structures, which
may remain intact on the globe, are the extrinsic muscles, orbital fat root
of the optic nerve, the eyelids, (including the third eyelid known as the
nictitating membrane) and the conjunctiva. Locate the positions of the eye
with reference to the six aforementioned aspects. Some clues, which may
help in this identification, are the shape of the pupil or cornea, the position
of the optic nerve and the location of the nictitating membrane. The pupil,
when contracted, and the cornea are pear shaped, with the broader end situated
medially or nasally. On the posterior aspect of the globe, the root of the
optic nerve is located somewhat ventral to the medial horizontal plane and
curves nasally. The nicitating membrane is located in the medial and ventral
quadrant when viewed from the front. In the human, a vestige of the nicitating
membrane (the plica semilunaris) can be seen in the medial corner of the
eye, situated behind the pink pad of tissue (the caruncle).
The specific extrinsic muscles need not be identified by name on your specimen,
since some of them have been torn away at the insertion. However, their
functions will be briefly described. There are six muscles concerned with
the orientation of the eye in space: 1.) the superior rectus, 2.) the inferior
rectus, 3.) the lateral rectus, 4.) the medial rectus, 5.) the superior
oblique and 6.) the inferior oblique. The first two muscles rotate the eye
upward and downward, the second two muscles rotate the eye nasally and temporally,
and the third two are responsible for tilting ht eye. Of course, any rotation
of the eye is due to an intricate coordination of all of these muscles responding
simultaneously. Three cranial nerves innervate these muscles- the oculomotor,
trochlear and abducens. There are two other muscles attached to the eye.
The levator is located above ht superior rectus, which upon contraction
raises the upper eyelid. Both the levator and the superior rectus are served
by the oculomotor nerve and they act conjointly. Thus when the superior
rectus contracts, rotating the eye upwards, the levator also contracts to
withdraw the upper eyelid from the field of vision. Rhythmic inhibition
of the levator causes blinking. The other aforementioned muscle is the retractor
surrounding the optic nerve and inserted around the equator of the globe
(the equator is defined with reference to the anterior and posterior poles).
The retractor, which is absent in humans, pulls the entire eye deeper within
the orbit. As a protective mechanism this action as results in the passive
partial closure of the nictitating membrane over the sclera and cornea.
The third eyelid has no muscle of its own, but is served indirectly by the
retractor. The abducens nerve innervates the retractor. The lower eyelid
has no separate muscle either, but a branch of the inferior rectus is attached
to it, providing some degree of movement.
The yellowish orbital fat in which these muscles are embedded functions
as a cushion absorbing any mechanical shocks to the eye and provides support
to the globe within the orbit.
Around the root of the optical nerve is the dural sheath. This sheath is
continuous with the dura matter of the brain, consistent with the embryological
development of the optic nerve and retina as an extension of the central
The conjunctiva is a delicate, folded membrane arising at the margin of
the cornea and sclera (called the limbus region), covering the sclera for
a short distance before folding backward to line the inner surface of the
eyelids. The conjunctiva secretes a watery fluid lubricates the area between
the eyelids and the cornea.
Now with the aid of a scalpel, scrape away the extrinsic muscles, orbital
fat and the eyelids, but preserve the root of the optic nerve. These structures
as well as any others surrounding the eye arte collectively termed the ocular
adnexa. After the adnexa is removed, the sclera will be completely exposed
over the entire surface of the globe. With a light pressure applies by the
fingers and thumb, note any differences in scleral thickness at different
points on the globe. Is the globe spherical or is one axis relatively short?
Note any pigmentation at the corneal margin and examine the corneal surface
for abrasions due to the loss of epithelium. If abrasions are not present,
make a few by scraping with the scalpel. If the cornea is not too cloudy,
the position and shape of the pupil should be noted. How does the cow pupil
differ from the human pupil? The innervation of the cornea is standard in
most mammals. Fine corneal nerve fibers emanate from a pericorneal plexus
deriving from the ciliary nerves. The latter are both sensory and motor
in function, supplying fibers to the ciliary muscle, iris, and cornea, and
constitute two twigs of the ophthalmic branch of the trigeminal nerve. In
the cornea both free nerve endings and terminals with rounded end bulbs
have been identified histologically. All of the cutaneous sensations can
be elicited from the cornea, but it is interesting that this sensitivity
is reduced in the infant during the first year of life.
After the cleaning of the globe, immerse it in water and observe the episcleral
fibers. These fibers connect the membrane loosely surrounding the globe
and are commonly known as Teron's capsule.
Mark the top and bottom by means of a notch and bisect the globe around
the equator. Hold the globe between the thumb and the forefingers of the
left hand and make an incision through the sclera with the scalpel, using
a sawing action. Upon penetration, registered by an escape of fluid, insert
the scalpel or a pair of scissors and cut all around the equator. Without
letting the two halves separate, take the scalpel and gently cut through
the vitreous humor with a sawing motion. Unless the vitreous humor is cut
first, it is liable to pull away as a whole, bringing the lens and retina
with it. This method of bisection avoids the lens, which is ordinarily hard
The Interior of the Globe:
is white film, black is pigment epithelium and blue spot (on left) is tapetum
Here is a sheep
eye showing the exit of the optic nerve.
An albino eye
cut around the orbit is better for showing ciliary muscle
Place the two halves in water to avoid drying out and to float out some
of the more delicate structures. Note the inequities in scleral thickness.
The appearance of the posterior half is similar to the view the ophthalmologist
receives when examining the eye. This hemispherical bowl is called the fundus.
Look for the optic nerve head or disc (devoid of receptor elements), the
filmy retina and vessels (folds in the retina are artifacts), the choroid
and any surface coloration it may have (the tapetum). Except in primate
mammals, no central retinal area, macula or fovea, is macroscopically distinguishable.
The choroid is a heavily pigmented layer behind the retina, providing a
dark chamber for the eye and absorbing incident light before it can be reflected.
On the inner surface of the choroid is a brilliant patch of colors, the
varying from yellow to blue to violet. It is responsible for the glow so
readily seen in the pupil of certain animals at night when they face a light
source. The tapetum is not found in the human eye. Although its function
is uncertain, it may act as a back reflector, increasing the amount of light
passing through the retina, and hence its sensitivity at low levels of illumination.
The receptor elements of the retina are located adjacent to the inner surface
of the choroid, with the bipolars, ganglion cells and blood vessels lying
over them. Light passing through the dioptric apparatus (cornea, aqueous
humor, lens and vitreous humor) must therefore pass through these retinal
layers before stimulating the photoreceptors or being absorbed by the choroid.
Gently pull apart of the retina away from the choroid and note its transparency.
Now examine the anterior half of the globe. Notice the sharp termination
of the retina along the circle of attachment called the ora ciliaris retinae
(ora serrata in humans). Beyond this point the choroid thickens to form
the ciliary body, which is roughly triangular in cross section. The ciliary
body tapers down from the iris and from it emanate the sensory ligaments
of the lens. The deeply pigmented sphere including the choroid, ciliary
body and iris is termed the uvea or middle coat of the eye. A number of
meridional folds, the ciliary processes, can be easily distinguished on
the inner surface of the ciliary body. The lens is located behind the iris.
Through it one may obtain "the cow's eye view" by holding the
anterior half towards the light.
After making these observations on the interior of the globe, the dissection
of the two halves may begin. First it will be necessary to remove the vitreous
humor, a gel-like substance having the consistency of raw eye white. With
a brushing motion, pull the vitreous from the posterior half. Of done properly,
the retina and choroid will remain intact and in position. Place a small
piece of the vitreous humor on a paper towel. Notice the gradual loss of
water content leaving the small solid residue. A watery fluid called the
aqueous humor fills the chamber between the lens and cornea. The vitreous
is similar to, if not identical to the aqueous humor. The major difference
between the two is that the vitreous contains a tenuous molecular sponge
or network of immense capacity for water absorption.
Dissection of the Posterior Half:
After removal of the vitreous replace the posterior half under water
and note how the retina comes away readily from the underlying choroid but
remains attached at the optic disc. The greater strength of the retina in
this area is due to the increased density of neuroglial cells around the
optic disc. As in the glial cells of the retina are supportive and connective
in function, as well as serving to insulate one neuron from another. The
blood vessels of the retina are visible only insofar as they retain some
blood in them, but tend to follow retinal folds.
With a scalpel, bisect the posterior half through the optic nerve root and
the optic disc. Note that it is narrowest where it transverses the scleral
foramen. With blunt forceps strip away the retina and examine the choroid
and tapetum. Scrape away some of the black pigment of the choroid near the
margin of the tapetum. How far does the tapetum extend beneath this pigment
layer? Again with forceps peel off the choroid. This tears away from the
sclera, the fibers of the suprachoroid, the outer cellular layer of the
choroid composed of pigmented connective tissue, and exposes a pigment layer
on the inner surface of the sclera upon which the imprint of ciliary nerves
and vessels are seen as the lightest
tracery. As described above, the ciliary nerves innervate the cornea,
iris and ciliary muscle. In the eye, nerves and blood vessels tend to follow
similar routes, frequently located adjacent to one another. Where this occurs
they are often named similarly, thus ciliary nerves and ciliary vessels.
The arterial circulatory system of the eye arises, in humans, from the internal
carotid artery. From this artery branches the ophthalmic artery, which subsequently
divides into the ciliary arteries and central retinal arteries. Venous blood
leaves the globe primarily via the large vorticose veins and smaller ciliary
veins between the choroid and sclera. These converge upward and downward
into superior and inferior ophthalmic vein that empty into the cavernous
sinus located in the floor of the cranium. The sinus is drained by the internal
jugular vein. The vascular system of the cow eye differs considerably from
the human eye. One of the major differences is that arterial influx and
venous efflux are provided primarily by the external carotid and external
jugular, respectively, rather then their respective internal branches as
Dissection of the Anterior Half:
Here is the lens
of the eye
Invert the anterior half over a dish of water. If the lens and vitreous
do not fall away, it will be necessary to cut through the suspensory ligaments
of the lens (attached to the ciliary body) by working a scalpel around,
between the lens equator and the ciliary processes, taking care not to damage
either. Detach the lens and note any differences in curvature between the
anterior and posterior surfaces. Then bisect the lens along the antero-posterior
axis. Note the laminated structure and harder central nucleus. Using your
fingers, break one of the lens halves into quarters. The laminations may
be more apparent now. A thin, transparent membrane, called the lens capsule,
should be visible at the point of fracture. The suspensory ligaments are
continuous with the lens capsule. The lens is the mechanism for ocular accommodation.
When the ciliary muscle, located on the broad anterior aspect of the ciliary
body, contracts in a sphincter like fashion, tension on the suspensory ligaments
is relieved, resulting in an increased curvature of the lens. Objects near
to the eye are thus brought into focus on the retina. The innervation for
the accommodation reflex is not well understood. It is thought to be primarily
a parasympathetic effect, initiated by a change in the focus of the light
incident on the fovea. Pathways to the visual cortex and superior colliculus
via the optic nerve and lateral geniculate bodies return to the ciliary
muscle is very small and probably has little significance. In considering
the role of this muscle in accommodation, how does its small size relate
to the location of the eyes on the head?
Now inspect the inner aspect of the remaining portion of the anterior half.
With blunt forceps test the attachment of the retina at the margin of the
ciliary body. Remove any vitreous that may remain. Strip off the anterior
uvea in one piece with the forceps and place it aside. Examine the inner
surface of the outer coat and then bisect it. At the edge of the cut, look
for differences in corneal thickness between the center and periphery. Note
the overlap the sclera over the cornea.
Now examine the anterior uvea. Note the white band denoting position of
the ciliary body (and muscle), the iris pattern and pigment, and the size
and shape of the pupil. Observe closely for traces of ciliary nerves and
vessels. When contracted the pupil assumes a somewhat pear shape shaped
horizontal slit with the broader end situated medially in conformity with
the shape of the cornea, but upon dilation, the pupil becomes more circular.
Changes in pupil size are a classical example of the antagonism between
the sympathetic and parasympathetic divisions of the autonomic nervous system.
The dilator muscles of the iris, oriented radially and innervated by the
sympathetic system, produce an increase in pupil size upon contraction.
Fibers from the superior cervical ganglion reach the trigeminal nerve via
the internal carotid nerve. The ciliary nerves of the ophthalmic branch
of the trigeminal then terminate on the dilator muscles. In opposite fashion
the sphincter muscles of the iris, oriented concentrically and innervated
by the parasympathetic system, cause the pupil to shrink upon contraction.
The sphincters are dominant over the dilators. Like the ciliary muscle sphincters,
the iris sphincters are innervated by the oculomotor nerve. Afferents from
the cornea, the ciliary muscle and the iris project into the trigeminal
nerve. Although the iris muscles respond to a wide variety of stimuli of
psychological origin, the simple and involuntary papillary light reflex
is relatively straightforward. In the optic nerve a discrete bundle of fibers,
the papillary fibers, are believed to be specialized for the papillary reflex
arc. These fibers semi decussate at the optic chiasm and terminate in the
pretectal nuclei of the midbrain. At this point additional crossing of fibers
occurs via the posterior commissure, other fibers from the petectal nuclei
synapse in the nuclei of the oculomotor nerves. As mentioned previously,
the oculomotor nerves then innervate the sphincters of the iris (but after
one synapse in the ciliary ganglion). The dilators of the iris remain in
relatively tonic contraction, exerting their influence when the sphincter
reflex arc is inactive. Finally, on the upper and lower edges of the iris
are located several deeply pigmented masses, the corpora nigra. These bodies
are arranged in such a way that when the iris sphincters contract strongly
the corpora interlace with one another, providing an additional degree or
closure of the pupil beyond that provided by the muscular response of the
Thanks to Christine Zelle, Lab coordinator for upper division biology labs,
for helping to prepare this lab, new Fall, 2004, for the General Physiology
This page was last updated 6/2/05
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